INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 39 (2006) R227–R244 doi:10.1088/0022-3727/39/14/R01

TOPICAL REVIEW

Rapid thermal processing of magneticmaterialsZ Q Jin and J P Liu1

Department of Physics, University of Texas at Arlington, Arlington, TX 76019, USA

E-mail: pliu@uta.edu

Received 21 November 2005, in final form 24 March 2006Published 30 June 2006Online at stacks.iop.org/JPhysD/39/R227

AbstractThe rapid thermal processing (RTP) technique features dynamic control oftemperature, which permits high heating and cooling rates that cannot bereached with conventional furnace treatments. In recent years, RTP has beenincreasingly applied to the processing of magnetic materials. Thecontrollable heating profiles provide an approach for the deliberateconstruction of materials structure via expediting phase transitions andtailoring materials morphology. In this review, the history, principles andfacilities of several types of RTP techniques are introduced, including laserand electron beam heating, lamp heating, Joule heating and pulse thermalprocessing techniques. RTP of various advanced magnetic materials arereviewed, including soft magnetic materials, spintronic materials, magneticrecording materials and nanocomposite hard magnetic materials.Advantages of RTP are highlighted and compared with conventional thermaltreatments.

1. Introduction

The development of materials has always been accompanied byadvancements in processing technologies. Recent progress inmagnetic materials research has also proved this phenomenon.For instance, high-energy ball milling has been used to producefine particles and metastable materials that are difficult orimpossible to produce by conventional melting and castingtechniques [1]. The synthesis of permanent magnets aswell as the fabrication of intermetallic compounds have beenextensively studied using mechanical milling and subsequentheat treatments [2–4]. Another technique widely used inmagnetic materials processing is rapid quench, by whichribbons or ribbon-like powders with amorphous structure ornanostructured grain morphology are produced. In melt-spinprocessing, very high cooling rates can be achieved when amelted alloy is ejected onto the surface of a rotating copperwheel. Depending on the quench rate which is proportionalto the speed of the rotating wheel, the microstructureof melt-spun materials can be adjusted. Combined withmodification of sample compositions and subsequent heat

1 Author to whom any correspondence should be addressed.

treatment conditions, the magnetic properties of resultantmelt-spun nanostructured or amorphous materials can betailored [5, 6].

Recently, ‘bottom–up’ approaches are adopted to producebulk nanocomposite magnetic materials [7–9]. In theseapproaches, morphology and property control of the materialsstarts from nanoparticle preparation. To retain the nano-scalemorphology in the final bulk magnets, dynamic compactionof rapidly quenched ribbons or nanoparticles is used toeliminate grain growth while achieving high density of thecompacts. The consolidated NdFeB-based nanocompositemagnets display remarkable magnetic performance. Effectiveinter-phase magnetic exchange-coupling has been achievedin the FePt–Fe3Pt nanocomposite magnets composed of theself-assembly of nanoparticles and 50 per cent energy-productenhancement was obtained compared with the single-phasecounterpart.

Since magnetic properties usually relate to both intrinsicand extrinsic structural parameters of materials, in mostcircumstances for materials processing, heat treatments arenecessary in order to realize desired phase transformationsand to tailor the material morphology. Traditionally, heattreatments were carried out in conventional electric-resistance

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High T

Low T

Adiabatic Mode Thermal flux Mode Isothermal Mode

(a)

(b)

(c)

Figure 1. Schematic figures showing three RTP modes. (a) Temperature distribution along the cross section. (b) Temperature distribution inthe sample surface. (c) Temperature profile.

furnaces with heating and cooling rates normally lower thanseveral degrees per second, though it has been known that thestructure and properties are sensitive to the heating and coolingrates. Grain growth is usually observed during conventionalheat treatments because the slow heating and cooling rates leadto prolonged heating times.

In the past several years, application of rapid thermalprocessing (RTP) techniques to magnetic materials has arousedconsiderable attention, particularly after the extensive develop-ment of low-dimensional magnetic materials, including rapidlyquenched ribbons, nanoparticles, nanowires and thin films.RTP has been used to control the crystallization of amor-phous phases, phase transitions, grain size reduction, grainisolation and crystallographic texture [10–17]. For instance,it was found that the crystallization behaviour of soft mag-netic alloys was remarkably affected by the heating rate. Inhard magnetic films, higher coercivity and remanence andmuch finer grain size were developed by RTP techniques.The RTP was effective in preventing soft magnetic phasesfrom coarsening during the process. RTP was also foundto be effective in promoting grain isolation, reducing themedium noise, lowering the phase transformation temperatureand improving the surface smoothness of magnetic recordingmedia. The crystallization behaviour and magnetic proper-ties of hard magnetic nanocomposites of various composi-tions have been investigated using RTP. Exchange-coupledhard magnetic nanocomposites exhibit improved properties,including higher remanence, larger coercivity and enhancedmaximum energy products. RTP is especially suitable for theprocessing of nanocomposite materials, as we will discuss indetail.

In this paper, an overview of the applications ofRTP to magnetic materials will be presented. First, theprinciple and heating modes of RTP are briefly introduced.RTP facilities used for processing magnetic materials arepresented. The advantages of RTP are described incomparison with conventional furnace processing. Wewill then discuss systematically the applications of RTP invarious advanced magnetic materials, including soft magneticmaterials, spintronic materials (magnetic semiconductors),magnetic recording materials and hard magnetic materials.

2. Development of RTP techniques

2.1. History of RTP technology

The emergence of RTP technology was closely related tothe development of semiconductors. In 1968, Fairfield

and Schwuttke employed an RTP technique to developsilicon diodes using laser irradiation [18]. After decades ofdevelopment, RTP has become one of the essential methodsin the manufacture of semiconductors. The widely used RTPheating sources include pulsed laser, electron beam sourcesand radiant heat sources. The RTP technique is generallyapplied to silicon wafers undergoing processing, such as thinfilm oxidation, silicidation and chemical vapour depositionand production of highly integrated interchanges with reducedlinewidths and thermal allowance. Upon integrating with othervacuum processing, this technique shows many virtues, such aslow thermal budget, reduced contamination, high throughputand flexibility of various wafer sizes. With the maturity of thistechnology, its application has also extended to other materialsincluding magnetic materials.

2.2. Heating modes and fundamental principles

RTP involves very short temperature ramp times and processcycle times. The processing duration can range fromnanoseconds in laser annealing and infrared beam annealingsystems to minutes in lamp systems. Three typical modesinvolved in RTP processes include adiabatic, thermal flux andisothermal annealing. Figure 1 shows schematic figures oftemperature profiles for these three heating modes.

Adiabatic annealing is a concept where an extremely highenergy within dozens of nanoseconds is employed initiallyonto the materials surface to produce a large thermal gradientwithin the materials, and subsequent annealing is realized bythermal conduction from the surface to the interior. Thismode is applicable to both pulsed laser and electron-beamheating. Thermal flux annealing utilizes a highly localizedenergy distribution, such as a spot or a line. Upon quicksweeping of this energy within several microseconds over asample surface, a huge vertical and lateral temperature gradientcan be produced. A scanned laser or electron-beam can be usedas the localized energy source. The large temperature gradientin these two modes is governed by thermal conductivity andcapacity. These two modes were applied to semiconductorprocessing, including annealing, implant regrowth, dopantdiffusion, selective-area heating and deposition. Since the RTPsystems adopting these two modes need complicated facilitieswith inconvenient operation, the third type of RTP was thusdeveloped, employing the isothermal annealing mode. For theisothermal mode, the heating time is usually several secondsto several minutes and the heating rate is up to 300 ◦C s−1,typically lower than the other modes. In this mode, theheating profile is much more uniform not only laterally on

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Figure 2. The time evolution of the average order parameter 〈ψ〉 atdifferent ramp rates. The value l reflects the strength of long-rangedinteraction with l = 0 for noninteraction and l �= 0 for existinginteraction. Courtesy of Dang and Gooding [21].

the surface but also throughout the samples. Tungsten–halogen lamps or graphite resistive strips can act as the heatingsources. Compared with the adiabatic mode and the thermalflux mode, the isothermal mode is more common in use dueto its uniformity in the temperature distribution and briefnessin operation. Newman et al [19] have made a comparison ofthe isothermal mode and the thermal flux mode employed inmagnetic recording materials treatments. Hart and Evans [20]also discussed different modes in detail. It can be seen fromthe above discussion on all the heating modes that RTP isgenerally suitable for low-dimensional materials such as thinfilms, ribbons and particles. Large bulk samples may not beheated homogeneously within a short time.

Dang and Gooding simulated thin-film crystallizationbehaviour under rapid thermal annealing conditions [21].They considered long-range interactions generated by elasticmisfit of inhomogeneous intermediate phase and established akinetic equation based on Langevin simulations. By numericalcalculation of the equation, they successfully reproduced theexpedited phase transition in lead zirconate titanate thin films.The dynamical phase evolution was described at differentheating rates as shown in figure 2. It is striking to see from thefigure that crystalline ordering is very strongly dependent onramp rate (heating rate).

