Magnetoresistance of magnetiteJ. M. D. Coey, A. E. Berkowitz, Ll. Balcells, F. F. Putris, and F. T. Parker Citation: Applied Physics Letters 72, 734 (1998); doi: 10.1063/1.120859 View online: http://dx.doi.org/10.1063/1.120859 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/72/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural and magnetic properties of magnetite-containing epitaxial iron oxide films grown on MgO(001)substrates J. Appl. Phys. 103, 043902 (2008); 10.1063/1.2840118 Microstructure and magnetic properties of magnetite thin films prepared by reactive sputtering J. Appl. Phys. 102, 113913 (2007); 10.1063/1.2817644 Large low field magnetoresistance in ultrathin nanocrystalline magnetite Fe 3 O 4 films at room temperature Appl. Phys. Lett. 91, 102508 (2007); 10.1063/1.2783191 Magnetoresistance in magnetite films prepared from aqueous solution at room temperature J. Appl. Phys. 87, 7127 (2000); 10.1063/1.372952 Transport and magnetic properties of epitaxial and polycrystalline magnetite thin films J. Appl. Phys. 83, 7049 (1998); 10.1063/1.367547

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Magnetoresistance of magnetiteJ. M. D. Coey,a) A. E. Berkowitz, Ll. Balcells,b) F. F. Putris, and F. T. ParkerCenter for Magnetic Recording Research, University of California, San Diego, La Jolla, California 92093

~Received 13 October 1997; accepted for publication 9 December 1997!

The magnetoresistance behavior of Fe3O4 in polycrystalline thin film, powder compact, andsingle-crystal form are compared. Negative magnetoresistance with peaks at the coercive field,observed in thin films and powder compacts but not in the single crystal, is due to field-inducedalignment of the magnetization of contiguous grains. The effect is associated with intergranulartransport of spin-polarized electrons. ©1998 American Institute of Physics.@S0003-6951~98!02406-1#

Magnetite is the best-known spinel ferrite. Its electronicstructure is characterized by 3d5 spin-polarized ferric ioncores, oppositely aligned onA and B sites, with a singleminority-spinB-site electron occupying a spin-polarizedt2g

band. At room temperature, the oxide is a poor metal. Theconductivity is 23104 V21 m21, which is an order of mag-nitude less than the minimum metallic conductivity.1 Theconduction electrons in magnetite are thought to form po-larons, or bipolarons. Transport properties were recently re-viewed by Brabers.2 The reported magnetoresistance data arequite inconsistent, with values differing by an order of mag-nitude between thin films and single crystals. There are evendisagreements regarding the sign of the effect. Singlecrystals3 and epitaxial films4 have been shown to exhibitlarge negative magnetoresistance around and below the Ver-wey transition at 120 K.

Recent work on ferromagnetic manganites5,6 and CrO2,7

which also have a spin-polarized 3d conduction band, hasrevealed a low-field magnetoresistance which has been un-ambiguously associated with the change of the relative ori-entation of ferromagnetic regions separated by a grainboundary8,9 or an interparticle contact.7 This ‘micromag-netic’ effect is related to the giant magnetoresistance ofmultilayers,10 granular ferromagnets,11 and spin-polarizedtunnel junctions.12,13 The common feature of all these sys-tems is that the magnetoresistance is greatest near the coer-cive field or switching field.

Here we revisit the magnetite problem, comparing themagnetoresistance of the material in three different forms: asingle crystal, a polycrystalline thin film, and pressed pow-der. The data reported are taken at room temperature and attemperatures above the Verwey transition. Details of thesamples are listed in Table I. The thin film was prepared byreactive dc sputtering onto a Si substrate at 500 °C.14 Itsthickness was 500650 nm and the grain size was determinedby etching to be 1–2mm. It has a slight in-plane 110 texture.The film is practically stoichiometric; a conversion-electronMossbauer spectrum shows an intensity ratior M of theB-site subspectrum to the total area of 0.64, the value nor-mally observed in stoichiometric material. The synthetic

single crystal and the powder are slightly cation-deficient,with r M50.58. The values of the resistivityr and magneti-zations in Table I reflect the nonstoichiometry of the crys-tal. The powder was composed of particles roughly 50mm insize, each of which was made up of a number of crystallites.It was cold-compacted in a die under a pressure of 1 GPa togive pellets with 40% of the full density. Other powderscomposed of smaller (;1 mm) cubic or acicular magnetiteparticles were found to show similar powder magnetoresis-tance~PMR! effects.

Resistivity measurements were made using a dc four-probe method. Room-temperature magnetoresistance curvesare shown in Fig. 1. Despite their very different resistivity,the thin film and pressed powder compact exhibit similarlyshaped curves, with maxima at the coercive field. Magne-toresistance ratios, defined asr 5(Rmax2R0)/R0, whereR0 isthe resistance in 0.5 T, are 1%–3%, with the largest valuesappearing for the acicular powder. There is little temperaturedependence. On this scale, there is no detectable magnetore-sistance in the single crystal.

The magnetoresistance in both compacted powder andthin film is related to the coercivity of the sample. This isseen in Fig. 2, where the coercivityHc is plotted against thefield Hm where the resistance is maximum. Data are for sev-eral different samples, at temperatures ranging from 100 to300 K. The slope of the line is 1.0560.05.

