JOURNAL OF APPLIED PHYSICS VOLUME 85, NUMBER 8 15 APRIL 1999 Anisotropy of the sublattice magnetization and magnetoresistance in Co/Re superlattices on Al2O3 112¯0... T. Charlton and D. Lederman Physics Department, West Virginia University, Morgantown, West Virginia 26506-6315 S. M. Yusuf and G. P. Felcher Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 Co(20 Å)/Re(6 Å) 20 superlattices were grown on a (112¯0) surface of a Al2O3 single crystal, with the 0001 direction of their hcp structure in the plane of the film. The Co layers were found to be antiferromagnetically coupled AF , with a saturating field of 6 kOe. Polarized neutron reflectivity was used to determine the direction of the sublattice magnetization. In zero applied field, the AF moments are aligned along the Co 0001 axis. In a magnetic field H perpendicular to the Co 0001 axis, the sublattices moments evolve to a canted arrangement, with the AF component always perpendicular to the field. With H along the Co 0001 axis, the AF moments flop in a direction perpendicular to Co 0001 axis. The spin flop transition is not abrupt, but can be described as a gradual rotation that is completed at 2 kOe. The anisotropy of the sublattice magnetization is related to the anisotropy of the magnetoresistance. This has the conventional dumbbell behavior with the field applied perpendicular to the Co 0001 axis, but exhibits an extended plateau near H 0, and a slight increase up to H 2 kOe, when H is parallel to Co 0001 axis. © 1999 American Institute of Physics. S0021-8979 99 18708-5 INTRODUCTION base pressure at West Virginia University.7 Prior to growth, Recently, major computer storage manufactures began to the substrates were acid etched and annealed in vacuum at use magnetoresistive read/write heads to increase the bit den- 575 °C. A nominally 50 Å Re buffer layer was then depos- sity in hard disk drives. Even larger gains are possible by ited at a substrate temperature of 560 °C, followed by the taking advantage of both giant magnetoresistance GMR growth of the 20 bilayer Co/Re superlattice at 158 °C. Two and anisotropic magnetoresistance AMR effects. GMR was nominally identical Co 20 Å /Re 5 Å superlattices were discovered approximately 10 years ago1 and has been thor- grown for separate neutron diffraction and magnetization oughly studied in several multilayer systems.2 This effect is measurements. The magnetization measurements were pre- driven by the antiferromagnetic arrangement of the magneti- formed at room temperature using the dc magnetooptic Kerr zation in adjacent magnetic layers of a metallic multilayer effect MOKE . Transport measurements were carried out on system.3 AMR, on the other hand, has been thoroughly stud- samples cut to approximately 3 3 mm2 squares at 10 K in a ied since the 1930's.4 In AMR the resistance with the current 5.5 T superconducting magnet using a four contact tech- applied parallel to the magnetization is greater than the re- nique. The samples were characterized by low angle and sistance with the current applied perpendicular to the mag- high angle x-ray diffraction using Cu K radiation from a netization. For this reason, systems with in-plane anisotro- rotating anode x-ray source. pies can be used to exploit AMR in technological devices. Magnetization and magnetotransport measurements can RESULTS be used to determine whether antiferromagnetic AF cou- Fits of an optical model8 to the low angle x-ray specular pling exists, but these measurements are indirect. Neutron reflectivity gave an actual layer thickness of Co 20 Å /Re 6 diffraction and polarization analysis gives a direct way to Å for the neutron sample and Co 17 Å /Re 8 Å for the sense the magnetic ordering of the samples.5 To study these magnetization sample. The thickness uncertainties were 2 effects, a system with an in-plane anisotropy and significant Å, the average interface roughness was 3.4 Å for both AF coupling is needed. For this reason, we chose a Co/Re samples. High angle x-ray