Although the simulation was based on parameters of theferroelectric lead zirconate titanate systems, the model whichconsidered the presence of an intermediate metastable phasecan be applicable to many magnetic materials systems, wherethe crystallization of the amorphous phase usually takes placein multiple stages, particularly in most rare-earth transition-metal systems where many intermediate states of metastablephases exist. This explanation also supports the observedphenomenon that the required annealing temperature andtime for phase transition from the disordered fcc structureto the required ordered fct structure in Fe(Co)Pt films canbe significantly reduced with an increase in the heatingrate [22, 23].

2.3. RTP experimental techniques

There are various types of equipment utilized for RTPtechnology such as laser systems, electron beam, ion beam

Figure 3. A schematic view of a typical tungsten–halogen lampRTP system showing several ways of temperature measurement,including thermocouple, photo-diode detector, pyrometer andradiometer.

and neutron beam systems, spike annealing systems, graphite-plate heaters, tungsten–halogen lamp systems and the recentlydeveloped pulse thermal processing (PTP) and Joule heatingtechniques.

2.3.1. Laser and electron beam heating systems. Laser andelectron beam RTP systems are based on the direct heating ofsamples by a laser or an electron beam. They are particularlysuitable for local heating treatments. The laser beam in theadiabatic mode has a small spot size (5–10 mm). In comparisonwith a laser beam system, an electron (or ion) beam system canprovide 5–100 times larger beam energy; thus, larger heatingregions up to 25 cm2 can be produced, as reported by Kaminsand Greenwald [24]. The high beam energy can significantlyincrease the scanning frequency to produce more uniformheating. Material modification using intense ion beams hasbeen reviewed by Penk et al [25]. However, due to therequirement for high vacuum, uniform scanning rate, precisecontrol of laser and beam resources output, these technologiesundoubtedly incur a high production cost.

2.3.2. Lamp heating systems. Lamp RTP systems useinfrared or tungsten–halogen lamps as the heat sources. Theheating rate for lamp systems is usually lower than the massdiffusion rate, and thus an isothermal mode is more suitablefor these systems. Radiation is the dominant heat transfermechanism in lamp systems. Figure 3 shows a schematicviewgraph of a tungsten–halogen lamp heating system. Thesystem typically consists of one or two arrays of lamps locatedaround a heating chamber. A horizontal wafer is placed in thecentre of the chamber. Heating power for a tungsten–halogenlamp up to 100 kW can be utilized to elevate the temperature upto 1200 ◦C with a heating rate up to 300 ◦C s−1. Samples canbe placed directly on the wafer or in a quartz tube on the wafer.The chamber also allows inert gas to flow through the chamber,providing an inert shield atmosphere and increasing the coolingrate. The wire thermocouples attached to the bottom side ofthe wafer can be used to detect the temperature. The wafertemperature can also be measured with a pyrometer.

For RTP systems, it is very important to measureand control temperature accurately. The measurement oftemperature is generally realized using wire thermocouples orpyrometer systems (including an optical pyrometer, a photo-diode detector or a radiometer). It should be noted that

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a point measurement (thermocouple) is not appropriate forthe adiabatic heating and thermal flux heating modes (asshown in figure 1). The pyrometer systems are employedin the noncontact mode, based on spectral radiometry. For apyrometer measurement system, the high background radiationoriginating from the lamps and walls affects the accuracy of thereal temperature and the temperature measurement deviationfor batch-to-batch samples. There are also some otherindirect temperature measurements, such as the utilization ofa ‘hotliner’ beneath the wafer as proposed by Nenyei et al[26] and the use of an acoustic thermometer developed byDegertekin et al [27]. The mechanism involved for the latteris the measurement of sound waves through the wafer whilstthe temperature is believed to be insensitive to the emissivityvariation, resolving the problem raised by optical pyrometry.

For lamp systems using the isothermal mode, Stuber andhis colleagues [28] have extensively investigated the heatingmode and controlling mechanism. The advanced multizonelamp system developed by Moslehi et al [29] improves theuniformity by using an overall cylindrical symmetry design.Recently, Edgar et al [30] pointed out that temperatureuniformity can only be achieved at the highest temperature inthe batch cycle because the diffusion process is an exponentialfunction of temperature. They also described another noveldesign of an RTP system configured with two controllable lampzones. One lamp zone located above the wafer is a high-powerzone essentially for heating over the whole wafer. Anotherrelatively low-intensity light zone, situated concentricallyaround the system, compensates for nonuniformities. Thetemperature uniformity can thus be further improved acrossthe entire heating system.

2.3.3. Pulse thermal processing. As a newly developed rapidthermal annealing technique, pulse thermal processing (PTP)provides a capability of processing nanomaterials at an evenhigher heating rate utilizing high-density infrared plasma arclamp technology [31]. The heating rate reaches 6 × 105 K s−1

and is far higher than most rapid thermal annealing systems.Moreover, the low thermal budget, increased throughputand the ability to process on low-temperature substrates arealso noticeable as its merits compared with other RTP andconventional furnace annealing. In the PTP system, powerdensities up to 20 kW cm−2 can be produced by a single lamp,which consists of a quartz tube sealed at two ends wherethe cathode and anode are located. Deionized water of highvelocities and pressure produced through high-velocity jetsimpinging on the tube wall enters at the cathode side andis impelled to the wall and spirals down the length of thetube, acting as an effective cooling and cleaning system. Theplasma gas is ionized with a capacitive circuit and maintainedas a plasma arc with a large direct current, which can reacha temperature in excess of 10 000 K. Shevchenko [32] hasinvestigated the basic characteristics of the radiation of flashlamps with microsecond discharge lengths and found thatthe parameters of electric discharge pulse, thermal inertiaof a heated gas and the region of transparency of a lampenvelope played important roles in the heating effect. ThePTP system can work in pulsed or continuous output modesand has a reflector to produce a line focus or an area ofuniform irradiance to meet diverse requirements. Usually, a

Figure 4. Schematic figure of Joule heating system.

low energy pulse is first employed to preheat samples to aspecific temperature and subsequent high energy multipulseswith short intervals are applied to heat the samples, leading torequired crystallization or phase transitions. The unique designof this system allows processing of thin films on polymersubstrate with little heating energy dissipated in the polymer,thus improving the packaging and device quality. It wasthought that in this approach crystallites can be nucleatedfrom the amorphous matrix of thin films in a single pulseand then grown in the following pulses. This particularlybenefits the production of photovoltaic thin-film transistors andmicroelectronic devices in which nanocrystalline Si particlesare distributed in an amorphous silicon matrix. The Oak RidgeNational Laboratory in the US has successfully applied thishigh-density infrared technology to the processing of Si thinfilms and other materials [31]. Work on the processing ofzirconium dioxide thin films on Si substrates has also beenreported [33].

2.3.4. Joule heating technique. Joule heating is a uniquerapid heating technique used for processing amorphous ornanocrystalline soft magnetic materials [34–37]. The Jouleheating effect is a common phenomenon of electric current-induced heat generation which can reach high heating andcooling rates. The application of Joule heating on magneticmaterials in 1990 [38] was reported earlier than the report ofother RTP techniques such as lamp heating of magnetic filmsin 1993 [39].

Joule heating processing is generally conducted by firmlyclamping dc or ac power sources at opposite ends of processedmaterial samples. The applied voltage is abruptly raisedfrom zero to the desired value and retained for a desiredduration and then turned off to zero by cutting the power;thus, self-heating can be realized due to materials’ electricresistance effect. Figure 4 shows a schematic figure of a Jouleheating system. Heat power and raised temperature are verysensitive to the applied current (or voltage) and the material’sresistance. Since there is no demand for the special deviceexcept for the power source, this technique is much moreeconomical than complicated lamp systems. Moreover, theheating instantaneously goes through the entire sample, whichleads to uniform material morphology. The heating rate canbe 300 K s−1 or even higher depending on the applied electriccurrent density.

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3. Magnetic materials processed by RTP

3.1. Soft magnetic materials

Soft magnetic materials are characterized by their highsaturation magnetization, large permeability and smallcoercivity. These materials can be used not only in manytraditional applications such as in transformers but also inadvanced devices such as in magnetic recording heads [40] andunderlayers below the recording films in magnetic recordingdiscs [41].

The soft magnetic Co–Zr–Nb amorphous underlayer ina double layer structure can increase the areal density ofperpendicular recording media by enhancing the perpendicularcomponent of magnetic flux [42]. However, the domain wall ofthe soft magnetic underlayer also generates spike noise, whichdeteriorates the recording performance. Ando and Nishihara[41] and Takenoiri et al [43] found that spike noise could bereduced by introducing a hard magnetic SmCo layer or anantiferromagnetic Ir–Mn layer. The reduction of spike noise isa consequence of the domain pinning via interlayer exchange-coupling between the ferromagnetic or antiferromagnetic layerand the soft magnetic layer, which reduces the domain walldensity and promotes the stability of signals. It is interestingto note that the spike noise of IrMn/CoZrNb films could becompletely eliminated using RTP performed at high coolingrates [43].

