exchange-bias and grain-surface relaxations in nanostructured nio/ni induced by a particle size...

Exchange-Bias and Grain-Surface Relaxations in Nanostructured NiO/Ni Induced by a Particle Size ReductionAleksandar Kremenovic,*,† Bostjan Jancar,‡ Mira Ristic,§ Milica Vucinic-Vasic,∥ Jelena Rogan,⊥

Aleksandar Pacevski,† and Bratislav Antic #

†Faculty of Mining and Geology, University of Belgrade, Đusina 7, 11000 Belgrade, Serbia‡Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia§Division of Materials Chemistry, Ruđer Boskovic Institute, POB 180, HR-10002 Zagreb, Croatia∥Faculty of Technical Sciences, University of Novi Sad, Trg D. Obradovica 6, 21000 Novi Sad, Serbia⊥Faculty of Technology and Metallurgy, University of Belgrade, POB 494, 11000 Belgrade, Serbia#Institute of Nuclear Sciences ”Vinca”, University of Belgrade, POB 522, 11001 Belgrade, Serbia

ABSTRACT: Transition-metal-oxide/transition-metal nanocomposites, such asNiO/Ni, FeO/Fe, and CoO/Co, have been the subject of much recentinvestigation (i) because of their potential applications and (ii) because they aregood model systems for studies of some effects on the nanoscale. They are used,for example, as catalysts, fuel-cell electrodes, magnetic memories, etc. When ananocomposite is composed of both ferromagnetic (FM) and antiferromagnetic(AFM) nanoparticles, interesting physical properties can occur, such as thephenomenon of exchange bias (EB). A Ni/NiO nanocomposite obtained by thethermal decomposition of nickel(II) acetate tetrahydrate, Ni(CH3COO)2·4H2O,at 300 °C is composed of NiO (62%) and Ni (38%) with crystallite sizes of 11and 278 nm, respectively. We observed an increase in the crystallite size for NiOand decrease of crystallite size for Ni, a decrease in the microstrain for both andan increase in the NiO phase content with thermal annealing in air, while high-energy ball milling leads to a decrease of the crystallite size, an increase in the sizeof the agglomerates, and microstrain as well as reduction, NiO→ Ni. The lattice parameters of the nanosized NiO and Ni show adeviation from the value for the bulk counterparts as a consequence of crystallite size reduction and the grain-surface relaxationeffect. The exchange bias found in a milled sample with particles of 10 nm (NiO) and 11 nm (Ni) disappears for larger particlesas a consequence of a coupling-area decrease between the antiferromagnetic and ferromagnetic particles. Due to reduction/oxidation (NiO ↔ Ni) and size as well as surface-relaxation effects the saturation magnetization value increases/decreases withmilling/annealing, respectively. Having in mind the effect of size on the exchange bias, coercivity, and magnetization values, it ispossible, by annealing/milling, to tailor the composition and particle size and then control the exchange bias and improve theother magnetic properties of the Ni/NiO.

1. INTRODUCTIONNanocomposites have attracted much attention recently owingto the synergistic properties induced by the interactionsbetween different nanometer-scale objects. Nanocompositescould show fascinating magnetic, magneto-optical, and semi-conducting properties that can be modulated by the interfacialinteractions between the different nanocomponents. Thisopens up a new opportunity to develop advanced, multifunc-tional nanomaterials for device concepts and applications.1

In particular, transition-metal-oxide/transition-metal nano-composites (such as NiO/Ni, FeO/Fe, and CoO/Co) haverecently been investigated because of (i) their potentialapplications and (ii) they are good model systems for studiesof some effects on the nanoscale. They are used in catalysts,fuel-cell electrodes, magnetic memories, etc.2,3 For nano-composite Ni/NiO, Lee et al.4 demonstrated the successful

application of NiO-coated Ni nanoparticles for the magneticseparation of Histidin-tagged proteins.When a nanocomposite is composed of both ferromagnetic

(FM) and antiferromagnetic (AFM) nanoparticles an interest-ing physical property can occur, for example, the exchange bias(EB) phenomenon. Such an EB is explained in terms of theexchange interactions between the FM and the AFM phases attheir interface. The main indication of the existence of an EB isthe shift of the hysteresis loop, HE, along the field axis after fieldcooling from above the Neel temperature, TN, of the AFM (andbelow the Curie temperature, TC of the FM) in materialscomposed of FM−AFM interfaces.5

