Spin-dependent tunneling in discontinuous Co–SiO 2 magnetic tunnel junctionsSandra Sankar, A. E. Berkowitz, and David J. Smith Citation: Applied Physics Letters 73, 535 (1998); doi: 10.1063/1.121924 View online: http://dx.doi.org/10.1063/1.121924 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/73/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetoresistance of spin-dependent tunnel junctions with composite electrodes J. Appl. Phys. 90, 6222 (2001); 10.1063/1.1419259 Asymmetric bias voltage dependence of the magnetoresistance of Co/Al 2 O 3 /Co magnetic tunnel junctions:Variation with the barrier oxidation time J. Appl. Phys. 89, 8038 (2001); 10.1063/1.1375805 Temperature dependent resistance of magnetic tunnel junctions as a quality proof of the barrier J. Appl. Phys. 89, 7573 (2001); 10.1063/1.1361055 Use of magnetocrystalline anisotropy in spin-dependent tunneling Appl. Phys. Lett. 75, 1941 (1999); 10.1063/1.124878 Spin-dependent tunneling junctions with hard magnetic layer pinning J. Appl. Phys. 83, 6685 (1998); 10.1063/1.367838

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Spin-dependent tunneling in discontinuous Co–SiO 2 magnetictunnel junctions

Sandra Sankara) and A. E. BerkowitzDepartment of Physics/Center for Magnetic Recording Research, University of California-San Diego,La Jolla, California 92093-0401

David J. SmithDepartment of Physics and Astronomy/Center for Solid State Science, Arizona State University, Tempe,Arizona 85287-1504

~Received 10 March 1998; accepted for publication 18 May 1998!

Discontinuous magnetic tunnel junctions~DMTJs! are an alternate system to the magnetic tunneljunctions~MTJ! currently being considered for magnetoresistance~MR! sensors. The DMTJs areeasier to fabricate and more robust than the MTJs. The nominal film structure isSiO2(20 Å)/Co(tCo)/SiO2(30 Å)/Co(tCo)/SiO2(20 Å), in which the thin Co layers arediscontinuous, in the form of nanoparticles. Magneto-transport measurements were madeperpendicular to the film plane on macroscopic junctions for in-plane applied magnetic fields. Theresults, for these films with only two magnetic layers, are similar to those of the discontinuousmultilayers previously reported. The MR response is relatively sharp and almost linear at lowmagnetic fields and is reproducible from one junction to another. This MR~defined asDV/Vmax! isweakly temperature dependent with a maximum between 4 and 300 K. The MR and themagnetization at low temperature suggest ferromagnetically coupled particles, which switch inlower magnetic fields than noninteracting particles. ©1998 American Institute of Physics.@S0003-6951~98!02530-3#

Magnetic tunnel junctions~MTJs! exhibiting magnetore-sistance~MR! due to spin-dependent tunneling were firstfabricated by Jullie`re in 1975.1 These junctions consist oftwo continuous magnetic electrodes, of different coercivities,separated by an insulating barrier. The MR is due to a tunnelconductance that varies with the angle between the magneti-zation directions in the two electrodes. The critical part ofthese MTJs is the quality~smoothness and dielectric proper-ties! of the insulating barrier. Aluminum oxide, Al2O3,formed by oxidation of sputtered Al, is a common choice ofbarrier material.2–4 However, the oxidation process requiresprecise control4 and the technique is limited to very fewoxides. For other materials, with direct deposition of the bar-rier, fabricating pinhole-free barriers and reproducing resultsremain major problems.5,6 Furthermore, there is usually asubstantial decrease of MR with increasing temperature.2–6

We have discussed discontinuous magnetic metal/insulator multilayers as an alternate system which minimizesthe above problems.7 However, for technological advantages,we have reduced the number of discontinuous metallic layersto two without diminishing the MR of multilayers with 20bilayers. The resulting structure, which consists of twoplanes of magnetic nanoparticles~discontinuous metallic lay-ers! in an insulating matrix, is referred to as a discontinuousmagnetic tunnel junction~DMTJ!. A DMTJ is easier to fab-ricate than a MTJ, since the system consists of thousands ofnanoscopic tunnel junctions with defects~if present! con-fined to individual junctions. It is also more robust since themagnetic particles are well protected by the insulator.

The films were sputtered from two separate targets ontothermally oxidized ~3000 Å oxide! Si~100! substratesmounted on a rotating substrate table. The metal~Co! was dcsputtered, and the insulator (SiO2) was rf sputtered from anoxide target. The base pressure was;1027 Torr. Deposi-tion was at room temperature with a 2 mTorr argon pressure.The nominal film composition is SiO2(20 Å)/Co(tCo)/SiO2(30 Å)/Co(tCo)/SiO2(20 Å), where the Co thickness,tCo, was varied between 16 and 20 Å. The nominal layerthickness was determined from the deposition rate~9–13 Å/min for the metal and 20–30 Å/min for the insulator!, cali-brated by low-angle x-ray reflection. When deposited uponSiO2, the thin Co layers grow by island formation and arediscontinuous.