The metastable amorphous Co–Zr–Nb alloys have someshortcomings that Co crystallites usually precipitate from theamorphous matrix at elevated temperatures, deteriorating thesoft magnetic properties and limiting the application at lowtemperatures. This has been overcome with the development ofnanocrystalline Fe-based materials, such as Fe–TM–(C,B,N)alloys where TM is a group IV or V element. Thesematerials have higher saturation magnetization and betterstability. The ultrafine grain size of Fe crystallites in theFe-based films is an assurance for low coercivity and highpermeability. By employing RTP in a static magnetic field,Roozeboom and Dirne [10] found that the degree of growthof α-Fe grains in FeNbN films was inhibited by promotingtheir nucleation and that a higher operation temperature canbe employed while retaining the grain size below 20 nm.Meanwhile, they also found that the crystallization of Co inCoZrNb films can be suppressed and that the permeabilitywas enhanced and the undesired magnetic anisotropy andmagnetostriction were reduced, leading to a ‘softer’ behaviour.The rapid thermal magnetic annealing (RTMA) reactor inthe presence of a magnetic field was introduced severalyears later [44]. This reactor consists of a halogen lampsystem and an external electromagnet that generates a uniformmagnetic field of 660 Oe with the field direction lying in thewafer surface. Compared with conventional vacuum furnaceannealed samples, the films processed by RTMA in a staticfield show a nearly two times larger permeability (as illustratedin figure 5) due to slightly finer microstructure. Theenhanced permeability is ascribed to the unique crystallizationkinetics induced by the fast heating and cooling rates ofRTMA. The RTMA with its inherent low thermal budget haspromising applications for processing soft magnetic materialsfor high-density magnetic data storage. Additionally, themagnetoimpedance effect of soft magnetic FeNi-based alloy

Figure 5. Relative permeability µr of 1 µm Fe77Nb11Si2N10-filmson GGG after rapid thermal magnetic annealing and furnaceannealing in a magnetic field. Courtesy of Roozeboom et al [44].

(Ni77Fe14Mo5Cu4) prepared by sputtering and RTP under amagnetic field has also been reported [40]. An inducedmagnetic anisotropy and an enhanced magnetoimpedancesensitivity were observed after the RTP in a field.

Fe-based Sendust alloy (Fe85Si10Al5) films may supersedeFeNi-based metallic alloy films with low saturationmagnetization to serve better as shielding materials in inductiveand magnetoresistive recording heads due to their highmechanical hardness, high saturation magnetization, largepermeability, low anisotropy and small magnetostriction.Generally, only after thermal treatment can these alloys exhibitgood performance since the as-deposited films have largecoercivity and low permeability. The thermal treatment maybe carried out using conventional thermal processing (CTP)at a temperature around 600 ◦C for several hours to inducethe phase transition from the disordered α phase to a highpermeability DO3 structured phase so as to optimize the softmagnetic properties. However, the high temperature impairsthe potentialities of the materials as shielding parts in the filmmagnetoresistance sensor [45]. Many studies have shown thatthe requisite temperature can be reduced to 450–500 ◦C andthe annealing time can be shortened down to several minutesby the RTP technique [46–48]. The annealing time has beenfurther reduced recently to 15 s by Macken et al [45]. Theyutilized the RTP to realize the phase transition mentioned aboveand found that RTP samples exhibit a very small coercivityof 0.4 Oe. Both high temperature (600 ◦C) incorporated withshort time (15 s) and low temperature (500 ◦C) incorporatedwith long time (120 s) can realize the goals of achievinghigh permeability and improving the anti-passivation abilityin the studied Sendust system. Recently, Zhuang et alreported a successful preparation of hexagonal barium ferriteincorporated with a Sendust underlayer [49]. Of most interestis the observation that the film retained proper soft magneticproperties while an out-of-plane texture of Ba ferrite wasdeveloped. The out-of-plane texture will be beneficial for thedevelopment of perpendicular recording materials to increasethe recording density.

It is important to eliminate stress and other lattice defectsin magnetic recording materials to reduce the noise. Daval et alfound that this could also be realized by using a cumulativeRTP technique [48]. After rapid thermal annealing of theadherent and compressive stress-induced films, a stress-freeSendust film was produced. Meanwhile, a small grain size

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and the presence of amorphous regions between grains havealso been observed.

For high frequency applications, the high resistanceoxide soft magnets arouse great attention. Huijbregtse et alinvestigated Fe55Hf17O28 films prepared by sputter-depositionand RTP [50]. It was found that this material exhibitsa combination of good soft magnetic properties and highresistivity (>1 m� cm) after rapid thermal annealing for 5 sat 400 ◦C in flowing argon in a dc magnetic field. The short-time annealing retains the grain size of α-Fe the same as theas-deposited samples (5–10 nm). These nanocrystals wererandomly distributed in an amorphous matrix, which providesan essentially high resistance.

Except for the soft magnetic intermetallic alloys, softmagnetic ferrites films, such as NiZn-ferrites films, alsohave extensive applications in microwave integrated circuits,electrodes, sensors and catalysts. When these films aredeposited on Si/SiO2 substrates, the materials may also besuitable for application in magnetoresistive recording heads[51]. In this case, the deposition temperature is required tobe restricted below 500 ◦C to protect the amorphous SiO2

substrate. However, the low deposition temperature is notsufficient for the crystallization of ferrites. In addition, theexistence of tension at the interface between the ferrite andSiO2 layers due to different expansion coefficients may resultin low mechanical cohesion [52]. This issue has been resolvedrecently by the combination of a photo-assisted, wet-chemicalmethod (sol–gel) and RTP in conjunction with the depositionof an FeOx buffer layer on the thermally oxidized siliconsubstrate [52]. The RTP was conducted in a commercial RTPfurnace using a thin graphite plate. The selected heating andinitial cooling rates were around 10 K s−1. They found thatconventional heating only led to the formation of Fe2O3 at400 ◦C, whereas the RTP promoted the formation of superiorferrite films at this low temperature. The FeOx buffer layer ledto an improved match between SiO2 substrate and crystallizedferrite films when RTP was performed at 500 ◦C. The magneticpermeability of the RTP-treated ferrite films exhibits a doublevalue compared with the permeability of films annealed usingphotothermal annealing.

3.2. Spintronic materials

As we mentioned earlier, the RTP techniques were first usedin processing semiconductors. Recent research continuouslyproves that RTP is important in improving the semiconductormaterials’ quality and performance [53, 54].

Magnetic semiconductors are a new class of semicon-ductor materials based on the spintronics concept, promis-ing for fabricating spin-dependent magnetoelectronic devices[55]. Mn-doped ferromagnetic semiconductors, such asGaMnAs [56] and InMnAs [57], are among those systemsthat have attracted great attention because of their good room-temperature ferromagnetic behaviour [58], better compatibil-ity with III–V heterostructures [59] or their co-contributions[60]. Other significant features of these materials include giantmagnetoresistance (GMR) [61] and giant magneto-opticaleffect [62].

Mn-doped GaAs semiconductors can be fabricated byannealing MBE-synthesized Ga1−xMnxAs films [63] or by Mn

ion implantation followed by annealing at suitable temperature[64]. Both the negative and positive magnetoresistance havebeen identified for the GaAs/MnAs nanostructure system [63,65]. Photo-induced ferromagnetism and magnetoresistanceeffects in this system have also been investigated. Continuedwork on the positive magnetoresistance effect was performedon GaAs films with nanoscale MnAs clusters. Yuldashevet al [61] reported an enhancement in the positivemagnetoresistance under light illumination. The film wasprepared by ion implantation and RTP at 920 ◦C for 5 s. Theyascribed the increase in the positive magnetoresistance tothe enhanced geometric magnetoresistance effect and largemobility of photo-excited electrons under light illumination.Pekarek et al [66] investigated the iron-cluster-embeddedGaAs and p-In0.53Ga0.47As semiconductor films prepared byion implantation and RTP and found Fe3GaAs clusters with aparticle size of 2–10 nm. The superparamagnetic clusters wereresponsible for the GMR effect (3.2% at 5 K in 0.5 T) whichmight have a linear relation with the cluster concentration.

Moreno et al [67] investigated the structure andmagnetism of the GaAs-based system with Mn dopingprepared using MBE and different annealing conditions. In situannealing at 600 ◦C for 10 min during the MBE processled to the formation of 10 nm crystallites with zincblende-type tetrahedral structure, which was coherent to the matrixwith ambiguous interface. In this case, the coercivity andremanence were lower than those samples produced by meansof RTP. When the in situ annealed samples were subjected toadditional long ex situ thermal treatment at 300 ◦C for 1 h, some5 nm clusters formed with stacking-fault. The nanoclusterfilm showed a soft magnetic behaviour. However, it wasobserved that RTP at 700 ◦C for 20 s led to the formation ofspherical hexagonal MnAs crystallites with a size of 10–50 nm,which had a well-defined cluster/matrix interface. Only inthe case with the formation of a hexagonal structure was astrong ferromagnetic behaviour achieved. Mn ion implantationfollowed by RTP was also carried out recently [68]. Theas-implanted films were found to be paramagnetic, and aparamagnetic-to-ferromagnetic transformation was observedafter RTP at 750 ◦C as a result of the precipitation of MnAsmetallic clusters, as detected by the resonant surface plasmaoscillation.