Received: July 13, 2011Revised: January 11, 2012Published: January 17, 2012

Article

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© 2012 American Chemical Society 4356 dx.doi.org/10.1021/jp206658v | J. Phys. Chem. C 2012, 116, 4356−4364

pubs.acs.org/JPCC

The EB in Ni/NiO has already been reported, e.g.,6−8 DelBianco et al.9 investigated EB in Ni/NiO composites with a Nicontent varying between 4 and 69 wt %. They showed the EBdependence on the Ni content and (micro)structural character-istics of both phases.9 From the magnetic point of view, NiOchanges the physical properties with the particle size: bulk NiOis antiferromagnetic with a Neel temperature TN = 523 K, whilethe nanosized counterpart is superparamagnetic or super-antiferromagnetic.10 Nickel is ferromagnetic with a TC of 627K.11

Ni/NiO nanocomposites were obtained by differentmethods, e.g., milling NiO under H2 atmosphere,2 bymicrowave irradiation of two different nickel organic salts(acetate and formiate),12 mechanical alloying of Ni and NiO,7

and by the sol−gel route.13 In this study we used a simplemethod to form a Ni/NiO nanocomposite by the thermaldecomposition of nickel(II) acetate tetrahydrate, Ni-(CH3COO)2·4H2O. The aims were (a) an integrated studyof the thermal decomposition process of nickel(II) acetatetetrahydrate, (b) an investigation of the influence of temper-ature and milling on the phase composition, structure/microstructure and magnetic properties, and (c) studies ofthe size effect on grain-surface relaxation and EB.

2. EXPERIMENTAL SECTION

2.1. Sample Preparation. The nanocomposite Ni/NiOwas obtained by the thermal decomposition of nickel acetatetetrahydrate, Ni(CH3COO)2·4H2O (Alfa Aesar). The startingcompound was annealed in air at 300 °C for 5 min and fastcooled to room temperature. The so-obtained sample, denotedas S300, was further annealed at different temperatures up to800 °C. The samples annealed at 550 (S550) and 800 °C(S800) were selected in order to analyze the effects of thermaltreatment on the structure, microstructure, magnetic propertiesand phase content ratio (NiO:Ni). The as-prepared sampleS300 was milled for 1 h using a planetary ball mill (FritschPulverisette 5). A hardened-steel vial of 500 cm3 volume, filledwith 40 hardened steel balls with a diameter of 13.4 mm, wasused as the milling medium. The mass of the powder was 20 gand the balls-to-powder mass ratio was 20:1. The milling wasdone in air atmosphere without any additives. The angularvelocity of the supporting disk and vial was 32.2 and 40.3 rads−1, respectively. The resulting sample, S300_HEBM, wascompared with unmilled S300.2.2. Experimental Methods. The thermogravimetric

(TGA) and differential thermal (DTA) analyses wereperformed simultaneously (30−800 °C range) on a SDTQ600 TGA/DSC instrument (TA Instruments). The heatingrates were 20 °C min−1 and the sample mass was less than 10mg. The furnace atmosphere consisted of air at a flow rate of100 cm3 min−1.The particle (agglomerate) size distribution was determined

using a Malvern Mastersizer 2000 Particle Size Analyzer,capable of analyzing particles between 0.01 and 2000 μm. Amicroprecision wet-dispersion unit, Hydroμp, was used. Themeasurement parameters were: pump speed = 2500 rpm;ultrasonic = on.FT-IR spectra were recorded at RT using a Perkin-Elmer

spectrometer, model 2000. The FT-IR spectrometer wascoupled to a personal computer loaded with the IRDM (IRData Manager) program to process the recorded spectra. Thesamples were pressed into discs using spectroscopically pure

KBr. Precautions were taken to eliminate the influence ofmoisture from the air on the samples.Electron microprobe analyses were obtained using a JEOL

JSM−6610LV scanning electron microscope (SEM) connectedwith a INCA 350 energy-dispersion X-ray (EDS) analysis unit.Acceleration voltages of 30 kV and 20 kV were used for theimages and the analyses, respectively.FE SEM images were recorded using a thermal field-emission

scanning electron microscope, JSM-7000F, Jeol Ltd., Japan.The FE SEM was coupled with an energy-dispersive X-rayanalyzer, EDS/INCA 350, Oxford Instruments, England. Thesamples inspected with the FE SEM were not coated with aconductive layer.Transmission electron micrographs and selected-area dif-

fraction patterns were collected with a Jeol JEM 2100transmission electron microscope operating at 200 kV. Thesamples were prepared by dispersing the powders in acetoneand dropping the suspension on a lacey carbon film supportedon a 300-mesh copper grid.The hysteresis loops were measured at 5 K after being zero-

