Exchange bias in ferromagnetic (FM)/antiferromagnetic (AF) bilayers is usually investigated in the longitudinal configuration with the exchange coupling established in the film plane. In this work, we report on the perpendicular exchange bias in FeMn(8 nm)/[FeNi(2 nm)/FeMn(8 nm)]n multilayers induced by perpendicular field cooling. The thin FeNi layers give rise to large values of the exchange field and coercivity, and n=15 allows a sufficiently large magnetization for the measurements. Even though the soft FeNi layers have an intrinsic in-plane anisotropy, perpendicular exchange bias has been observed after cooling in a perpendicular external field. The exchange field in the perpendicular configuration is about 0.85 that of the longitudinal case. In both the longitudinal and perpendicular configurations, the exchange field decreases quasilinearly with temperature. The squareness of perpendicular hysteresis loops decreases with increasing temperature

Chien, C.-L.

Searson, P.C.

Sun, L.

Zhou, S.M.

JScholarship

JOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 10 15 MAY 2003Longitudinal and perpendicular exchange biasin FeMn Õ„FeNiÕFeMn…n multilayersL. Suna)Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218S. M. ZhouDepartment of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218P. C. SearsonDepartment of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218C. L. ChienDepartment of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218~Presented on 12 November 2002!Exchange bias in ferromagnetic~FM!/antiferromagnetic~AF! bilayers is usually investigated in thelongitudinal configuration with the exchange coupling established in the film plane. In this work, wereport on the perpendicular exchange bias in FeMn~8 nm!/@FeNi~2 nm!/FeMn~8 nm!] n multilayersinduced by perpendicular field cooling. The thin FeNi layers give rise to large values of theexchange field and coercivity, andn515 allows a sufficiently large magnetization for themeasurements. Even though the soft FeNi layers have an intrinsic in-plane anisotropy, perpendicularexchange bias has been observed after cooling in a perpendicular external field. The exchange fieldin the perpendicular configuration is about 0.85 that of the longitudinal case. In both the longitudinaland perpendicular configurations, the exchange field decreases quasilinearly with temperature. Thesquareness of perpendicular hysteresis loops decreases with increasing temperature. ©2003American Institute of Physics.@DOI: 10.1063/1.1544447#thxiernaclalmthintilaia. IbleliulMxtuc-ex-ngol-ul-de-cularicu-in aub-le in-o-theldrncetedhevi-When ferromagnetic~FM!/antiferromagnetic~AF! bilay-ers are cooled in an external magnetic field from aboveNéel temperature of the AF layer, both unidirectional echange anisotropy and uniaxial magnetic anisotropy areduced at the FM/AF interface, resulting in a shifted hystesis loop and a coercivity enhancement. Longitudiexchange coupling has been explored in a wide rangematerials and structures with in-plane anisotropy.1–5 Re-cently, several groups have shown that exchange biasalso be established in multilayers with perpendicuanisotropy.6–8 Multilayered CoxFe12x/Pt films show squarehysteresis loops when the field is perpendicular to the fiplane, indicating that the magnetic easy axis is alongsurface normal. The shifted square loops have been obtawhen the multilayers are in contact with antiferromagneCoO, FeMn, or FeF2 after field cooling.In contrast to the studies on films with perpendicuanisotropy, no observations of perpendicular exchange bing have been reported for films with in-plane anisotropyis interesting to explore whether perpendicular exchangeasing exists in samples with in-plane anisotropy. In principexchange bias can be induced regardless of the field coodirection, however, it remains to be seen how perpendicexchange bias is different from the in-plane case.Here, we report the exchange bias study in NiFe/Femultilayers, in which both longitudinal and perpendicular echange coupling have been observed at room temperaa!Electronic mail: li.sun@jhu.edu6840021-8979/2003/93(10)/6841/3/$20.00e-n--lofanreedcrs-ti-,ngarn-rewith the cooling field applied along the corresponding diretions. The temperature dependence of the perpendicularchange field and coercivity with perpendicular field coolihas been investigated. Compared to longitudinal field coing, the perpendicular exchange bias in the NiFe/FeMn mtilayers is always smaller than the in-plane values. Withcreasing temperature, the squareness of the perpendihysteresis loops increases, indicating a stronger perpendlar anisotropy induced by the cooling field.Samples were fabricated by dc magnetron sputtering6 mTorr Ar atmosphere with a base pressure of 831028Torr. Si wafers with a native oxide layer were used as sstrates. Two samples were fabricated:@FeNi~2 nm!