Recently, the RTP technique has been used for tailoringthe optical and magnetic properties of Mn-doped CdTe-basedquantum dots (QDs) [69, 70] and InAs/GaAs self-assembledQD structures (SAQDs) [71]. The preliminary studies showedthat RTP led to an increase in the average dot size accompaniedby narrowed emission linewidth of the individual magneticdot. It was suggested that the RTP technique would be apotential method for tuning the spin properties of magneticQDs. For InAs/GaAs SAQDs, the samples processed by RTPshowed an intermixing of the dot interfaces, which resultedin a reduction in the confinement potential strength of thedots. This reduction allows the detection of the magneto-photoluminescence spectra and examination of the electronicstructure. RTP-induced enhancement of the ordering degreehas also been reported on MBE-grown τ -MnAl films onGaAs/AlAs substrate [72]. The increase in the crystallineorder was responsible for the increase in magnetization andthe decrease in coercivity. Additionally, RTP can also create

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Figure 6. (a) Carrier concentration, (b) Hall mobility as a function of x, (c) hysteresis loops (d) temperature dependence of magnetizationmeasured in a field of 0.5 T for as-deposited and annealed Zn1−x(Co0.5Fe0.5)xO films at 300 K. Courtesy of Cho et al [76].

defects at boundaries of QDs and their surrounding materials,leading to the release of the strain within the materials. Thisstrain release effect induces a change in polarization-selectiveresponses due to the transition from the S-like ground state tothe strain-induced splitting of P-like first excited states [73].

Although the III–V heterostructured materials mentionedabove have great potential application in magnetoelectricdevices, these materials have low Curie temperature (∼110 K),which is not favourable for practical device applications.Another kind of semiconductor, ZnO, is attracting increasingattention due to its unique properties. It has promisinguse in several areas, including ultraviolet light emitter, gassensors and transparent electronics. ZnO-based magneticsemiconductors doped with Mn, Fe, Co, Ni [74–77] have largemagnetization and high Curie temperatures, which makes themappropriate candidates for novel spintronic materials, thoughthere are many different explanations about the origin of themagnetism. Cho et al [76] thought that the magnetism of thewurtzite structure Zn1−x(CoFe)xO films originated from thecontribution of Co2+ and Fe2+ ions. They prepared the CoFe-doped magnetic semiconductor films using co-sputtering andRTP. The solubility of Co and Fe in the ZnO lattice couldbe as high as 15% when RTP was performed at 600 ◦C for10 min. Figure 6 shows the effect of RTP on the electricalproperties and magnetic properties of films with differentCoFe content. Obviously, the RTP resulted in a remarkableincrease for all the compositions not only in the electricalproperties, such as the electronic concentration and the Hallmobility (figures 6(a) and (b)), but also in magnetic properties,including saturation magnetization (figure 6(c)) and Curietemperature that was apparently above 300 K (figure 6(d)).The enhancement was ascribed to the formation of oxygenvacancies due to the occupation of Fe2+ and Co2+ in theZnO lattice. The carrier concentration in this system was

comparable to that (7.8 × 1017–1.6 × 1018 cm−3) in Mndoped GaAs nanostructure [61], while the latter had muchlarger mobility (543–784 cm2 V−1 s−1) at room temperature.The large difference in mobility might be attributed to theoxide characteristic. Although it is possible that Fe andCo atoms substituted the Zn site in ZnO, Norton et al[78] demonstrated using four-circle x-ray diffractometry thatthe magnetic properties were more preferentially associatedwith the existence of hcp Co nanocrystal precipitates. Intheir experiments, Co was implanted into the (110)-orientedSn-doped single-crystal ZnO substrates. The subsequent RTPwas carried out at 700 ◦C for 5 min. The concentration ofCo near the surface region of the single-crystal bulk wasaround 3–5 at.%. The estimated size (∼3.5 nm) of the epitaxialnanocrystal Co was smaller than the superparamagnetic criticalsize of 8 nm.

Except for the metal-doped magnetic semiconductorsdiscussed above, the magnetic tunnel junction (MTJ) is anotherkind of spintronic material. Magnetic random access memory(MRAM), based on the MTJ, has the potential of taking theplace of modern DRAM, SRAM and flash memory [79].The magnetic performance of these materials is also verysensitive to thermal treatments, which is required to procurehigh tunnelling magnetoresistance (TMR), large exchangebias (Hex) field and good thermal stability. There arenumerous reports on this subject. However the applicationof the RTP technique in the processing of MTJs wasdisclosed only recently in the reports of Lee et al [79,80]. They systematically investigated the effect of RTPon the tunnelling magnetotransport and thermal propertiesof MTJs, which had a very complex structure described asNiFe/FeMn/CoFe/Al2O3/CoFe/NiFe. The RTP was carriedout using a radiating heating furnace with an infrared lamp.Low annealing temperatures of 200–400 ◦C were selected

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Figure 7. TMR curves and M–H loops for the MTJs (a) as-grown,(b) annealed at 300 ◦C by RTP and (c) annealed by CTA and (d) byCTA after RTP, respectively. Courtesy of Cho et al [80].

in their experiments for both RTP and conventional thermalannealing (CTA) while the ramp-up, annealing and cool-downtimes were 10 s, 10 s and 6 min for the former and 15 min, 1 hand 1.5 h for the latter, respectively. Of most importance intheir observations is that the TMR and exchange bias field andthermal stability were significantly enhanced after the RTP.Figure 7 shows the different effects of RTP and CTA on theM–H loops. It can be seen that both the values of TMR andHex increase after RTP, while they decrease after CTA (TMRare 27%, 46%, 14% and Hex are 180 Oe, 230 Oe, 104 Oe foras-grown, RTP and CTA processed samples, respectively).They explained this phenomenon in consideration of oxygenredistribution and homogenization at the initial stage of RTPand the interdiffusion of Mn to the adjacent layers duringlong time CTA processing. The oxygen redistribution andhomogenization and the improvement in interface smoothnesswere thought to contribute to the increase in TMR. In addition,the RTP also improved the thermal stability as a result of theimpediment to the interdiffusion of Mn and reduced structuraldefects in the FeMn pinning layer, which is superior to theconventional thermal treatments. Rickart et al [81] used RTPtechnology to investigate phase transition of tunnel junctionscontaining an antiferromagentic MnPt layer. The effectiveexchange-coupling and high blocking temperature of thismaterial can be used to stabilize low and high resistancestates in magnetic fields and enhance thermal stability andexchange bias field due to an accompanying effect of reducedinterdiffusion. It was also found that this technique promotesstructural phase transitions and strengthens exchange-couplingin antiferromagnetic/ferromagnetic layers.

3.3. Magnetic recording media

In the past two decades, magnetic recording density hasbeen increased remarkably. This increase is accompanied byprogress in realizing grain refinement and grain isolation inmagnetic recording media. RTP plays a unique role in theprogress. In this section we restrict ourselves to discussionson RTP applications in some magneto-optical media, FePt-and CoPt-based thin films and nanoparticle materials which

are considered as the future recording media. The effect ofRTP on grain isolation in CoCrPt-based media will also bementioned.

Bi-substituted iron garnet films present large magneto-optic Faraday effects at wavelengths less than 550 nm [82]and exhibit high carrier-to-noise ratio (CNR ∼ 54 dB) [83].The large CNR value was obtained in the expensive singlecrystal substrates. In order to reduce the production cost,other inexpensive substrates, such as glass discs and quartzplates, were explored. However, the films deposited on thesesubstrates exhibited low CNR’s due to unfavourably increasedmedia noise. The low CNR may be due to the inability to writecircular domains in these substrate materials [84] or due to itsorigin from grain-boundary light diffusion [85].

Generally, an ordered structure in these oxides is required,and this can be produced by a conventional annealing process.The films presenting optimal magneto-optical properties aregenerally prepared by annealing at a temperature above 650 ◦Cwhile the noise during readout caused by grain and surfaceroughness is a serious issue for use of such storage media.Many attempts were made to control the grain size throughvarious dopants [86, 87], underlayers [88] and annealingschedules [89]. Early work also shows that in situ depositionmay improve the surface smoothness [90]. However, extensiveannealing usually results in grain growth, which increases themedia noise of magneto-optical materials. The RTP was thusconsidered to be able to avoid the side-effect.