field-cooled (ZFC) and field-cooled (FC) for the selectedsample using an MPMS XL-5 SQUID magnetometer.For the collection of the X-ray powder-diffraction (XRPD)

data a Philips PW1050 (phase identification) and Bruker D8DISCOVER (Rietveld and size-strain refinement) automatedX-ray powder diffractometers were used. The Philips PW1050diffractometer was equipped with a Cu-tube, Ni filter and a Xe-filed proportional counter. The generator was setup at 40 kVand 32 mA. The divergence and receiving slits were 1° and 0.1mm, respectively. The scanning range was 30−110° in 2θ, witha step of 0.05° and a scanning time of 30 s per step. The BrukerD8 DISCOVER diffractometer was equipped with a Cu tube,Ge primary beam monochromator (yielded strictly pure CuKα1 radiation), LYNXEYE PSD detector (3° opening). Thegenerator was setup at 40 kV and 40 mA. The scanning rangewas 10−110° in 2θ, with a step of 0.02° and a scanning time of3 s per step.A microstructure determination from the XRPD data is one

of the most frequently used techniques. The microstructuraleffects that are responsible for the profile shape of thediffraction peaks are the finite size of the crystals or domainsand the microstrain. The crystallite size and microstrain valuesof phases NiO and Ni in the samples were determined with thestandard procedure for extracting the crystallite size andmicrostrain parameters from the diffraction data based on theintegral breadth of the line profiles.14 The microstructuralanalysis was performed by considering both the size and straineffects as isotropic, by using the Fullprof computer program15

The instrumental broadening was fully characterized throughthe instrumental resolution function that was obtained using astandard specimen of LaB6.

3. RESULTS AND DISCUSSION

3.1.1. Thermal Decomposition of Nickel(II) AcetateTetrahydrate. TGA/DTA Analysis. The thermal decom-position of nickel(II) acetate tetrahydrate occurs in two steps: I,the dehydration of the Ni(CH3COO)2·4H2O and the partialhydrolysis of the acetate groups to the basic nickel acetate withgeneral formula (1 − x)Ni(CH3COO)2·xNi(OH)2, and II, thefurther decomposition of the dehydrated intermediate generat-ing solid products such as Ni, NiO or a mixture of Ni and NiO.The range of temperatures of such thermal events and the

The Journal of Physical Chemistry C Article

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resulting solid products may depend on the atmosphere, theheating rate and the origin of the powder.16,17

The TGA and DTA curves for Ni(CH3COO)2·4H2O arepresented in Figure 1. The DTA curve shows an endothermic

peak at 113 °C and an exothermic peak at 380 °C. Theendothermic DTA peak is due to the dehydration and thehydrolysis up to about 170 °C, with the weight loss found to be33.0%. This process continues up to 294 °C in a subsequent,almost horizontal step with an additional 2.0% weight loss. Theoverall weight loss of 35.0% up to 294 °C is much larger thanthe theoretical value for four water molecules (about 29%).Therefore it should be ascribed to the hydrolysis of the acetategroups during the dehydration, resulting in the simultaneousevolution of acetic acid to the gas phase. According to thisresult, the formula of the obtained intermediary solid phase is0.82Ni(CH3COO)2·0.18Ni(OH)2. This formula is very similarto the formula 0.86Ni(CH3COO)2·0.14Ni(OH)2 reported byDe Jesus et al.16

The second decomposition step corresponds to the majordecomposition of the dehydrated intermediate and to theexothermic DTA peak (Figure 1), which is most probably thecombustion of organic residues in the air. The weight lossfound (71.8%) up to 381 °C is consistent with the valueexpected for the formation of a mixture of Ni and NiO (about72%).After these fragmentations at temperatures up to 800 °C, no

thermal effects were observed in the DTA curve (Figure 1).The TGA and DTA curves show that the temperature of 381°C (Figure 1) corresponds to a complete decomposition ofnickel(II) acetate tetrahydrate with a residual mass of about28%, which was due to the mixture of Ni and NiO. Thismixture was also verified by the XRPD patterns (for details, seesection 3.1.3).In conclusion, the partial hydrolysis of Ni(CH3COO)2·4H2O

can be described by the general equation:

·

= − ·

+ − +

x x

x x

Ni(CH COO) 4H O(s)

(1 )Ni(CH COO) Ni(OH) (s)

(4 2 )H O(g) 2 CH COOH(g)