Samples for transport measurements were deposited withshadow masks to allow four terminal measurements with thecurrent perpendicular to the film plane. The top and bottomelectrodes~;1500 Å Cu! formed a cross pattern sandwich-ing the DMTJ. The effective junction area was 0.09 mm2.Thirty-seven junctions were simultaneously deposited upon a3 in. diameter substrate with a variation in junction resis-tances due to a small gradient in the barrier thickness. TheMR measurements were made with a constant current formagnetic fields applied in the film plane.

The microstructure of the films was characterized bytransmission electron microscopy~TEM!. Cross-sectionalsamples were investigated to determine the film integrityalong the growth direction, and in-plane images were takenof single discontinuous Co layers, sandwiched between pro-tective SiO2 layers. Figures 1~a! and 1~b! show electron mi-crographs for films withtCo520 Å. The plan-view micro-graph of the single Co layer in Fig. 1~a! shows that the Coa!Electronic mail: sandra@physics.ucsd.edu

APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 4 27 JULY 1998

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~dark contrast! forms short chain-like clusters with widths ofapproximately 35 Å, with SiO2 ~light contrast! filling thespace between Co particles. The clusters appear to be com-prised of touching particles with diameters of about 35 Å. Inthe high-resolution cross-sectional image of the DMTJ@Fig.1~b!#, the strong speckled contrast is due to the amorphousSiO2, and the darker areas to the crystalline Co. The layersappear to be well defined with reasonably sharp interfaces,and the actual layer thickness of a nominal 20 Å Co layerappears to be about 30 Å. The increase of the layer thicknessis expected since the layer becomes discontinuous afterdeposition. However, the contrast in cross-sectional imagesis a result of projection through the specimen thickness sothe layers may actually be thinner or more discontinuousthan they appear.

Although the DMTJs exhibited some MR as deposited,the MR increased considerably after annealing at tempera-tures in the range of 300–350 °C for 1 h in a vacuum of;1028 Torr. Cross-sectional TEM images of these annealedfilms were qualitatively similar to as-deposited films with nopronounced structural changes. However, with similar an-nealing of multilayers with 20 bilayers, the number of super-

lattice reflections visible in electron diffraction patterns in-creased. Furthermore, annealing at higher temperatures~>400 °C! led to a decrease in the number of superlatticereflections. These results suggest a sharpening of the metal/insulator interfaces with moderate annealing~300–350 °C!,but deterioration of the interfaces~most likely due to diffu-sion! with overannealing~>400 °C!. All the data discussedhere were taken on samples annealed at 350 °C.

The conductivity of these DMTJs is similar to that ofdiscontinuous multilayers7,8 in that there is an energy gapdue to the Coulomb charging energy,Ec , which is requiredto create a pair of charged particles.9,10 I (V) curves mea-sured at 4 K show the gap to be 20 meV fortCo516 Å and6 meV for tCo520 Å which is consistent with the chargingenergy being smaller for larger particles. The gap disappearsat higher temperatures askBT becomes comparable toEc .As a result of the charging energy, the resistivity of a DMTJexhibits a stronger temperature dependence than that of acontinuous MTJ.

The films with tCo520 Å have a larger low-field MRresponse. Figures 2~a! and 2~b! show typical MR curves, at 4K and room temperature~RT!, after annealing at 350 °C for1 h. The MR ~left ordinate! is plotted as the voltage mea-sured at a fixed current of 2mA normalized to the maximumvoltage (Vmax). The shapes of the MR curves suggest super-paramagnetic particles that are blocked at 4 K and unblockedat RT. However, other magnetic features are shown byM (H) at 4 K which is also plotted~right ordinate! in Fig.2~a!. M (H) is comprised of two parts: magnetically coupledregions with a coercivity of 1050 Oe; and possibly un-coupled blocked particles with a distribution of coercivitiesgreater than 1600 Oe. The peak in MR occurs at 1600 Oe,corresponding to the state at which the magnetically coupled

FIG. 1. Transmission electron micrographs of:~a! plan view of single dis-continuous layer of composition SiO2~30 Å!/Co~20 Å!/SiO2~30 Å!, and ~b!cross section of DMTJ of composition SiO2~20 Å!/Co~20 Å!/SiO2~30 Å!/Co~20 Å!/SiO2~20 Å!. Both films were annealed at 350 °C for 1 h invacuum.

FIG. 2. ~a! MR andM (H) at 4 K of anannealed DMTJ~350 °C for 1 h invacuum! with tCo520 Å. The peak in MR occurs at 1600 Oe which is notthe coercivity (Hc51050 Oe).~b! MR at room temperature. The high-fieldslope is extrapolated to they axis to get the low-field MR. The inset is anexpanded scale at low fields, including data for two additional samples.