Using rf magnetron sputtering, in combination with theRTP technique with a heating ramp rate from 1 to 50 ◦C s−1 ina heat pulse apparatus, Suzuki et al prepared Bi-substituted(BiDy)3(FeGa)5O12 garnet films [90]. No significantdifference in magnetic and magneto-optical properties wasobserved between RTP and CTA. However, the surfaceroughness was drastically improved with the RTP technique.Meanwhile, a remarkable reduction in the grain size withincreased heating rate was also observed. The grain refinementwas verified in later experiments [91,92]. Suzuki demonstratedthat the multilayer garnet films which are crystallized by theRTP consist of far smaller crystallites than those in a singlelayer film and exhibit higher coercivity Hc [93]. The RTP isalso effective in promoting the crystallinity of epitaxial Bi-YIGfilms without significantly degrading the structural integrity ofthe film [94]. Eppler et al [92] found that the grain size canbe reduced even by an order of magnitude in comparison withthe grain size of the film crystallized in a conventional furnace.They also pointed out that the improved regularity of writtendomains in their RTP experiments was effective in reducingthe noise.

Ce-substituted CeDyGaIG film has large magneto-optical properties in the range of near infrared. Bechevetet al [85] investigated as-deposited amorphous CeDyGaIG(CeDy2Ga0.4Fe4.6O12) films crystallized using an RTP furnacewith 1 kW lamp power with variable pulse width (0.5–2.5 s).The coercivity of films processed under vacuum was increasedafter prolonged RTP under ambient atmosphere. Grainrefinement is one of the attributions to the enhancementof coercivity, which may be related to the change in thecomposition of oxide films too.

Shen et al [95] also reported that no evidence of preferredorientation of the grains could be observed in the crystallized

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(Bi,Al)-doped Dy3Fe5O12/T (T = Fe, Co, Dy) multiplayerfilms prepared by magnetron sputtering and multipulse rapidthermal annealing, which is similar to the work of Bechevetet al [85]. The crystallized film had an average grainsize close to 10 nm and a surface roughness smaller than5 nm. The largest Kerr rotation obtained was around 2.0◦.However, in the same year, Duan et al [96] synthesized thegarnet Bi2DyFe3.5GaO12 films, using pulsed laser ablationand RTP methods, and claimed the existence of perpendicularanisotropy. They also reported that the incorporation ofpulsed laser ablation and RTP methods promoted the formationof stoichiometry composition, which was difficult to realizesolely using the deposition technique, due to the large vapourpressure of Bi, one of the important elements in the garnetfilm used for large Faraday rotation. They also observed thatthe film had good surface smoothness and a Faraday rotationof 0.78 ◦ µm−1. The distinction in the observations of theanisotropy or the texture in these studies may be attributedto the different composition and preparation methods, whichis similar to the results for PtMnSb films, where the formationof crystalline texture depends on the chemical composition andthe RTP time [97].

For barium hexaferrite films, there exist many preparationtechniques, including dc and rf sputtering, electron-beamevaporation, pulsed laser deposition and molecular beamepitaxy. The as-synthesized thin films are usually amorphous,nonmagnetic and oxygen deficient. The RTP techniques withheating durations of several to tens of seconds can be used toobtain designed phase structures and morphologies. In 1993,a modulated heating profile of the RTP technique was firstsuccessfully utilized to develop well-ordered barium ferritefilms within 1 min [39]. It was observed that the structure andmagnetic properties of samples were sensitive to the heatingprofile. In 1994, Gagosandoval et al prepared BeFe12O19 filmswith dominant perpendicular magnetic anisotropy using thelaser radiation RTP technique [98]. A significant reduction inthe crystallite size was also observed. Chen et al investigatedthe nucleation rate Rn and the growth rate Rg for barium ferritethin films during the RTP and concluded that Rn and Rg arearound 9 grains/s in an area of 3 µm × 3 µm and 69 nm s−1

in the early stages of crystallization, respectively [99]. Amultitarget scanning co-sputtering process has been used inconjunction with the RTP to produce a series of barium ferritefilms in which Co, Cr, Mn and Ni are selectively introducedto substitute Fe from 5 to 20 at.% [100]. The RTP wasconducted at a linear heating rate 35 K s−1 up to 810 ◦C. Thesubstitutions improved the magneto-optic properties at a shortwavelength (∼670 nm), particularly in the case of Co andNi, while no explanation associated with the enlargement ofsaturation polar Kerr rotation was given. Chen et al [101]investigated the intergranular interaction in barium ferrites.They observed that after RTP at 710 ◦C for 30 s, highergrain isolation could be realized in comparison with thatof a sample heated at 750 ◦C for 30 s. Higher temperatureled to full crystallization of the amorphous films. Whenthe amorphous films were annealed using RTP for a shorttime, they transformed into partially crystallized Ba-ferritewith columnar grain morphologies with preferential out-of-plane c-axis orientations [102]. The samples had a muchsmoother surface than fully crystallized BaM thin films.

The remaining nonmagnetic amorphous phase isolated thecrystallites and reduced the magnetostatic intergrain couplingand noise. Recently, Scherge et al prepared Co and Tidoped barium ferrite films by sputter deposition and comparedthe tribological performance of these materials annealed ina conventional furnace or by RTP techniques [103]. It wasshown that RTP plays an important role in the improvement ofsurface smoothness and tribological performance through thereduction in grain size and the formation of more continuousparticle size distribution with minimized voids among theneedle-like particles.

High magnetic anisotropy is required for media forhigh density recording because a large anisotropy meansa small critical size for superparamagnetism. For thisreason, FePt and CoPt systems have aroused considerableattention. Nanoparticles of FePt and CoPt compounds havepotential application in magnetic recording with extremelyhigh recording density [104, 105]. The ideal mode for therecording medium is an array of uniformly sized noninteractingsingle domain particles with one bit per particle. Generally,as-synthesized FePt particles and thin films have a disorderedfcc structure, which has no magnetic anisotropy at roomtemperature. It is required to realize an fcc–fct phasetransformation at an elevated temperature above 600 ◦C toobtain the ordered L10 hard magnetic phase. RTP has beenapplied to achieve this phase transition. Zeng et al [23]found that chemical ordering of 4 nm FePt nanoparticle self-assemblies can be improved by rapid thermal annealing, witha significantly lowered annealing temperature and a shortenedtime. Jeong et al [106] found that RTP at 700 ◦C for 18 mininduced a strong perpendicular anisotropy in FePt and CoPtpolycrystalline films. In their experiments, a polycrystallineMgO layer was used as an underlayer. After proper RTP, a fullyordered L10 structure, with a grain size of 10 nm, was obtained.Zeng et al [22] recently reported the preparation of highlyoriented and nearly perfect (001)-textured CoPt and FePt thinfilms by direct deposition on thermally oxidized Si substratesand subsequent rapid thermal annealing. This result has beenrepeated by Yan et al [107,108] with glass substrates. Recently,Ito and Kusunoki [109] discussed the ordering mechanism inFePt thin films by RTP. Yokota et al [110] also investigatedthe difference in magnetic textures and properties of FePtfilms prepared by RTP and high current-density ion-beamirradiation. It was reported that RTP induced (001) textureand strong magnetic perpendicular anisotropy while the latterproduced samples with more isotropic behaviour due to a (111)texture.

In 2002, Jeong et al [111] successfully prepared in situordered polycrystalline FePt L10 (001) films with textureat an elevated substrate temperature. The average grainsize of the as-deposited films was 10–15 nm. Except forthe L10 tetragonal phase, nanosized fcc clusters and less-ordered regions in the films were also observed. When aZn or CrMn top layer was deposited on the films followinga RTP treatment, the coercivity was increased remarkably by40–50%. Figure 8 shows the RTA effect on the hysteresis loopand the angle dependence of coercivity. The angle dependenceof the coercivity does not follow a domain wall motion mode(1/ cos θ ), and Hc decreases with increasing deflection of fielddirection with respect to the easy axis, revealing exchange-decoupling behaviour.

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Figure 8. (a) Hysteresis loops of Zn/FePt/MgO films before (i) andafter RTP at 180 ◦C (ii) and 250 ◦C (iii) for 1 min. (b) Angulardependence of Hc of the CrMn/FePt/MgO films before and afterRTP at 450 ◦C for 1 min. The angle is defined between the easy axesand the applied field. Courtesy of Jeong et al [111].

High squareness of magnetization loops is important formagnetic recording media. The highest squareness ratio, i.e.the remanence ratio Mr/Ms, can reach up to 0.99 in CoPtnanowire materials embedded in anodic aluminium oxide afterapplication of RTP [112]. In 1998, a combined multilayer ofCoPt/DyFe garnet was prepared by rf-magnetron sputteringand RTP, followed by coating of Co/Pt multilayers whichprovided a low-noise smooth surface [113]. The magneto-optic Kerr rotation of such a system was around 1.8–2◦.This investigation shows the possibility of improving therecording performance with a multi-component structure,through improvements in the RTP technique.