3 2 2

3 2 2

2 3

Based on the TGA/DTA and XRPD results, the reactionsoccurring during the thermal decomposition of Ni-(CH3COO)2·4H2O are as follows:Step I, partial hydrolysis (up to 294 °C)

·

= · +

+

Ni(CH COO) 4H O(s)

0.82Ni(CH COO) 0.18Ni(OH) (s) 3.64H O(g)

0.36CH COOH(g)

3 2 2

3 2 2 2

3

and step II, oxidation (294−381 °C)

· +

= + +

x0.82Ni(CH COO) 0.18Ni(OH) (s) O (g)

0.28Ni(s) 0.72NiO(s) product(g)3 2 2 2

where x depends on gaseous products, which can be verydifferent.16

3.1.2. XRPD Analysis. The process of the thermaldecomposition of nickel(II) acetate tetrahydrate was monitoredby XRPD. The starting compound was annealed at differenttemperatures in air for 5 min and each resulting sample wasanalyzed using the XRPD technique. Figure 2 shows the

process of the thermal decomposition of nickel(II) acetatetetrahydrate and the formation of the Ni/NiO composite, aswell as the process of the partial oxidation of nickel. The X-raydata were recorded for lowest temperatures until the completedecomposition of nickel acetate. Evidently, only slight changeof intensity ration of main peaks in diffraction patterns of nickelacetate could be observed before structure collapse. It ispossible that basic nickel acetate is formed as amorphous likephase. The inset of Figure 2 shows the process of nickeloxidation in air after the formation of Ni/NiO at 300 °C. Thephase evolution of the Ni/NiO composite was studied by

Figure 1. TG and DTA curves for nickel(II) acetate tetrahydrate.

Figure 2. (a) Thermal degradation of nickel(II) acetate tetrahydratemonitored by XRPD (see text). Diffraction patterns for nickel(II)acetate tetrahydrate at ambient temperature (at the bottom) and afterannealing (main panel) and after the formation of the nanostructuredNi/NiO (the inset). (b) Change in sample color after selectedtemperatures.



Rietveld refinement using the Fullprof computer program15

Details of the refinement procedure are given in section 3.3.Table 1 shows the percentage of Ni and NiO in the Ni/NiO

composite. At 300 °C there is 38(1) % Ni, but with an increase

of the temperature due to the oxidation of nickel, the NiOcontent increases up to 94(2) % at 800 °C. We made aninteresting observation in the milled sample previouslyprepared by the decomposition of Ni(CH3COO)2·4H2O at300 °C, i.e., the phase content ratio NiO:Ni was found to be48(2):52(2). It is worth mentioning that the inverse process,the reduction of NiO into Ni with the formation of the Ni/NiOcomposite, was achieved in different ways, e.g., by mechanicalmilling under a H2 atmosphere.2,12 The initial microstructure ofthe NiO powder is shown to play an important role in thereduction rate and in the final microstructure of thenanocomposites. Already-published results indicate that themechanically induced defects have a strong influence on thekinetics of the reduction process.12 Furthermore, it should benoted that the color of the sample was dark-green after thethermal treatment, but before the thermal treatment the samplecolor was light-green (see Figure 2b).3.1.3. FTIR Analysis. To better understand the possibilities of

the obtained nanosize composites by high-energy ball milling,the compound obtained after the exposure of the startingmaterial at 240 °C (NiAc-240) was milled for 1 h. Theinfluence of the milling was studied with the aid of the FTIRspectra.The FTIR spectrum of the milled NiAc-240 (mNiAc-240) is

similar to the spectra of the starting NiAc-240 sample and nosignificant difference could be obtained (Figure 3). As weexpected, the corresponding bands of the starting materials inrelation to the milled ones are always stronger and betterdefined. The only apparent difference is the existence of astrong band at 3478 cm−1 for NiAc-240. This band arose fromthe ν(O−H) stretching vibrations and confirmed theintermediate basic nickel acetate up to 240 °C due to thedehydration and the hydrolysis of acetate groups in the firststep of the thermal decomposition. The formation of basicnickel acetate up to 170 °C was also confirmed by the TGanalysis. The presence of H2O molecules caused theappearance of a strong and broad ν(O−H) stretching bandcentered at about 3500−3000 and 3300−3000 cm−1 in theFTIR spectra of mNiAc-240 and NiAc-240, respectively. Theshape and position of these bands indicate that both samplesdid not dehydrate totally, but the hydrolysis had started only inthe case of NiAc-240. The reason for such behavior is the factthat milling inhibits the hydrolysis.Additionally, two very strong and broad bands in the 1600−