/FeMn~8nm)]15 multilayers and Cu~30nm!/FeMn~8 nm!/@FeNi~2 nm!/FeMn~8 nm)]15 /Cu~30 nm! multilayers for structural andmagnetic measurements, respectively. The second sampcludes an extra FeMn~8 nm! layer such that every FeNi~2nm! layer is sandwiched between two FeMn~8 nm! layers.The bottom Cu layer is included to stabilize the antiferrmagneticg phase of FeMn and the top Cu layer protectsmagnetic layers from oxidation. No external magnetic fiewas applied during deposition.Figure 1 shows a small angle x-ray reflectivity pattefor the @FeNi~2 nm!/FeMn~8 nm)]15 multilayer sample withCu Ka radiation. Figure 1 clearly shows the superlattipeaks associated with the multilayer structure. The calculabilayer period of 10.2 nm is in good agreement with tdesigned period of 10 nm.The magnetic properties were characterized using abrating sample magnetometer@~VSM! ADE model 10# from1 © 2003 American Institute of PhysicsrtinnauresempaldAeblsd-Igeeder-rg/a-wnf abiasthelarldiasig.ol-oughthewithed2geias.ivitycu-lon-w azerothetheheex-tureangelueys/Ptsx-eental-6842 J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Sun et al.100 K to room temperature. Lower-temperature measuments, down to 5 K, were performed on a superconducquantum interference device~SQUID! magnetometer. Allsamples were heated to 428 K in the VSM and cooled i2.0 T external field. For measurements taken between 5100 K, the sample was first field cooled to room temperatin the VSM and then field cooled in the SQUID magnetomter. At each temperature, the sample was cycled four timethe external field and the fifth hysteresis loop was recordThe FeMn~8 nm!/@FeNi~2 nm!/FeMn~8 nm)]15 samplewas first cooled with the field applied parallel to the filplane. Measurements were taken with the applied fieldallel and perpendicular to the film. Figure 2~a! shows aroom-temperature hysteresis loop with longitudinal fiecooling and measured along the cooling field direction.exchange bias field of2824 Oe and a coercivity of 238 Owere measured. Note that the multilayer structure enahigh accuracy measurements even though the NiFe layeronly 2 nm thick. The multilayer structure is particularly avantageous for studying perpendicular exchange biassmall magnetic fields where the signal is even smaller.FIG. 1. Small angle x-ray diffraction pattern for@FeNi~2 nm!/FeMn~8nm)]15 multilayers.FIG. 2. Hysteresis loops of FeMn~8 nm!/@FeNi~2 nm!/FeMn~8 nm)]15 mul-tilayers. ~a! and ~c! longitudinal field cool,~b! and ~d! perpendicular fieldcool.e-gande-ind.r-nesareinnaddition, the multilayer structure also provides exchancoupling from both interfaces of the FM layer as opposone interface in FM/AF bilayers.The longitudinal exchange energy per unit area (Ds i) ofthe FeMn~8 nm!/@FeNi~2 nm!/FeMn~8 nm)]15 multilayer canbe calculated from:Ds i52HEitFMS/2, ~1!whereHEi is the measured exchange field,tF is the singleFM layer thickness,Ms is the saturation magnetization of thFM layer, and the factor of 2 is due to the two FM/AF intefaces. For the FeMn~8 nm!/@FeNi~2 nm!/FeMn~8 nm)]15multilayer sampleDs i has been calculated to be 0.065 ecm2 usingMs5785 emu/cm3 at room temperature.When the longitudinally field cooled sample was mesured with the field perpendicular to the film plane, as shoin Fig. 2~b!, we observed a slanted loop, characteristic omagnetic hard axis. As expected, there is no exchangefield since the exchange coupling has been established infilm plane.The same @FeNi~2 nm!/FeMn~8 nm)]15 multilayersample was then cooled with the applied field perpendicuto the film plane from 428 to 300 K. After perpendicular fiecooling, the hysteresis loop is shifted with an exchange bfield of 2676 Oe and coercivity of 203 Oe, as shown in F2~c!. This result clearly shows that perpendicular field coing has established perpendicular exchange bias even thit is the hard axis of the thin ferromagnetic layers. Whenperpendicular exchange-coupled sample is measuredthe field in the film plane, there is no loop shift but enhanccoercivity, as shown in Fig. 2~d!.A comparison of the four hysteresis loops in Fig.hows that the values of exchange bias fieldHE and thecoercivityHC at room temperature for longitudinal exchanbias are larger than those for perpendicular exchange bFigure 3 shows the temperature dependence of the coercand the exchange field for both longitudinal and perpendilar field cooling. Both the coercivityHC and the absolutevalue of exchange bias fieldHE display similar temperaturedependences regardless of the field cool direction. Thegitudinal and the perpendicular exchange bias fields