The reduction in media noise is another important issueto be explored in the recording industry. It is well knownthat the recording noise originates from thermal fluctuationsas well as intergranular exchange-coupling. The thermalfluctuation effect can be reduced through using high anisotropymagnetic materials. The exchange interaction is related to thedegree of grain isolation. It was found that the segregationof the grains can lower the media noise [114] and that RTPis effective in promotion of the segregation [115]. Thesegregation may also lead to an increase in coercivity as aconsequence of the reduction in exchange-coupling betweenneighbouring magnetic grains. Because of the reductionin intergranular exchange-coupling, the collective magneticreversal via domain wall motion across grain boundariescan also be suppressed [116]. Controlling grain boundarydiffusion by RTP is an ideal way of improving the segregationbecause, upon subjecting the film to a high temperaturefor a long time in conventional annealing, the segregationwill be destroyed [117]. This has been demonstrated inthe work of Mukai et al [114]. An increase in coercivityHc of CoCrPt/Cr films and a decrease in the annealing

temperature, at which the maximum Hc value was obtained,were realized upon RTP for 20 s at temperature as low as460 ◦C. Meanwhile, Cr as a nonmagnetic element was found todiffuse from the underlayer into the magnetic layer during thisRTP, resulting in better segregation at the grain boundariesand improved isolation of CoPt magnetic grains. SimilarRTP effects were also observed in CrMn/CoCrPt/CrMn/NiAland Cr/CoCrPt/Cr/NiAl thin films with CrMn and Cr asintermediate layers and overlayers. The overlayers usedhere are effective in preventing oxidation and promotingthe interdiffusion. It is seen that the films with a CrMnintermediate layer have a microstructure characterized by asmaller and a more evenly distributed grain size. Subsequentworks performed by Zou et al [116] revealed a significantincrease in coercivity by 38% for CoCrPt/CrTi/NiAl film andeven by 60–73% in Mn-capped CoCrPt/CrTi/NiAl film. Thereason for the increase of coercivity was attributed to bettergrain isolation since the RTP promotes the diffusion of adjacentMn layers into the grain boundary, resulting in the breakupof the exchange-coupling. The grain boundary diffusionwas verified by a series of experiment results. Analogically,Jeong et al [118] prepared the in situ ordered polycrystallineFePt thin films with a MgO underlayer and a CrMn or Zntop layer. They also observed the exchange-decoupling andthe enhanced magnetic hardening as a consequence of grainboundary diffusion after subsequent RTP.

In situ processing protects the recording media filmsfrom oxidation since the metallic surface is not exposed tothe environment prior to overcoating. Using this method,incorporating ex situ RTP technology, Harkness et al [119]prepared a CoCrPt film with a CrMn caplayer of 0.5–5 nmthickness. The combination of in situ annealing and ex situ RTPled to an increase in coercivity and a remarkable improvementin signal-to-noise ratio by 216%. The increase in coercivitywas not due to grain growth but was more likely due to theformation of single-domain particles.

Because of the very high uniaxial anisotropy of the rare-earth intermetallic magnetic materials, exchange-decoupledSmCo- [120] and PrCo5-base [121] materials have also arousedgreat attention recently for magnetic recording applications.In order to reduce the exchange-coupling between thePrCo5 particles, for instance, Chen et al [121] developednovel nanocomposites (Pr0.17Co0.83)69C31 with nonmagneticC matrices isolating the hard magnetic PrCo5 particles.The materials were prepared by pulsed filtered vacuum arcdeposition and subsequent application of the RTP technique. Acarbon overcoat layer was used to provide resistance to surfaceoxidation so as to extend the lifetime. A combination of a largecoercivity of 5.2 kOe, a small mean grain size of 7.8 nm andthe elimination of exchange-coupling due to isolated singledomain particles shows that the nanocomposite films may beanother candidate for future high-density recording media.

3.4. Hard magnetic materials

3.4.1. Rare-earth transition-metal magnetic materials. Theemergence of high-performance Nd2Fe14B materials in the1980s resulted in a significant advancement in the developmentof rare-earth transition-metal permanent magnetic materials.For permanent magnets, coercivity is an essential property

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which is dependent on both intrinsic and extrinsic structuralparameters of the materials. To obtain desired intrinsicproperties (e.g. phase constitution) and to tailor morphology(e.g. grain size), heat treatments are normally necessary.Among all the heat treatment techniques, RTP plays animportant role in heat treatment of rapidly quenched ribbonsand thin films. The early application of RTP in permanentmagnetic materials was in 1989, when rapid crystallizationand grain refinement of the melt-spun NdFeB samples wereexploited [122]. To obtain very fine-grained material, all thesamples were crystallized by heating to 925 ◦C for 5 s usinga modified current pulse heating device and then quenched toroom temperature. This process increased the coercivity byabout 30%.

Yu et al reported the preparation of high performanceNdFeB films utilizing the RTP technology [11]. The filmsof thickness of 500 nm prepared using magnetron sputteringconsisted mainly of amorphous phase with α-Fe as the minorphase. Upon annealing at a high heating rate of 200 K s−1 inflowing Ar, isotropic NdFeB films were developed. Propercontrol of heating temperature and time could produce highperformance NdFeB films with coercivity up to 20 kOe andremanence ratio up to 0.8. The high coercivity was attributedto the formation of single-domain particles. The remanenceratio of 0.8 was apparently larger than the predicted Stoner–Wohlfarth value of 0.5 for isotropic single-phase magnets[123]. Since the particles size was around 50 nm, which wasfar below the observed domain size of 500 nm, it was suggestedthat these hard magnetic nanocrystallites were exchange-coupled. The lower crystallization temperature (500 ◦C) in thisstudy compared with the typical temperatures (600–650 ◦C) inconventional heating methods was thought to be due to thedifferent Nd concentrations and high heating rates as statedabove. Heat treatments with higher temperature and longertime only led to degraded magnetic properties due to graingrowth and minimization of defects.

Through the adjustment of buffer layer, sputter rates,film thickness, composition and annealing parameters, variousmagnetic properties of NdFeB thin films have been obtainedwith coercivity as high as 20 kOe [11] and energy products upto 22.8 MGOe [124]. Recently, Jiang and O’Shea prepared thinNdFeB films (<200 nm) and various buffer and cover layers ofCr, Mo, Nb, Ta, Ti, V [125]. Subsequent RTP was performedusing halogen lamps under flowing nitrogen. Their resultsrevealed a smaller grain size in RTP samples in comparisonwith CFA samples. The RTP-treated Nb/Nd2Fe14B/Nb filmhad a coercivity of 26.3 kOe, which was 30% larger than theconventionally annealed samples.

Similarly to the structure of Nd2Fe14B, the Nd2Fe14Ccompound has also been investigated using the RTP technique,while this compound presents high-temperature instabilityand decomposes above 880 ◦C, leading to the formation ofNd2Fe17Cx [126]. For melt-spun ribbons, however, theNd2Fe14Cx was formed by a solidification reaction via a phasetransformation from Nd2Fe17Cx [127]. When Nd2Fe17Cx

transforms into Nd2Fe14Cx , more α-Fe precipitates. Forsingle-phase magnets, the precipitates of Fe should beminimized in order to obtain optimal magnetic properties. Thework of Daniil et al revealed that this could be realized usingRTP technology [128]. It was found that hysteresis loops

of almost all the conventionally annealed ribbons showed akink around zero field due to the existence of large size α-Fecrystallites. However, the kink could be diminished, eveneliminated, using the method of rapid annealing due to theabatement of unfavourable large α-Fe grains.

Except for the 2 : 14 : 1-type rare-earth transition-metalcompounds, the Sm-Fe-(N,C)-based 2 : 17-type interstitialcompounds have also attracted much attention due to theirexcellent properties as permanent magnet materials [129–132].These compounds are metastable and decompose into RN andFe upon heating to 700 ◦C. With the substitutions of Ga, Cr, Al,Zr and Si for Fe, the temperature stability of the Sm2Fe17Cy

compounds can be significantly improved. The Curietemperature of Sm2Fe14Ga3Cy can be achieved above 600 Kand its room temperature anisotropy field is higher than 90 kOe.Si-substituted compounds even have a room temperatureanisotropy field of up to 110 kOe. However, it is necessaryto retain the single-phase structure or a nanoscale compositestructure consisting of the 2 : 17-type phase and ultrafine α-Feto obtain high magnetic properties. However, it is an arduoustask to reduce the grain size. The decomposition of thesesystems at high temperatures and the grain growth suggesta requirement for low processing temperature. Recently, theannealing of melt-spun Sm2(Fe,Ga)17Cx carbides conductedusing RTP technology in a flowing Ar atmosphere was reported[133]. The as-spun amorphous samples were annealed in aninfrared furnace at ramp rates as high as 600 ◦C min−1 andat various temperatures for various times. The crystallizationbehaviour and magnetic properties of the materials have beenexamined. The annealing at temperature as low as 600 ◦Cresulted in the crystallization transformation to the 2 : 17 phase,which remained undecomposed up to 900 ◦C.