1360 cm−1 region are known as asymmetric, νas(COO), andsymmetric, νs(COO), stretching vibrations of the acetatogroups.18 The comparison of their difference, Δν, with thevalue for a “purely ionic” salt, NaCH3COO, (Δνi = 164 cm−1)

indicates the nature of the carboxylate coordination.19 It isevident that the Δν values for mNiAc-240 and NiAc-240 aresmaller than Δνi: 149 cm−1 [νas(COO) = 1569 cm−1 andνs(COO) = 1420 cm−1] and 131 cm−1 [νas(COO) = 1551 cm−1

and νs(COO) = 1420 cm−1], respectively. Such behavior is inaccordance with the presence of chelating COO groups in themNiAc-240 and NiAc-240. The very similar intensity and theshape of the COO bands also support this conclusion.Characteristic C−H deformational vibrations in the range

750−700 cm−1 were observed for both samples.3.2. Morphology, Microstructure, and Agglomeration.

We investigated the samples with SEM and FE SEM at differentmagnifications. The characteristic FE SEM images of theinvestigated samples S300 (prepared at 300 °C) and milledS300_HEBM (after high-energy ball milling) are presented inFigure 4a,b. At low magnification the FE SEM showed particlesin the form of aggregates of irregular shape in the micrometersize range (not shown). The size of these aggregates increaseswith the temperature of the thermal treatment. At highermagnification it can be seen that they are built from muchsmaller particles. The size of these smaller particles varies from15 to 20 nm. Consequently, the bigger particles are in factsecondary particles, built up from smaller, pseudosphericalprimary particles. A smaller number of larger secondaryparticles, up to 500 nm, are also visible. If we compare theFE SEM images of the samples S300 and S300_HEBM it seemsthat the particles are approximately the same in size, but theaggregation is more pronounced in the milling process. As theXRPD analysis showed, the thermal decomposition productscontain two kinds of particles, NiO and Ni, which are ofsignificantly different sizes (for details, see section 3.3). Itshould be noted that the FE SEM cannot distinguish these twokinds of particles with any certainty, i.e., it is not possible todistinguish between the secondary NiO particles and the largerprimary particles of metallic nickel. The elemental compositionof the samples was additionally determined by EDS analyses atdifferent positions on the samples. However, the EDS analysesobviously show the increase in the NiO content as thetemperature of the thermal treatment increases from 300 to 800°C, which can be related to the XRPD results.

Table 1. Crystallite Size (εβ), Strain (e), and Percentage ofEach Phase (wt %) for Ni/NiO

NiO Ni

sample εβ [nm] e·104 % εβ [nm] e·104 %

S300 11 27 62(1) 278 10 38(1)S550 67 12 67(1) 254 6 33(1)S800 166 8 94(2) 185 5 6(2)S300_HEBM 10 43 48(2) 11 47 52(2)

Figure 3. FTIR spectra for nickel(II) acetate tetrahydrate afterannealing at 240 °C NiAc-240, and after its milling, mNiAc-240.



The particle size analyses, performed with the Mastersizer2000, gave the particle volume percentage in 100 discrete sizeranges between 0.02 and 2000 μm. However, any comparisonbetween different particle-sizing techniques is difficult, andsometimes meaningless. The electron microscopy results could,fortunately, be compared with a number based on the resultsobtained from the laser diffraction. For this reason the resultswere, after the measurement, recalculated as the particlenumber percentage. The number-based results of the particlesize distribution (PSD) analysis are shown in Figure 4c. Thisnumber-based PSD showed that the number fraction ofparticles (agglomerates) smaller than 0.2 μm is negligible(<1%) for both samples and that in both samples agglomeratesgreater than 1 μm do exist. Such determined agglomerates

consist of many crystallites or particles, and thus are manyorders of magnitude larger than the monocrystalline andpolycrystalline particles. One of the PSD parameters, namedd(0.5)n.b., indicates that 50% the particles (agglomerates)measured on the number basis were smaller than, or equalto, the size stated. For sample S300 this value was ∼280 nm,approximately the same as the average crystallite size of the Niobtained from the XRPD data (the average crystallite size isgiven in Table 1). As can be seen from Figure 4c, the particle-size distribution curve for S300 was monodispersed, with themost common value of the frequency number distributioncurve (mode) being 252 nm. An increase in the degree ofagglomeration caused by the milling is evident: the distributioncurve for the milled sample S300_HEBM was shifted towardhigher values and the fraction of agglomerates greater than 1μm increased from 2.2% (for S300) to 17.1% for S300_HEBM.As a result of this agglomeration, the formation of three sizefraction is evident. The mode values of these fractions were 279nm, 365 and 489 nm, in increasing order. The increase in thedegree of agglomeration caused by the milling was evaluatedquantitatively using the ratio between the number-weighedmeans: D[1,0]S300_HEBM/D[1,0]S300 = 2.08.The significant agglomeration in all the samples (especially