536 Appl. Phys. Lett., Vol. 73, No. 4, 27 July 1998 Sankar, Berkowitz, and Smith

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regions have switched but the blocked particles have not.Structurally, the two discontinuous layers are quite similarand such behavior is also seen in multilayers, suggesting thatthe magnetically coupled regions are distributed in both dis-continuous layers. At RT, the MR appears to be due to su-perparamagnetic particles~no hysteresis!, but M (H) ~notshown! shows a partly ferromagnetic behaviour with a coer-civity of 44 Oe. Comparing with films in which all the par-ticles are superparamagnetic, e.g., granular metal/insulatorfilms and DMTJs with thinner Co layers,8 the MR responseof this film is considerably sharper suggesting that both su-perparamagnetic and magnetically coupled particles contrib-ute to a sharp MR response.

The MR in these DMTJs is reproducible from one junc-tion to another despite the variation in sample resistances. Asshown in the inset of Fig. 2~b!, the MR responses for threedifferent samples are identical. The resistances of thesesamples as deposited are 43, 162, and 305V, and they in-crease to 310, 1070, and 2220V after annealing at 350 °Cfor 1 h, thus maintaining the relative ratios. Also seen in theinset of Fig. 2~b!, most of the MR at RT occurs for fields<1kOe with an almost linear response up to 200 Oe. Thus, theseDMTJs are potentially useful for applications, with appropri-ate materials to optimize the low-field response.

The MR curves have a high-field slope at all tempera-tures. For comparison at different temperatures, the low-fieldMR is defined as (Vmax2Vsaturation)/Vmax5DV/V after thehigh-field slope is subtracted. Thus the low-field MR is 3.9%and 2.7% at RT and 4 K, respectively, after extrapolation ofthe high-field slope to they axis as illustrated in Fig. 2~b!. Incontinuous MTJs, there is a monotonic decrease of MR withincreasing temperature, and, in numerous cases, no MR isobserved at RT.5 However, in the DMTJs, there is a broadmaximum of the low-field MR between 4 K and RT asshown in Fig. 3~a!. For further insight, we plot separately

DV andV ~at 2mA! as shown in Fig. 3~b!. The solid line for

V(T) represents a continuous measurement as the tempera-ture is ramped slowly from 4 K to RT, and theopen circlesrepresent discrete points extracted from MR curves at fixedtemperatures.V(T) drops off rapidly up to about 70 K,which corresponds to the charging energy~6 meV! deter-mined fromI (V). In this temperature region,DV increases,which is not yet fully understood. The net result is the in-crease in MR with increasing temperature. At higher tem-perature, the MR decreases slightly becauseDV falls offfaster thanV.

For the voltage range of the above measurements, therewas little change of MR withV. However, at larger voltages~0.1–0.5 V!, there was a decrease in MR with increasingV,similar to what is reported for the MTJs.2–6 At RT, the MRdecreases from 4% at lowV to 2% at 0.4 V, whereas at 4 K,the MR decreases from 2.6% to 2%. Thus, the MR at highvoltages seems to be temperature independent. In theseDMTJs, there are two ways to overcome the insulator energybarrier—with temperature or with voltage. With high volt-ages~0.4 V!, one would expect temperature to play a smallrole in the conduction, as observed.

In conclusion, discontinuous magnetic tunnel junctions~DMTJs! offer an alternate system to the continuous mag-netic tunnel junctions~MTJs! currently being developed formagnetic sensors. These DMTJs are easier to fabricate thanMTJs. Masks are changed only between the noncritical Cuelectrodes and the junction~discontinuous magnetic layersand insulating matrix!. These MTJ offer reproducible MRwhich is monotonic and approximately linear in the low-fieldregion, with a temperature dependence which can be opti-mized for appropriate applications by changing the particlesize ~charging energy!. Furthermore, magnetic interactionswithin the discontinuous layers seem to play a significantrole in achieving a sharp MR response in low magneticfields.

This work was supported primarily by NSF Grant No.DMR-9400439. Electron microscopy was conducted at theCenter for High Resolution Electron Microscopy at ArizonaState University. The authors thank Fuding Ge for assistancewith TEM sample preparation.

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FIG. 3. ~a! Low-field MR of annealed DMTJ~350 °C for 1 h invacuum! asa function of temperature withtCo520 Å. ~b! V andDV. The solid line forV represents a continuous measurement as the temperature is increased andthe discrete points are taken from the MR data at constant temperature. Thedashed line forDV is just a guide to the eye.

537Appl. Phys. Lett., Vol. 73, No. 4, 27 July 1998 Sankar, Berkowitz, and Smith

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