3.4.2. Nanocomposite magnets. In 1989, Coehoorn andhis co-workers discovered a remanence enhancement effect inisotropic Nd2Fe14B/Fe3B composite magnets and ascribed itto intergrain exchange-coupling [134]. Since then numerousstudies have been focused on the lean rare-earth exchange-coupled nanocomposite magnets, including the Nd2Fe14B/α-Fe type nanocomposite [135]. These magnets generally consistof one magnetically hard phase providing large coercivity andone or two soft phases contributing to high magnetization.When the grain size of these magnetic phases is reduced downto nanoscale dimension and when these grains are in intimatecontact with each other, the magnetic moment of soft magneticphase with relatively small anisotropy aligns with the overallmagnetization direction of the neighbouring hard phase.Theoretical calculations predicted a high isotropic remanenceratio as well as a single-phase-like magnetization behaviourand therefore large energy products in the nanocompositesystems due to the inter-phase exchange-coupling [136, 137].The magnetic properties of such nanocomposite magnetssignificantly depend on the grain size [138, 139]. In the casewhere the grain size is too large to effectively exchange couple,a kink or a step will show up on demagnetization curves,leading to a deterioration of the energy products. The requiredsoft-phase dimension is in the order of the exchange lengthLex ≈ π(Aeff/Keff)

1/2, where Aeff represents the effectiveexchange stiffness and Keff is the effective anisotropy constantthat originates from the contribution of the hard and soft

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magnetic components of nanocomposite magnets [140]. Othersimulations gave the critical soft-phase dimension as twicethe domain wall thickness in the hard phases [136–138]. Sunet al [141] reported that the ultrafine grain size D (≈ 2Lex)

for the optimization of magnetic properties should be in therange 15–25 nm for the volume fraction of α-Fe in the range30–70%. Recent modelling analyses showed that the soft-phase critical length in a nanocomposite is also related to thesoft-phase itself [142, 143]. A ‘semi-hard’ phase will givea better exchange-coupling. It has also been found that theinter-phase exchange interaction is strongly related to the grainboundary conditions [144, 145].

Based on the above concepts, sophisticated constructionof the phase constitution and morphology is essentialfor fabricating the nanocomposite magnets. Grain sizereduction is the first challenge to realizing the desiredmorphology. Doping of selected elements into the NdFeBbased nanocomposites such as the addition of Co, Cu, Nb, Crhas been investigated [146–148]. It was found that introducingCo into the nanocomposite inhibits the precipitation of α-Feand Fe3B. While Cu addition leads to the formation of ahigh number density of Cu clusters prior to crystallizationof the amorphous phase, these ultrafine clusters promotethe nucleation of the Fe3B phase. Nb addition induces theformation of another type of soft magnetic Fe23B6 phase.The addition of Cr either induces the desired decompositionof the metastable Nd2Fe23B3 phase into the Nd2Fe14B/Fe3B[149, 150] or incorporates into Fe3B and lowers the freeenergy of metastable Fe3B, thus resulting in the microstructuralmodification [151].

As we have discussed in the preceding sections RTP iseffective in grain size reduction. Recently, the RTP has beenextensively applied to the processing of the Nd2Fe14B-basednanocomposites [13,17,152–154]. Suzuki et al studied theCr-added Nd2Fe14B/Fe3B nanocomposite by the applicationof RTP at a heating rate up to 5.3 K s−1 (∼320 K min−1) underan argon atmosphere [154]. The amorphous Nd5Fe77xCrxB18

(x = 0, 3, 5) ribbon was prepared by melt spinning. For Cr-free samples there was no remarkable effect of the heating rateon the crystallization behaviour. However, the crystallizationof Cr-added amorphous Nd5Fe74Cr3B18 depends strongly onthe heating rate. When the Cr content is higher (x = 5),the heating rate dependence of crystallization behaviour wasnot obvious again. These results suggested that crystallizationbehaviour relates to both heating rate and composition, whichhad been explained hypothetically by the consideration of theminimum free energy curve [148]. The results hinted that thechange in composition might vary the activation energy of thephase transformation [154].

Wu et al investigated the effect of heating rates(up to 5 K s−1) on the crystallization of amorphousNd4.5Fe73B18.5Co2Cr2 ribbons [13]. Differential scanningcalorimetry (DSC) analysis (figure 9) revealed a two-stagecrystallization behaviour at low heating rate (<100 K min−1),with the formation of Fe3B and Nd2Fe23B3 and a one-stage crystallization of Nd2Fe14B phases at high heating rate.This has been interpreted in lines of a time–temperature-transformation diagram. Since higher heating rate resultedin finer microstructure of approximately 15 nm grain sizecompared with 30 nm for a low heating rate, higher remanence,

Figure 9. DSC traces of amorphous Nd4.5Fe73B18.5Co2Cr2 ribbons.Courtesy of Wu [13].

coercivity and energy product were obtained at a higherheating rate.

Apparently, the heating rate employed in theseexperiments was still not very high (1–5 K s−1). It remainsunclear whether the grain size has a monotonous relationshipwith the heating rate. Bernardi et al [155] studied the Cr-,Co-, Si-added (Nd,Tb)2Fe14B/Fe3B nanocomposites preparedby splat cooling and fast annealing at a heating rate of20 K s−1. In these samples, the grain size was found tobe smaller than 20 nm for Cr added samples. In theirfurther studies, the RTP was carried out especially for theCo added nanocomposite alloys with nominal compositionNd3.25Tb1Fe72.75Co5B18 [156]. The amorphous ribbons wereheated to the desired temperature with a heating rate of15–25 K s−1. This type of thermal treatment resulted in bettermagnetic properties at higher heating rates.

To answer the question how fast the magnetic hardeningcan be realized in given magnetic materials, recently Chuet al [153] used the RTP technique to process the melt-spunNdFeB-based nanocomposites. This is a single wafer processinvolving radiative heat transfer from an array of halogen lampsto the wafer. Figure 10 shows the representative heating profile,including four heating stages, i.e. delay (idle phase), rampup (heating phase), steady (hold phase), ramp down (coolingphase). By switching on and off the tungsten–halogen lamps,the high heating (∼200 K s−1) and cooling rates (∼150 K s−1)can be realized. They systematically investigated the effectof different heating methods on the magnetic properties ofthe NdFeB base melt-spun nanocomposite ribbons. The as-spun samples showed magnetically soft behaviour due to theamorphous structure. After the RTP at 600 ◦C, the magnetic

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Figure 10. Heating profile of the halogen lamp RTP system.

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Figure 11. The coercivity as a function of the annealing time forthe NdFeB-based nanocomposite samples after RTP treatment at600 ◦C with low (40 ◦C s−1) or high (200 ◦C s−1) heating rates.NFe9, NFe15, NFe20 denote three different compositions,Nd9Co10Ti2B10.5Fe68.5, Nd2Fe14B/Fe3B (15 vol.% Fe3B) andNd2Fe14B/α-Fe (20 vol.% α-Fe), respectively.

(This figure is in colour only in the electronic version)

hardening was developed within the preserved heating durationas short as 1 s, as shown in figure 11. The highest coercivitywas obtained in 1 s annealed samples, indicating an almostcomplete crystallization process in such a short time. Figure 12shows the grain morphology comparison of samples annealedat 600 ◦C in a conventional furnace and in the RTP system.Apparently, the average grain size (40 nm) of RTP sampleswas much more uniform and finer than that (60 nm) of theconventional furnace-annealed sample. Correspondingly, ahigh coercivity Hc of 10.2 kOe and a large remanence ratioMr/Ms = 0.78 were obtained with RTP, which are largerthan their respective values (Hc = 9.5 kOe, Mr/Ms = 0.75)obtained for the furnace-annealed samples. The shortest timeinterval which can be achieved in the halogen system is 1 s. Itcan be inferred that if an even shorter annealing time can be set,an even faster crystallization and magnetic hardening would beobserved. The finer grain size of soft magnetic phase due tohigh heating rate was responsible for the optimal magneticproperties.

Jiang and O’Shea reported the annealing effect onthe magnetic properties and exchange spring behaviour of

(a)

(b)

Figure 12. TEM images of melt-spun NdFeB-based nanocompositesamples annealed at 600 ◦C in (a) conventional furnace and (b) RTPsystem.

Nd2Fe14B/α-Fe thin films [2]. The springboard in their studywas that the rapid heating rate allowed a larger amount ofnucleation centres to form and grow than the lower heating rate,which is consistent with the analysis discussed in section 2.2(figure 2). As a consequence, a more uniform grain sizedistribution would be obtained and magnetic properties couldbe improved. Their experiments satisfied the consideration.It was also found that the rapidly annealed sample exhibitedmore irreversibility in recoil magnetization curves than theconventionally annealed sample.

Jin et al [157] successfully prepared hard magneticPr2Fe14B/α-Fe nanocomposities by employing Joule heatingon amorphous Pr7Tb1Fe85Nb0.5Zr0.5B6 ribbons and simultane-ously monitoring the crystallization behaviour through mea-suring the room-temperature electrical resistance R versuscurrent curves. The relationship between resistance and cur-rent during the Joule heating provided accurate control of theamorphous-to-nanocrystalline phase evolution and helped toidentify the optimal thermal annealing conditions for improve-ment in magnetic properties. The amorphous ribbons with across section of 3 × 10−8 m2 were subjected to ac Joule heat-ing in an Ar atmosphere. A rapid reduction in resistance wasobserved at the start of the Joule heating. The variation inresistance was directly related to the crystallization of amor-phous ribbons and the formation of nanocrystallites. Since theheating power and temperature rise depended on the resistanceof the materials during the Joule heating, this technique was anonisothermal heating process [158,159]. Of most interest wasthe observation that the variation of resistance was very similar

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u.)