the milled samples) was confirmed by the bright-field TEM(transmission electron microscopy) images (Figure 5a,b). Theselected-area electron diffraction (SAED) pattern collectedfrom agglomerate (Figure 5a) revealed the presence of both Niand NiO nanoparticles. The experimental and simulatedpatterns of the Ni and NiO, calculated using the data obtainedfrom ICSD #162279 and #87108, respectively, are shown inFigure 5c. Apart from the composite agglomerates containingboth Ni and NiO, separate agglomerates containing eithermostly Ni or mostly NiO were found.Furthermore, carbon traces in the form of graphite sheets

(Figure 6) were observed among the particles within theagglomerate, which we suggest were formed during thedecomposition of the nickel(II) acetate tetrahydrate.

3.3. Structure and Microstructure Analysis: Rietveldand XRPD Line-Broadening Analysis. The collected XRPDdata were used to refine the crystal structure and microstructureof the samples using the Fullprof program.15 The diffractionpatterns of the samples produced after the thermal treatment at300 °C, 550 °C and 800 °C as well as the milled sampleshowed the presence of the NiO and Ni phases only (Figure 2).A two-phases structural model, NiO S.G. Fm3m,20 and Ni S.G.Fm3m,21 was used for the crystal-structure refinement. TheRietveld refinement results are shown in Figure 7 for the S300and S300_HEBM.The microstructure analysis results are given in Table 1. It is

clear that the crystallite size effect is the main source of thepeak broadening. However, a non-negligible microstrain ispresent. Interestingly, it was found that the decompositiontemperature did not influence the crystallite size of both phasesto the same extent. The difference between the diffractionpeaks’ breadth for the NiO and Ni is clearly evident for theS300 sample’s diffraction pattern (Figure 7a). The values of thehalf width at half maxima (HWHM) are greater for the NiOthan for the Ni. The diffraction peak’s HWHM difference is aconsequence of the different microstructure of the NiO and Ni.The lower decomposition temperatures resulted in the largestdifference between the crystallite sizes of the NiO and Ni: theratio between the crystallite sizes of the phases (εβNi/εβNiO)decreased from approximately 25 to approximately 1 with an

Figure 4. FE SEM images of S300 (a), S300_HEBM (b), andaggregate distribution (c) (see text).



increase of the temperature within the interval 300−800 °C.For the sample annealed at 300 °C the difference in theHWHM diffraction peaks disappeared after milling, as can beseen from Figure 7b. This is a consequence of a significantcrystallite-size decrease for Ni (from ∼280 nm to ∼11 nm) aswell as of the microstrain increase of both the NiO (from ∼27× 10−4 to ∼43 × 10−4) and Ni (from ∼10 × 10−4 to ∼47 ×10−4).

The lattice-parameter value for NiO decreases with thecrystallite-size increase associated with the temperatureincrease, Figure 8. The lattice-parameter values for the Ni inS550 and S800 match, within the range of three estimatedstandard deviations (e.s.d.) and they are close to the bulk value

Figure 5. Bright-field TEM images of S300 (a and b), and SAEDsimulated and experimental (c) (see text). The circle in image (b)marks the area from which the SAED pattern was collected.

Figure 6. Bright-field TEM image of interparticle carbon inS300_HEBM.

Figure 7. Result of the Rietveld refinements for: (a) S300 and (b)milled S300_HEBM. Dots denote the observed step intensities; theline represents the corresponding calculated values. The vertical barsindicate the reflection positions for each phase separately; the upperones for NiO, while the lower ones are for Ni. The difference patternappears below.