shoquasilinear temperature dependence and extrapolate toat 424 and 410 K, respectively. At each temperature,perpendicular exchange bias field is always smaller thanlongitudinal exchange bias field, however, the ratio of tperpendicular exchange bias field to the longitudinalchange field remains at about 0.85 with a weak temperadependence, as shown in Fig. 4. The coercivityHc shows amuch stronger temperature dependence than the exchbias field and decreases rapidly with temperature. The vaof HC for the perpendicular exchange bias is also alwasmaller than that for the longitudinal exchange bias.For the case of perpendicular exchange bias in Comultilayers pinned by CoO,7 the perpendicular exchange biafield was also found to be smaller than the longitudinal echange bias field with a ratio of about 0.5. This fact has battributed to the special spin structure and the strong crysline anisotropy of~111! texture of CoO. Unlike CoO andinomrlleatiobehed,In-thetheter-tioForisinngn aoer-ersbiasx-ithIngi-in-eldingy bek-1-Re-ett.ys.yan-r6843J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Sun et al.FeF2 used in the perpendicular anisotropic multilayers,6–8FeMn does not have a bulk spin structure with alternatoppositely aligned spins. Antiferromagneticg-FeMn has aface-centered-cubic structure where the Fe and Mn atrandomly occupy the lattice.9,10 FeMn has a noncollineaspin structure and the crystalline anisotropy is much smaFIG. 3. Temperature dependence of the exchange bias and coercivitlongitudinal ~a! and perpendicular~b! field cool.FIG. 4. Ratios of exchange bias field and coercivity for perpendicularlongitudinal field cool in FeMn~8 nm!/@FeNi~2 nm!/FeMn~8 nm)]15 multi-layers as a function of temperature~solid line!. Also shown is the temperature dependence of the hysteresis loop squareness for perpendiculachange bias~dotted line!.gsrthan that of CoO. These differences probably cause the rof the exchange bias field in FeMn/FeNi multilayers todifferent from those in other systems.A comparison of Figs. 2~c! and 2~d! show that eventhough perpendicular exchange bias has been establisthere remains substantial in-plane magnetic anisotropy.deed, the magnetization saturates at a smaller field withfield applied in-plane than when applied perpendicular tofilm plane. It is also noted that the squareness of the hysesis loop in both Figs. 2~c! and 2~d! is small. For the un-shifted loop in Fig. 2~d!, the squareness is taken as the raof remanent magnetization and saturation magnetization.the shifted loop in Fig. 2~c!, the remanent magnetizationtaken as the value atHE . While perpendicular squarenessFig. 2~c! is small, its value increases rapidly with decreasitemperature at low temperatures as shown in Fig. 4, imanner similar to the temperature dependence of the ccivity. The remanence becomes quite substantial~about 0.5!at low temperatures. This suggests that if thinner FM layhad been used, one may achieve perpendicular exchangewith perpendicular anisotropy.In summary, we have shown that perpendicular echange bias can be established in FM/AF multilayers win-plane anisotropy through perpendicular field cooling.the FeMn~8 nm!/@FeNi~2 nm!/FeMn~8 nm)]15 multilayer, theperpendicular exchange bias field is about 0.85 of the lontudinal exchange bias field at all temperatures. Althoughplane magnetic anisotropy remains after perpendicular ficooling, the rapidly increasing squareness with decreastemperature, suggests that perpendicular anisotropy mapossible with different ferromagnetic layers with small thicnesses.ACKNOWLEDGMENTThis work was supported by NSF Grant Nos. DMR001814 and DMR00-80031.1R. Jungblut, R. Coehoorn, M. T. Johnson, J. Van de Stegge, and A.inder, J. Appl. Phys.75, 6659~1994!.2O. Allegranza and M. Chen, J. Appl. Phys.73, 6218~1993!.3J. Nogues and I. K. Schuller, J. Magn. Magn. Mater.192, 203 ~1999!.4A. E. Berkowitz and K. Takano, J. Magn. Magn. Mater.200, 552 ~1999!.5T. Ambrose, R. L. Sommer, and C. L. Chien, Phys. Rev. B56, 83 ~1997!.6B. Kagerer, Ch. Bink, and W. Kleemann, J. Magn. Magn. Mater.217, 139~2000!.7S. Maat, K. Takano, S. S. P. Parkin, and E. E. Fullerton, Phys. Rev. L87, 087202~2001!.8F. Garcia, G. Casali, S. Auffret, B. Rodmacq, and B. Dieny, J. Appl. Ph91, 6905~2002!.9Magnetic Properties of Metals: d-Elements, Alloys and Compounds, e -ited by J. P. J. Wijn,~Springer, Berlin, 1991!, pp. 49–50.10W. J. Antel, Jr., F. Perjeru, and G. R. Harp, Phys. Rev. Lett.83, 1439~1999!.fordex-

Longitudinal and perpendicular exchange bias in FeMn/(FeNi/FeMn)n multilayers

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Longitudinal and perpendicular exchange bias in FeMn/(FeNi/FeMn)n multilayers

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