Magnetic anisotropy, coupling, and transport in epitaxial Co/Cr superlattices on MgO(100) and (110) substrates J. Johanna Picconatto and Michael J. Pechan Department of Physics, Miami University, Oxford, Ohio 45056 Eric E. Fullerton Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 Superlattices of Co/Cr have been epitaxially sputtered onto MgO 100 and 110 substrates coated with epitaxial Cr 100 and 211 buffer layers. The Co thickness is fixed at 20 Å and the Cr thickness varied from 7 to 22 Å. On the MgO 110 /Cr 211 substrates, coherent hcp-Co 11¯00 / bcc-Cr 211 superlattice structures are formed. On MgO 100 /Cr 100 , x-ray-diffraction results suggest strained hcp-Co 112¯ /bcc-Cr 100 superlattices. Magnetization measurements show fourfold magnetic in-plane anisotropy for the MgO 100 orientation and twofold for the MgO 110 . By utilizing a simple model based upon perpendicular uniaxial anisotropies, we have concluded that the fourfold anisotropy has its origin in the second-order uniaxial Co anisotropy energy. The antiferromagnetic interlayer coupling strength exhibits a maximum value of 0.15 erg/cm2 at a Cr thickness of 13 Å in the MgO 110 orientation. The MgO 100 orientation exhibits its strongest coupling of 0.55 erg/cm3 at 10 Å Cr thickness. Modest giant magnetoresistance values no larger than 3% are observed and we find no evidence of enhanced anisotropic magnetoresistance effects recently reported for Co 11¯00 /Cr 211 superlattices. © 1997 American Institute of Physics. S0021-8979 97 49508-7 I. INTRODUCTION and Cr layers. This growth procedure has been successfully Co/Cr alloys and superlattices have long been the subject used to grow Fe/Cr and Co/Cr superlattices.2,5 On the of study as candidates for increasing information density in MgO 110 /Cr 211 substrates, coherent hcp-Co 11¯00 /bcc- recording media. This is in part due to the uniaxial crystal- Cr 211 superlattices are formed, with x-ray diffraction, line anisotropy inherent in hcp Co which gives it fundamen- similar to those of Ref. 4. The in-plane epitaxial relationship tally unique magnetic properties in comparison with cubic Fe is MgO 001 Cr 01¯1 Co 0001 . On MgO 100 /Cr 100 sub- and Ni. Recently, investigators1­4 have utilized epitaxial strates, the expected Co structure depends upon the Co layer growth techniques to deposit coherent hcp Co layers with thickness.3 For thick layers, the films are epitaxial hcp- in-plane c-axis orientation. By choosing appropriate sub- Co 112¯0 which grow with a bicrystal microstructure.6 The strate and buffer layer crystalline orientations, one can con- in-plane twin directions are MgO 001 Cr 011 Co 0001 trol the symmetry of the in-plane magnetic anisotropy and and MgO 001 Cr 011¯ Co 0001 . For thinner layers the Co the interlayer coupling strength and therefore the magne- strains toward a bcc 100 layer. The x-ray-diffraction data of toresistance in Co-based superlattices. Of particular techno- the present samples are similar to those reported in Ref. 2 logical interest are the small Co domains arising from per- suggesting strained hcp-Co 112¯0 layers. pendicular c axes oriented in registry with a fourfold III. RESULTS AND DISCUSSION symmetric substrate lattice. We have prepared epitaxial Co/Cr superlattices on MgO substrates for the purpose of Magnetic hysteresis measurements were performed us- investigating the anisotropy, coupling energies, and magne- ing a vibrating sample magnetometer VSM at room tem- totransport properties. Particularly noteworthy is our conclu- perature with the field in plane. Magnetotransport properties sion that both the direction and magnitude of the magnetic were measured from room temperature to 5 K using a stan- anisotropy associated with the fourfold symmetry samples dard four-terminal dc technique with a constant current and can be understood as arising from the second-order Co H in plane. To characterize the magnetic anisotropy we mea- uniaxial anisotropy energy. sured the remnant magnetization normalized to the satura- tion magnetization as a function of the azimuthal angle of II. EXPERIMENTAL DETAILS the applied field. The results are shown in Fig. 