of 3.523,20 while for S300 and S300_HEBM the unit-cellparameter values for Ni are slightly higher. The same trend wasalso found for the lattice parameters of NiO, Figure 8. Thedifference in the lattice parameters for NiO and Ni between theas-prepared and annealed samples is clearly evident. Consid-ering the microstructure results, this difference is mostly due tothe crystallite-size effect associated with the grain-surfacerelaxation (GSR). The crystallite size for both phases inS300_HEBM is the smallest, and consequently the latticeparameter differs the most from the bulk counterpart. Anincrease in the lattice parameter of NiO with the decrease in theparticle size was also reported in:22 the lattice parametersobtained from the Rietveld fits were 4.178, 4.182, 4.192, and4.211 Å for the 24, 12, 7, and 3 nm nanoparticles, respectively.The increase of the lattice parameter induced by milling (a(NiO)= 4.1919(5) Å ; a(Ni) = 3.5360(4) Å) is a consequence of thecrystallite-size reduction produced during the milling.The unit-cell parameters of most nanocrystalline materials

are significantly different from the coarse-grain or single-crystalcounterparts. However, a sound explanation is not alwaysavailable, even if the phenomenon is well-known. For example,to interpret the experimental evidence, stoichiometric effects,e.g., related to vacancies or oxygen in excess, are frequentlyused. However, a GSR effect connected to the displacement ofatoms next to the surface of a grain from an equilibriumposition is not usually considered. In NiO/Ni nanocomposites,which are the subject of our investigation, this effect seems tobe important. However, it should be taken into account thatthis effect is more a rule than an exception in nanocrystallinepowders. A large number of publications related to severeplastic deformation show an evolution of the unit-cellparameters with the grain-size change23−29 Analogous effectsare observed in finely dispersed catalysts and crystallinepowders produced from amorphous precursors,29,30as well asin nanocrystalline CeO2.

14 In the literature there are theoreticalmodels that explain the GSR effect.31,32 An exponentialdependence between the unit-cell parameter and the meancrystallite diameter is common for most theoretical models.Such dependence could be observed in Figure 8 for both NiOand Ni. However, a more detailed interpretation of thisexponential dependence is complex and beyond the scope ofthis manuscript.3.4. Magnetic Behavior of Ni/NiO Nanocomposites. In

order to study the exchange bias in our composites we selected

samples with different particle sizes and different Ni/NiOratios: S300, S300_HEBM and S550. As was described insections 3.1 and 3.2, after the milling of the as-prepared samplea partial reduction NiO → Ni occurs, and the crystallite sizerapidly decreases (Table 1). When the sample S300 wasannealed at 550 °C a partial oxidation Ni → NiO took placeand crystallites size increased (Table 1).From the magnetic point of view, the samples are composed

of two magnetic phases, i.e., ferromagnetic Ni and anti-ferromagnetic NiO. A bulk antiferromagnetic, such as NiO, hasa zero net magnetic moment. However, an ensemble ofnanoparticles NiO exhibits superparamagnetism33 and spin-glass properties.34 This is because the uncompensated surfacemoments of NiO particles have nonzero net magneticmoments. To study the magnetic behavior of Ni/NiOcomposites with an emphasis on the FM-AFM interactions,we carried out magnetization measurements, M(H) at 10 Kafter being zero-field-cooled (ZFC) and field cooled (FC). Theresults of the measurements are presented in Figures 9a,b,c. Inthe insets are given to enlarge the low-field regions after the FCmeasurements. The obtained hysteresis loops are smooth,without any sign of the presence of more than one phase,indicating strong magnetic interactions between the Ni andNiO, so the composite Ni/NiO behaves magnetically as a singlephase.The exchange bias (EB) was found for sample S300_HEBM,

characterized by crystallite sizes of 10 nm (NiO) and 11 nm(Ni), the inset of Figure 9b. The EB expressed as HE = (HC− +HC+)/2 is a measure of the hysteresis-loop shift along the fieldaxis (HC− and HC+ being the points where the loop intersectsthe field axis). The HE value was found to be 890 Oe at 10 K. Inthe S300 and S550 samples with larger crystallite sizes, no EBwas found (the insets of Figure 9a,c). The explanation for thisresult is based on the physical mechanism of the EB effect, i.e.,exchange coupling between the FM and AFM material. Theinterparticle AFM-FM interactions depend on the coupling areabetween the Ni and the NiO. For a small particle the surface/volume ratio is larger than for bulk materials, and interparticleinteractions are more favorable. It is obvious that there is acritical particle size for an observation of the EB. The results ofM(H) show that for the S300 and S550 samples, the radii ofthe particles are probably above the limit for EB. Here, it isworth referring to some literature results. The effect of size onEB was investigated by Yin et al.35 in Fe-doped CuOnanoparticles. It was found that EB strongly depends on theparticle size, i.e., with the particle size increase the EB decreasessharply.We also considered the size and composition effect on the

basic magnetic characteristics, i.e., saturation magnetization andcoercivity. The results in Figure 9 show nonsaturation of themagnetization up to 5T for the S300 and S300_HEBMsamples, while the magnetization is almost saturated for theS550 samples. Magnetization nonsaturation is a typicalobservation in small particles due to the existence ofdisordering in the particle shell. Namely, in a particle shell alarge number of bonds are broken. Therefore, the symmetry ofa core is different from the shell symmetry and from themagnetic point of view the surface shows a spin-glass-likebehavior. The surface/volume ratio is significant for smallparticles, and less important in bigger particles. The magnet-ization of S550, composed of large particles (see Table 1), ismore like the magnetization of a bulk material. The saturationmagnetization depends on the magnetically “dead” shell, the