Although the thin film is stoichiometric, its resistivity istwice that of the crystal, and six times as great as the intrinsicresistivity of bulk stoichiometric material. This suggests thatgrain-boundary resistance is the dominant contribution. Sup-posing that the resistance of the film can be written asRi

1Rgb, where Ri is the intrinsic resistance andRgb5n3r gb/A is the extra resistance due ton grain boundaries in asample of cross-section areaA, we deduce r gb51 – 2310210 V m2. This may be compared with the value 4.1

a!Permanent address: Physics Department, Trinity College, Dublin 2, Ire-land. Electronic mail: coey@martech.fsu.edu

b!Permanent address: ICMAB, Campus UAB, E-08019 Bellaterra, Spain.

TABLE I. Characteristics of magnetic samples used in this work. All dataare at 20 °C.

Samplea0

~nm!r~20 °C!

~Vm!Dr/r~%!

m0Hc

~mT!s

(JT21 kg21) r M

Crystal 0.837 19031026 — 20.0 84.7 0.58Polycrystalline film 0.839 35031026 1.7 23.4 94.5 0.64Compacted powder 0.839 50031023 1.2 22.8 87.3 0.58

APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 6 9 FEBRUARY 1998

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310210 V m2 recently determined by Steenbecket al.8 for aferromagnetic manganite. There is also a possibility that an-tiphase boundaries15 contribute to the excess resistance ofmagnetite films.

The resistivity of the pressed powder is over a thousandtimes greater than that of the single crystal or thin film. Thehigh resistance arises from contacts between the particles.That average contact resistance is roughly estimated from theparticle sized as r/d'10 kV at room temperature; it ismuch greater at low temperatures where it far exceeds thequantum limit RQ5h/2e2512.9 kV. Figure 3 shows thehigh-field magnetoresistance of the powder compact at 130K. Current–voltage curves, taken at 130 K in fields of 2 T,where the low-field magnetoresistance is saturated and in220 mT where the resistivity is maximum, are shown in Fig.4. These have the nonlinear form expected for tunnelingthrough a barrier. An estimate of the number of junctions ina path between the voltage probes is 20, so the applied volt-

age per junction in Fig. 4 needed to increase the conductivityby a factor 10 is approximately 300 mV. Fitting to the Sim-mons model for a square barrier16 gives a barrier height of410 mV and a thickness of 0.21 nm. These are plausibleparameters, but the barrier height deduced from the nonlinearI –V curve varies inversely as the estimated number of con-tacts. Other possible intergranular transport mechanisms thatcan give rise to nonlinearI –V curves include inelastic tun-neling via localized states~Glazman–Matveev model! andvariable-range hopping among defects.17 It may be possibleto distinguish between these models by detailed measure-ments of the temperature dependence of the grain boundaryconductance.

In the polycrystalline film, the intergranular resistance isestimated fromr gb to be of order 0.2 kV, which is much lessthan RQ . The I –V curves are linear. The grains are notelectrically isolated, but they are unlikely to be stronglyexchange-coupled on account of the nearest-neighbor char-acter of the dominantA-B exchange. The spin-polarizedminority-spin conduction electrons encounter a change ofquantization axis which is sharp on the atomic scale as theymove from one grain to the next, whether in the polycrystal-line film or in the powder compact. In either case the prob-ability of transfer of the spin-polarized electron across thegrain boundary will be proportional to cos2 u/2,18 whereu isthe angle between the magnetization of adjacent grains.Domain-wall effects are evidently unimportant since there isno comparable effect in the single crystal. The domain-wall

FIG. 1. Room temperature magnetoresistance of~a! single crystal,~b! poly-crystalline film, and~c! powder compact. Note the different resistivityscales.

FIG. 2. Relation between the coercivity and the field for maximum resis-tance for several powder~s! and polycrystalline film~d! samples. Data aretaken at temperatures ranging from 130 to 300 K.

FIG. 3. Magnetoresistance of a magnetite powder compact at 130 K.

FIG. 4. I –V curves for a pressed powder compact at 130 K taken in appliedfields of 2 T ~s! and 220 mT ~d!, together with the fit for tunnelingthrough a square barrier.

735Appl. Phys. Lett., Vol. 72, No. 6, 9 February 1998 Coey et al.

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width in magnetite is some hundreds of interatomic spacings.In conclusion, we have shown that the magnetoresis-

tance of a polycrystalline magnetite film is qualitativelysimilar to that of a cold-pressed powder compact, and quitedifferent from that of a single crystal. In both cases the mag-netoresistance arises from misaligned magnetization in adja-cent ferromagnetic grains which are exchange-decoupled.Discrepancies in the previous reports of the magnetoresis-tance of magnetite may be related to the grain structure ofthe samples. The results suggest that magnetite films mightbe usefully developed as magnetoresistance sensors, with ap-propriate control of the coercivity, grain size, and grain-boundary structure. Likewise, powders with suitable particleshape and size may be formed into thick films or compositesfor the same purpose.

This work was supported by NSF Grant No. DMR-9400439. The authors are grateful for help from Lee Rudee,Sandra Sankar, Jae Yi, and Chris Platt. The film was pre-pared by David Margulies.

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736 Appl. Phys. Lett., Vol. 72, No. 6, 9 February 1998 Coey et al.

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