1 for ferro- magnetically coupled superlattices from each series. The Two series of Co 20 Å /Cr tCr 20 superlattices with remnant magnetization for the Co 11¯00 layers reflect the 7 tCr 22 Å were epitaxially sputtered onto single-crystal expected twofold uniaxial anisotropy with the easy axis MgO 100 and 110 substrates. The substrates were along the MgO 001 direction. The Co 112¯0 layers show a mounted side by side onto the sample holder and simulta- fourfold anisotropy with the easy axis along the MgO 011 neously deposited. A 100 Å Cr layer was initially deposited and 01¯1 directions. This high degree of in-plane anisotropy at a substrate temperature of 600 °C resulting in epitaxial confirms the epitaxial nature of the layers. The strong four- Cr 211 and 100 buffer layers on MgO 110 and 100 , fold anisotropy observed for the 100 samples is due to the respectively.5 The substrate was then cooled to 150 °C and twinned uniaxial microstructure. If we assume that the the samples were grown by sequential deposition of the Co anisotropies from the twinned region are added a reasonable 5058 J. Appl. Phys. 81 (8), 15 April 1997 0021-8979/97/81(8)/5058/3/$10.00 © 1997 American Institute of Physics Downloaded¬21¬Mar¬2001¬to¬148.6.169.65.¬Redistribution¬subject¬to¬AIP¬copyright,¬see¬http://ojps.aip.org/japo/japcr.jsp FIG. 1. Polar plot of the normalized remnant magnetization vs azimuthal in-plane angle for ferromagnetically coupled Co layer samples. Co/Cr 20 Å grown on MgO 100 exhibits fourfold symmetry squares whereas Co/ Cr 16 Å on MgO 110 exhibits twofold symmetry triangles . assumption if the crystalline domains are small and exchange FIG. 2. Magnetic hysteresis loops for samples with peak AF-coupled Co coupled then the effective anisotropy energy equals layers: a Co/Cr 13 Å on MgO 110 orientation and b Co/Cr 10 Å MgO 100 orientation. Solid and dashed lines are data in the easy and hard in-plane directions, respectively. The top plot has been expanded to show E K1 sin2 K2 sin4 K1 cos2 K2 cos4 easy axis details, but along the hard axis one observes the magnetization reach saturation at a field of approximately 8 kOe. K1 12K2 1 cos2 2 , 1 where K1 and K2 are the first- and second-order uniaxial function overlap. When H is applied perpendicular to the anisotropy constants and is the angle of the magnetization easy axis, the Co layers coherently rotate to saturation at a with respect to the MgO 001 direction. Therefore, we ex- field pect an effective cubic anisotropy with MgO 011 and 01¯1 easy axis directions and strength determined by the second- Hs 4J1 8J2 2tCoK1 4tCoK2 /MstCo , order uniaxial anisotropy constant. where J2 is the biquadratic interlayer coupling.9 Magnetic hysteresis loops and giant magnetoresistance For the superlattices on MgO 100 , the strongest AF GMR curves measured with the applied field along the easy coupling was observed for tCo 10 Å in agreement with Ref. and hard axis for samples with the strongest antiferromag- 2. The magnetization and MR show the expected dependence netic AF coupling strength in each series are shown in Figs. on the in-plane direction of the applied field see especially 2 and 3, respectively. We first focus on the superlattices on Figs. 8 a and 9 a in Ref. 7 . The saturation fields for the MgO 110 . The strongest AF coupling is observed for Cr applied field parallel to the easy and hard axes are 4.7 and layer thickness of 13 Å. The shape of the loop is character- 11.3 kOe, respectively. The difference in saturation fields istic of AF-coupled superlattices with strong uniaxial in- reflects the fourfold anisotropy. Assuming a fourfold anisot- plane anisotropy7 as seen previously in Fe/Cr 211 and ropy described by Eq. 1 , the expected saturation fields are CoFe/Cu 110 superlattices on MgO 110 .5,8 For the applied Hs (4J1 8J2 2K2)/MstCo where the and corre- fields parallel to the easy axis, the system undergoes a meta- spond to the hard and easy axes, respectively. From the dif- magnetic transition from an antiparallel to parallel configu- ference in the Hs values, the estimated value for K2 is ration at a switching field given by Hs 2J1/MstCo , where 