Figure 8. Lattice parameter versus crystallite size for NiO and Ni incomposite Ni/NiO. Solid symbols this work; open symbols data fromrefs 22 (□) and 27 (○).



interparticle interactions and the Ni/NiO ratio. The saturationmagnetization MS was obtained by extrapolating the M(1/H)dependence on 1/H = 0 using M(H) values above H = 7000Oe. So the obtained value of the S300 sample (MS = 27.5 emu/

g) is higher than the value for S550 (MS = 22 emu/g), mainlydue to the increasing ratio NiO/Ni (i.e., the content of NiO)with annealing. A more detail study of change of saturationmagnetization with increasing annealing temperature in Ni/NiO was reported by Kar et al.36 It was observed that themagnetization values were decreasing as the annealingtemperature was increasing due to an increase in the fractionof AFM phase of NiO. Also, interparticle interactions are moreintensive between smaller particles in the S300 sample. Thevalues for the coercivity are very similar (HC(S300) = 175 Oeand HC(S550) = 160 Oe). If we compare the magneticparameters of S300 with those of the milled sampleS300_HEBM a significant enhancement in the value of thesaturation magnetization and the coercivity was obtained. Thevalues of MS and HC for S300_HEBM were found to be 42emu/g and 400 Oe, respectively. Based on our findings duringthe crystal-structure refinement, a reduction process NiO → Nihappened during the milling. Increasing the ferromagnetic Nicontent as well as the interparticle interactions clearly results inan increase in the magnetic parameters. Those interparticleinteractions can have a significant influence on the coercivity, aswas reported by Frandsen et al. in nanocomposite γ-Fe2O3/NiO.37 They found a lower coercivity in the composite than inpure γ-Fe2O3, and the opposite ratio in the composite γ-Fe2O3/CoO due to exchange interactions.37

4. CONCLUSION

There has been an increasing interest in the past few years instudies of ferromagnetic (FM) - antiferromagnetic (AFM)systems because of the exchange coupling between the FM andAFM phases that influences the physical properties, as well asbecause of the unresolved scientific issues. Therefore, there is achallenge to find a simple and controlled procedure for thepreparation of FM-AFM nano systems with a technologicalimpact. We report on a simple and cheap method for thefabrication of Ni/NiO nanocomposites that show the exchange-bias effect. The method is based on using commercial nickel(II)acetate tetrahydrate, its degradation at 300 °C and milling theobtained precursor for one hour. Although the process of thethermal decomposition of nickel(II) acetate tetrahydrate hasbeen described in the literature, we performed an integratedstudy of this thermal decomposition using the SEM/FESEM,HRTEM, TGA/DTA, FTIR, and XRPD techniques. It wasfound that milling provoked a reduction, NiO → Ni. Thelattice-parameter values show a size effect: for particles with acrystallite size smaller than 10 nm (NiO) and 11 nm (Ni) thelattice parameters are higher than those for their bulkcounterparts. This was interpreted as a grain-surface relaxationeffect in nanoparticles. Based on the effect of size on theexchange bias, coercivity and magnetization values, it is possibleby annealing/milling to tailor the composition and particle sizeand then control the exchange bias and improve the othermagnetic properties of the Ni/NiO.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

NotesThe authors declare no competing financial interest.

Figure 9. Hysteresis loops at 10 K for as-prepared Ni/NiO (S300) (a),after milling of as-prepared sample (S300_HEBM) (b) and afterannealing at 550 °C of the as-prepared sample (S550) (c). The insets:low-field region after FC cooling. The exchange bias effect: the inset of(b), see text.



mailto:[email protected]

■ ACKNOWLEDGMENTSThe Serbian Ministry of Education and Science has financiallysupported this work under contracts No. 45015 and No.176016 as well as Swiss National Science Foundation; SCOPESproject (Grant No. IZ73Z0_1 27961). We thanks to Prof.Gerado F. Goya for performing magnetization measurements aswell as to Prof. Volker Kahlenberg and Dr. Richard Tessardi forXRPD data collection on BRUKER D8 DISCOVER.

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