Reorientational transition of the magnetic anisotropy in Co/Cr„001… superlattices Th. Zeidler, F. Schreiber, and H. Zabel Institut fu ¨r Experimentalphysik/Festko ¨rperphysik, Ruhr-Universita ¨t Bochum, D-44780 Bochum, Germany W. Donner Universita ¨t Wuppertal, Fachbereich 8, D-42097 Wuppertal, Germany N. Metoki Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan Received 23 June 1995 We have measured the magnetic anisotropy of Co/Cr001 superlattices as a function of the Co thickness using magneto-optical methods and a torsion magnetometer. The samples grown by molecular-beam epitaxy exhibit a reorientational transition of the easy axis from in-plane for large Co thicknesses ( t Co  15 Å to out-of-plane for 10 Å t Co  15 Å and back again to the in-plane orientation for t Co  10 Å . We provide evidence that this reorientational transition of the magnetization direction is due to a sign change of the interface anisotropy constant induced by a concomitant structural phase transition of the Co layers from hcp to bcc with decreasing Co layer thickness. I. INTRODUCTION Using advanced growth methods such as molecular-beam epitaxy MBE or seeded epitaxial sputtering interfacial ef- fects in heteroepitaxial systems can be studied with unprec- edented accuracy. One of the important issues in the field of thin magnetic films is the investigation of magnetic surface or interface anisotropies. For a clear understanding of these anisotropies sharp interfaces and well defined structural properties are of paramount importance. Perpendicular magnetic anisotropies induced by the inter- face have been observed for Co/Pd, Co/Pt, and Co/Ru, 1–3 and they are of fundamental interest as well as of highly practical importance in magnetic storage devices. Here we report on the magnetic anisotropy in Co/Cr001 superlat- tices in conjunction with the interface-induced structural phase transition of Co on bcc Cr001. While hcp Co exhibits a perpendicular anisotropy similar to the systems cited above, we have observed a reorientational transition of the easy axis back into the film plane as a result of the interface driven structural phase transition of Co. In this paper we treat the magnetic interface anisotropy of Co/Cr001Refs. 4 – 6 superlattices, while the interlayer exchange coupling proper- ties will be described in a forthcoming publication. II. SAMPLE PREPARATION AND STRUCTURAL PROPERTIES High-quality single-crystal Co/Cr001 superlattices with Co layer thicknesses of 5–50 Å were grown by molecular- beam epitaxy MBE with a base pressure of about p 5 10 -11 mbar. The superlattices were grown on a bilayer buffer system consisting of a 500 Å thick Nb001 layer grown on Al 2 O 3 112 ¯ 0 at 900 °C and on top a 500 Å thick Cr001 layer grown at 450 °C. The growth temperature for the subsequent superlattice was lowered to 300 – 350 °C. All growth temperatures were optimized by x-ray scattering and in situ high-energy electron-diffraction RHEED measure- ments. At lower substrate temperatures we observed an is- land growth mode and at higher temperatures interdiffusion between the adjacent Co and Cr layers occur. The crystal structure of Co/Cr001 superlattices has been described in detail elsewhere. 7–10 Therefore in this section we provide a brief summary of the most important structural properties. The structural properties have been investigated by x-ray and RHEED experiments. It was shown that the c axis of the hcp Co layers is oriented in the film plane with the Co0001 axis oriented parallel to the Cr110 axis. This orientation provides the smallest misfit of about –0.2% and +6% along the Co00.1 and Co11.0 axes, respectively. As a consequence of this rather large misfit the epitaxial strain gives rise to a continuous structural phase transition from hcp to bcc with decreasing Co layer thickness. The ideal bcc structure is, however, never realized even down to t Co =5 Å. The observed out-of-plane d spacing of d =1.3 Å for Co11.0 at t Co = 10 Å is too large to be explained by the Poisson expansion from in-plane strains, while it is too small to be taken as evidence for the existence of a meta- stable bcc phase. Therefore we observe an intermediate structural phase between bcc and hcp in the lower Co- thickness regime. At the same time, the out-of-plane lattice parameter of Cr does not change significantly, indicating that no relaxation processes of the chromium structure takes place with increasing Co thickness. At t Co = 48 Å we ob- serve a Co d spacing of d=1.26 Å close to the bulk value d =1.25 Å . The cubic symmetry of Cr001 causes the ex- istence of two structural domains with the c axes of hcp Co perpendicular to each other and aligned parallel to the Cr110 and Cr11 ¯ 0 axis, respectively. III. EXPERIMENTAL TECHNIQUES A. MOKE measurements The magnetic hysteresis curves of the superlattices were measured by the magneto-optical Kerr effect MOKE. The PHYSICAL REVIEW B 1 FEBRUARY 1996-II VOLUME 53, NUMBER 6 53 0163-1829/96/536/32567/$06.00 3256 © 1996 The American Physical Society