diffraction confirmed the superlat- superlattice grown on a (112¯0) Al2O3 substrate. Both Co tice epitaxy, with the 101¯0 direction of Co and Re along and Re have hcp crystal structures and their 0001 axis is in the direction of growth, and in the plane Co 0001 axis lying the film plane. For small Re thicknesses, the superlattice is parallel to the c axis of the substrate.7 antiferromagnetically ordered. In addition, a small GMR has MOKE measurement showed M ­ H loops with no rem- been observed previously6 in 0001 Co/Re multilayers. anent magnetization indicating AF alignment Fig. 1 . Notice however, that in the loop corresponding to H c there is a EXPERIMENT sudden change in the slope at 1.1 kOe. Superlattices of the type Co 20 Å /Re 15 Å are uncoupled,7 with the easy The samples were grown on (112¯0) Al2O3 substrates, in axis along the 0001 direction. On that basis, it is reasonable a dc magnetron sputtering system with a 3.0 10 7 Torr to surmise that in Co 17 Å /Re 8 Å the sublattice magne- 0021-8979/99/85(8)/4436/3/$15.00 4436 © 1999 American Institute of Physics Downloaded 01 Mar 2001 to 148.6.169.65. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Charlton et al. 4437 FIG. 1. MOKE hysteresis loops preformed at room temperature for the FIG. 2. a Integrated antiferromagnetic peak intensity and b antiferromag- Co 17 Å /Re 8 Å sample with the applied field, H, parallel and perpendicu- lar to the easy axis, denoted by c. netic moment angle with respect to H when H c and H c . Inset shows the direction of H with respect to the AF moment M 1 M 2 , where M 1 and M 2 are the magnetizations of adjacent Co layers. tization also points along the 0001 direction in zero field, MF as a function of H. For the H c case, a continuous and it spin flops at 1.1 kOe. Similar results have been transition from MAF at H 0 to MF as H increases is ob- recently observed in Fe/Cr9,10 and Co/Cr11 multilayers. served, as expected in a regular antiferromagnet. For the H c Spin dependant neutron reflectivity measurements were case, the SF transition between MAF and MAF is gradual. carried out on the Co 20 Å /Re 6 Å sample at Argonne National Laboratory on the POSY1 system. Over a momen- tum range up to the Bragg reflection of the superlattice, the measurements revealed an AF peak position corresponding to twice the superlattice period. Plotted in Fig. 2 a is the integrated AF peak intensity as a function of H. H was ap- plied c and c, always in the plane of the sample. The integrated AF peak intensity is proportional to the square of the AF component of the sublattice magnetization (M2AF). MAF has components parallel (MAF ) and perpendicular (MAF ) to H. These two components were separated by ana- lyzing the spin of the neutrons reflected at the AF peak.12 The scattering associated with MAF does not change the spin orientation of the neutron, while MAF causes the neutron to flip its spin. From these two components we plot the angle the AF moment makes with respect to H in Fig. 2 b . Notice that when H c, the AF moments are always H. When H c, the AF moment rotates from being c to being c. Above 2 kOe, MAF is essentially H regardless of the direc- tion of H. A more detailed picture is obtained assuming that the total magnetic moment per Co atom is Mtot 1.47 B /Co, and a homogeneous model such that M2 2 2 AF M AF M F M2 2 2 2 tot when H c, and that M AF M F M tot when H c. MF is the ferromagnetic component of the magnetization, FIG. 3. M which is not measured directly, but is derived from the val- AF , M AF , and M F obtained from neutron diffraction with spin polarization analysis for the cases H c and H c. Lines are guides ues of MAF and MAF . Figure 3 shows MAF , MAF , and to the eye. Downloaded 01 Mar 2001 to 148.6.169.65. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html 4438 J. Appl. Phys., Vol. 85, No. 8, 15 April 1999 Charlton et al. positive contribution to the magnetoresistance from both the GMR and AMR. CONCLUSIONS The magnetic anisotropy in Co/Re (101¯0) multilayers causes the MOKE hysteresis loop to have a plateau near H 0, and either subtracts from or adds to the magnetoresis- tance depending on the direction of the c axis with respect to I. This behavior is a result of the gradual SF transition de- duced from neutron diffraction measurements. To com- pletely understand this, more detailed measurements are un- derway to determine the role of the surface magnetization and the sensitivity of the SF transition to the angle between the c axis and H. Only then will it be possible to maximize the d( / )/dH for this kind of anisotropic system ACKNOWLEDGMENTS FIG. 4. Magnetoresistance measurements preformed at 10 K using a four contact technique for the Co 17 Å /Re 8 Å sample in the H c, H I and The authors thank J. McChesney for performing some of H c, H I configurations. I denotes the direction of the current. the MOKE measurements. This work was supported by US- DOE, BES-MS Contract No. 31-109-ENG-38 at ANL, and Below 2 kOe, the spins rotate in a canted arrangement while NSF CAREER Award No. DMR-9734051 at WVU. the angle between them decreases until at 2 kOe no AF com- ponent parallel to the field is observed. Above 2 kOe this 1 M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dan, F. Petroff, P. angle decreases until the spins are parallel to each other at Eithenne, G. Chreuzet, A. Friedrich, and J. Chazelas, Phys. Rev. Lett. 62, 2472 1988 . high field. One reason why a smooth rotation is observed, 2 For a review, see A. Fert, P. Gru¨nberg, A. Barthe´le´my, F. Pertroff, and W. instead of a first-order SF transition like in traditional anti- Zinn, J. Magn. Magn. Mater. 140­144, 1 1995 . ferromagnets, is that a surface SF transition occurs and then 3 P. Gu¨nberg, R. Shreiber, Y. Pang, M. Brodsky, and H. Sowers, Phys. Rev. propagates layer by layer toward the center of the sample as Lett. 57, 2442 1986 . 4 S. Chikazumi, Physics of Magnetism Wiley, New York, 1964 , pp. 419­ the field is increased.10 Other possibilities are a small mis- 421. alignment of the sample's c axis13 with H and the interface 5 J. B. Hayter, in Neutron Diffraction, edited by H. Dachs Springer, New disorder causing a distribution of the antiferromagnetic cou- York, 1978 , pp. 47­48. 6 pling strengths throughout the sample. P. P. Freitas, L. V. Melo, I. Trindade, M. From, J. Ferreira, and P. Mon- teiro, Phys. Rev. B 45, 2495 1992 ; L. V. Melo, I. Trindade, M. From, P. The magnetotransport data shown in Fig. 4 agree well P. Freitas, N. Teixerira, M. F. da Silva, and J. C. Soares, J. Appl. Phys. 70, with the neutron diffraction measurements. In the H c, H I 7370 1991 . configuration, where I is the direction of the current, the 7 T. Charlton, J. McChesney, D. Lederman, F. Zhang, J. Z. Hilt, and M. J. resistivity increases as the field is lowered from saturation. Pechan unpublished . 8 B. Vidal and P. Vincent, Appl. Opt. 23, 1794 1984 . This can be associated with the increase in the AF alignment 9 E. E. Fullerton, M. J. Conover, J. E. Mattson, C. H. Sowers, and S. D. observed via neutron diffraction, resulting in the GMR ef- Bader, Phys. Rev. B 48, 15755 1993 . 10 fect. As H is lowered from 2 kOe to zero, the magnetoresis- R. W. Wang, D. L. Mills, E. E. Fullerton, J. E. Mattson, and S. D. Bader, Phys. Rev. Lett. 72, 920 1994 . tance dips because of the AMR effect, caused by the rotation 11 Th. Zeidler, K. Theis-Bro¨hl, and H. Zabel, J. Magn. Magn. Mater. 187, 1 of M from being to being to I. In the H c,H I configu- 1998 . ration, the spins start out ferromagnetically aligned and 12 I at S. Adenwalla, G. P. Felcher, E. E. Fullerton, and S. D. Bader, Phys. Rev. saturation, and then gradually become antiferromagnetically B 53, 2474 1996 . 13 S. Foner, in Magnetism, edited by G. T. Rado and H. Suhl Academic, aligned and I as H approaches zero. In this case, there is a New York, 1963 , Vol. 1, pp. 388­389. Downloaded 01 Mar 2001 to 148.6.169.65. Redistribution subject to AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html