2.3 106 ergs/cm3, which is comparable to the room- J1 is the bilinear interlayer coupling, and Ms is the saturation temperature value of K2 for bulk Co, 1.5 106 ergs/cm3. This magnetization of the Co layer. Commensurate with the tran- suggests that the fourfold anisotropy results from the sition in the magnetization is a transition in the MR. Using uniaxial structures oriented perpendicularly within a layer the center of the offset hysteresis loops as an estimate of Hs rather than intrinsic anisotropies arising from a coherent bcc 930 G , the interlayer coupling equals J1 0.13 erg/cm2. crystal structure. The value of the exchange coupling, deter- The coupling strength is considerably weaker in this mined from the average saturation field, is J1 2J2 0.55 Co hcp /Cr bcc system than in epitaxial Fe bcc /Cr bcc erg/cm2. systems, which may result from the reduced symmetry of the The GMR values are generally quite small in compari- bilayer structure and the resulting decrease in electron wave- son to Fe/Cr superlattices, room-temperature values of 0.8% J. Appl. Phys., Vol. 81, No. 8, 15 April 1997 Picconatto, Pechan, and Fullerton 5059 Downloaded¬21¬Mar¬2001¬to¬148.6.169.65.¬Redistribution¬subject¬to¬AIP¬copyright,¬see¬http://ojps.aip.org/japo/japcr.jsp allel to the Co 0001 , the authors report MR ratios up to 18%. For other orientations of the applied field and current, MR values 1% were reported. Modeling of both the magnetization and the MR data is underway using approaches described in Refs. 7 and 9. In addition, torque measurements are planned to better isolate anisotropy from coupling effects and to provide a more in- sight into the relationship between K1 and K2 in each of the MgO 110 and 100 series. A key question underlying the magnetic behavior of these systems is the role played by micromagnetics. In other words, to what extent is the as- sumption leading to Eq. 1 of small bicrystalline domains with strong interdomain exchange valid in our system? Mi- cromagnetic modeling, such as that successfully employed recently by Peng et al.,10 may help in addressing such ques- tions. ACKNOWLEDGMENTS We wish to thank Zachary Hilt for taking the data for Fig. 1 and C. H. Sowers for sample preparation. This work was supported by U.S. Department of Energy, Basic Energy Sciences­Materials Sciences, under Contracts No. W-31- 109-ENG-38 ANL and No. DE-FG02-86ER45281 Mi- ami . 1 F. Schreiber, Z. Frait, Th. Zeidler, N. Metoki, W. Donner, H. Zabel, and J. FIG. 3. In-plane magnetoresistance data for samples shown in Fig. 2: a Pelzl, Phys. Rev. B 51, 2920 1995 . Co/Cr 13 Å on MgO 110 and b Co/Cr 10 Å on MgO 100 . Dashed 2 G. R. Harp and S. S. P. Parkin, Appl. Phys. Lett. 65, 3063 1994 . solid lines represent data taken with the field in the hard easy in-plane 3 N. Metoki, W. Donner, and H. Zabel, Phys. Rev. B 49, 17 351 1994 . direction. 4 J. C. A. Huang, Y. Liou, Y. D. Yao, W. T. Yang, C. P. Chang, S. Y. Liao, and Y. M. Hu, Phys. Rev. B 52, R13 110 1995 . 5 E. E. Fullerton, M. J. Conover, J. E. Mattson, C. H. Sowers, and S. D. and 1.2% for the AF-coupled superlattices on MgO 110 and Bader, Phys. Rev. B 48, 15 755 1993 . 6 100 , respectively, increasing to 2.0% and 3.2% at 5 K. The A. Nakamura and M. Futamoto, J. Appl. Phys. 32, 1410 1993 . 7 W. Folkerts, J. Magn. Magn. Mater. 94, 302 1991 . results are only weakly dependent on the relative direction of 8 K. Inomata and Y. Saito, Appl. Phys. Lett. 61, 726 1992 . the current with respect to the field and crystallographic axis, 9 M. Grimsditch, S. Kumar, and E. E. Fullerton, Phys. Rev. B 54, 3385 characteristic of the anisotropic MR of Co. We did not ob- 1996 . 10 serve the large transverse MR reported for Co 11¯00 /Cr 211 Q. Peng, H. Neal Bertram, Nina Fussing, Mary Doerner, Mohammad Mir- samaanj, David Margulies, Robert Sinclair, and Steven Lambert, IEEE superlattices.4 For H along the Co 112¯0 and the current par- Trans. Magn. MAG-31, 2821 1995 . 5060 J. Appl. Phys., Vol. 81, No. 8, 15 April 1997 Picconatto, Pechan, and Fullerton Downloaded¬21¬Mar¬2001¬to¬148.6.169.65.¬Redistribution¬subject¬to¬AIP¬copyright,¬see¬http://ojps.aip.org/japo/japcr.jsp