T (oC)

Figure 13. The curve of R/R0 versus the applied current I and thedifferential curve ∂(R/R0)/∂(I )–I . The result of DTA analysis onamorphous samples is shown in the inset. Here R0 and R denote theroom temperature electrical resistances of amorphous ribbon beforeand after Joule heating, respectively.

to the results for differential thermal analysis (DTA), as shownas an inset in figure 13. The structural transformation startedwith a significant drop in resistance R(I), where three dropson the differential curve ∂(R/R0)/∂(I )–I were consistent withthe exothermic reactions. The first one represents the crystal-lization of α-Fe and metastable TbCu7 structure and the othertwo reflect the formation of 2 : 14 : 1 and Pr1+δFe4B4 phases,respectively [160]. The two endothermic peaks at high tem-peratures were related to the melting points of 2 : 14 : 1 phaseand iron, respectively. Thus, it is suggested that the charac-teristic R value which varied with respect to applied current I

was closely related to the amorphous-to-nanocrystalline phasetransformation and can be used as a guide for studying thecrystallization behaviour in an attempt to optimize magneticproperties. Further investigation revealed that the magneticproperties of as-prepared samples could be well controlled bythe optimization of heating power and duration. Magnetic per-formance up to 30% higher than the values for conventionally(furnace) heated samples were obtained in the Joule-heatedsamples. The better magnetic properties were attributed tothe higher loop squareness originating from stronger exchangeinteraction between the hard Pr2Fe14B and soft α-Fe phasesbecause the rapid heating rate led to a large number densityof nuclei, which may be helpful for obtaining a much finerand more uniform nanostructure. From the TEM observationas shown in figure 14, it can be seen that the traditionallyannealed (Pr,Tb)2(Fe,NB,Zr)14B/α-Fe samples have a hetero-geneous morphology with irregular shape and coarse grainsabout 50 nm; however, the nanocomposite samples preparedusing Joule heating present much finer and more uniform mor-phology with a grain size of 30 nm.

Compared with the NdFeB-based nanocomposites,Sm–Co materials have higher Curie temperatures and largeranisotropy fields. These materials are particularly suitablefor high-temperature applications and the production ofnanocomposite magnets with high coercivity. Liu et al[139] conducted systematic research on the SmCo- and PrCo-based multilayer nanocomposite films. One of the mostimportant figures in the work was the remarkable increase

(a) (b)

Figure 14. Bright-field (top) and dark-field (bottom) TEMmorphology of conventionally annealed nanocomposite (a) and thesamples prepared using Joule heating (b).

in coercivity after applying RTP. The SmCox /Co multilayerthin films with Cr underlayer and cover layer were preparedusing a multiple-gun dc and rf sputtering system by alternativedeposition of SmCox (or PrCox) and Co layers onto glass orsilicon substrates. The RTP was employed with temperaturesup to 800 ◦C and a heating rate up to 200 K s−1. It wasinteresting to observe that a two-step heat treatment gavethe best hysteresis loops. Figure 15 shows an exampleof comparison among different annealing processes for theSmCox–Co thin film. The two-step treatment, a combinationof an RTP and a conventional furnace annealing, was foundto enhance the effective inter-grain exchange-coupling asevidenced by the smallest kinks on the hysteresis loop sincedirect conventional annealing led to excessive grain growth.We attribute the improved magnetic properties achieved by themultiple treatments to the fact that RTP led to reduced grainsize and the subsequent furnace treatment resulted in betterinterface conditions, which are important for the exchangecoupling [144, 145].

Another class of magnetic phases with very high magneticanisotropy is chemically ordered FePt and CoPt tetragonalphases (K1 = 6.6×107 erg cm−3 for FePt, 4.9×107 erg cm−3

for CoPt) [161], as we discussed in the section on magneticrecording media. The FePt system is naturally also agood candidate for permanent magnets with large energyproducts, high Curie temperature and good resistance againstoxidation. FePt single phase and nanocomposite systemshave been widely studied in recent years [7,162–170]. Theenergy products of this type of permanent magnetic materialshave been improved from 30 [162] to 50 MGOe [163]. Afirst-principles calculation using linear-muffin–tin-orbitals andGreen function methods showed the possibility of achievingan energy product of 90 MGOe in a perfect FePt/Fe two-phasestructure [164].

There are diverse methods for preparing the FePt-basedfilms. The general technologies include molecular-beamepitaxial growth [165], dc and rf plasma sputtering [166],chemical synthesis and self assembly [23] in combination

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Figure 15. Comparison of hysteresis loops of SmCo films treatedwith different processes. (a) RTP at 500 ◦C, (b) conventionalfurnace annealing at 500 ◦C for 20 min and (c) combined processingof (a) and (b).

with thermal annealing. RTP plays a very important rolein the processing of the nanocomposite systems because ofthe necessity of controlling the grain size during the heattreatments to realize magnetic hardening by phase transitionfrom the disordered fcc structure to the fct structure. Zhou et al[167], Mahalingam et al [168], Chu et al [169] and Liu et al[163, 170] have investigated the FePt-based nanocompositethin film systems using sputtering and RTP. Liu and hiscolleagues found that by applying RTP of the nanoscaleFe/Pt multilayers, very fine and homogeneous nanocompositemorphology was obtained. Figure 16 shows the TEM imageof the morphology. The nano-sized Fe-rich soft phase grains(5–8 nm) were embedded in the large grains of the hard phasewith grain size (50–100 nm) with coherent grain boundaries.The hard phase formed a ‘network’ with orientation as shownby the out-of-plane anisotropy. This morphology resulted inan energy product exceeding 40 MGOe. It was found thatthe homogeneous nanocomposite morphology could not beobtained by means of conventional furnace annealing of thesamples with the same composition.

4. Summary

In this paper, fundamental aspects of RTP and the applicationof RTP in advanced magnetic materials have been discussed.A comparison between the RTP and conventional furnace

Figure 16. TEM bright-field image of the Fe(2.1 nm)/Pt(1.5 nm)16

after RTP. The ‘white grains’ with diameter less than 10 nm are fccphase which are embedded in the FePt matrix grains with the lateralgrain size about 100 nm.

annealing has been made in various circumstances. Among avariety of effects of RTP on materials structures and properties,expedited and facilitated phase transitions under increasedheating rates are the most important effect which form thebasis for application of RTP. Both theoretical and experimentalinvestigations have proved this principle. Another mostpronounced RTP effect is the grain size reduction comparedwith conventional heat treatments. Rapid heating promotesnucleation and increased number of nuclei leads to reducedaverage grain size. This effect is extremely importantfor processing advanced nanostructured magnetic materials.These two effects are related to each other in some cases.The latter has more significance in the morphology control ofmaterials. In addition to these two effects, RTP is also effectivein controlling diffusion, especially grain boundary diffusion,as we have seen from the application of RTP in promoting grainsegregation.

Magnetic materials, particularly ferromagnetic materials,have very close correlation between their properties andtheir extrinsic morphology because of two major factors.(1) Magnetic dipolar interactions are directly related tothe shape and size of magnetic moment holders. Thistype of interaction leads to the demagnetizing effect andthe magneto-static energy change. (2) Magnetic coercivityis highly dependent on materials morphology. Therefore,deliberate manipulation of materials morphology is of vitalimportance for ferromagnetic materials. In this sense, anyprocessing techniques that can be applied to tailoring andcontrolling material morphology will be significant for theresearch in magnetic materials. As we have reviewed in thepreceding sections, RTP has played a unique role in controllingmorphology of various soft and hard magnetic materials andtherefore improving the magnetic performance.

The application of RTP in magnetic materials is stillin its initial stage, based on the fact that low-dimensionalmagnetic materials (thin films, ribbons, nanoparticles andother nanostructured magnets) have been developed only

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for a short time compared with other traditional magneticmaterials. Understanding of RTP effects on magnetic materialsprocessing is far from the end. There are still many unansweredquestions and unsolved problems. For instance, it has beenobserved that effects of RTP on the thermodynamics andkinetics of materials are not universal. They strongly dependon materials composition and structure. To understand thecorrelation between the RTP effects and specific compositionsand structures, more work needs to be done. In this review, wehave mainly collected the experimental results without makingtoo much effort in interpreting each result. Accumulation ofthe results will benefit our future research. On the other hand, itis worthwhile to note that currently available RTP facilities areonly suitable for low-dimensional magnetic material sampleson a laboratory scale. Large scale applications in industrieshave not yet been started.

It is nevertheless clear that RTP has become increasinglyimportant for advanced magnetic materials research anddevelopment and is a key approach to sophisticated materialsdesign and morphology control in the future.

Acknowledgments

This work is supported by the US DoD/MURI program underGrant No N00014-05-1-0497 and DoD/DARPA through ArmyResearch Office (ARO) under Grant No DAAD19-03-1-0038.The authors thank Mr K-T Chu for his work in the literaturesearch for this review.

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