Magnetic-circular-dichroism microspectroscopy at the spin reorientation transition in Ni„001… films W. Kuch, J. Gilles, and S. S. Kang Max-Planck-Institut fu¨r Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany S. Imada and S. Suga Osaka University, Graduate School of Engineering Science, 1 – 3 Machikaneyama, Toyonaka 560-8531, Japan J. Kirschner Max-Planck-Institut fu¨r Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany ~Received 10 November 1999! The spin reorientation transition in fcc Co/Ni/Cu~001! epitaxial ultrathin films as a function of Co and Ni film thickness is studied by the combination of photoelectron emission microscopy and x-ray magnetic- circular-dichroism spectroscopy at the Ni L 2,3 edge. This microspectroscopic technique allows one to extract local quantitative information about the Ni magnetic properties on a submicrometer scale. Domain images in the thickness range of 1.4–2.6 atomic monolayers ~ML! Co and 11–14 ML Ni show that the spin reorientation occurs as a function of both Co and Ni thicknesses. Increasing the Co thickness or decreasing the Ni thickness leads to a switching of the magnetic easy axis from @ 001# out-of-plane to ^ 110& in-plane directions. A constant effective Ni spin moment similar to the bulk magnetic moment is observed. The Ni orbital to spin moment ratio shows distinctly different values for out-of-plane magnetization (0.08060.005) and in-plane magnetiza- tion (0.05360.005). This is discussed in terms of the connection to the Ni magnetocrystalline anisotropy. The domain density of the perpendicular magnetization increases towards the spin reorientation transition line. I. INTRODUCTION A considerable portion of current research on ultrathin magnetic films is focused on the direction of the easy axis of magnetization. It is determined by the magnetic anisotropy energy ~MAE!, which in epitaxial thin films contains impor- tant thickness-dependent contributions connected to the pres- ence of a surface or interface, or to the elastic strain. The minimum of the sum of the MAE and the magnetostatic demagnetizing energy ~shape anisotropy! defines the easy axis of magnetization. Spin reorientation transitions of the easy axis of magnetization in ultrathin films may occur as a function of film thickness, temperature, or composition. Both thin films with a magnetization in the film plane and perpen- dicular to it have important technological relevance. There- fore measurement and control of the MAE are important technical issues. The MAE is related to the anisotropy of the orbital mag- netic moment, as discussed by Bruno, 1 and later experimen- tally verified. 2–5 The orbital moment should be higher for a direction of magnetization preferred by the MAE. 6–8 This opens the possibility of determining the angular dependence of the MAE in an element-selective way by measuring the orbital magnetic moment by magnetic circular dichroism in soft x-ray absorption ~XMCD!. XMCD probes the spin and orbital asymmetry of the unoccupied part of the band struc- ture just above the Fermi level. 9 Transitions of spin- and orbit-polarized core level electrons into the unoccupied part of the exchange split valence bands are excited by circularly polarized x rays. The dichroism, i.e., the difference in ab- sorption intensity upon reversal of helicity, thereby depends on the projection of the direction of the incoming photons onto the magnetization direction. The intensity of photoemit- ted electrons from a sample surface is most often used as a convenient measure for the absorption. Sum rules have been proposed to deduce quantitative in- formation from XMCD spectra. 10,11 Although there has been some dispute about the applicability of these rules, 12–15 they seem to yield reasonable results for the 3 d transition metals. 14–16 These sum rules allow one to extract numbers for the spin and orbital magnetic moments from a compari- son of the absorption cross section at the L 3 and L 2 edges of transition metals. They enable thus the separate determina- tion of magnetic properties of different elements in the same sample. This element specificity has been used by Du ¨ rr et al. to study element-resolved anisotropies of a stack of three atomic monolayer ~ML! Co on top of 30 ML Ni on Cu~001!. 4 Epitaxial Ni films on Cu~001! show a spin reori- entation transition from an easy axis parallel to the film plane at film thicknesses below ’8 –10 ML to an easy axis per- pendicular to the film plane at thicknesses between ’8 ML and 56–75 ML. 17–25 The perpendicular magnetization is at- tributed to a magnetoelastic contribution to the MAE caused by substrate-induced strain in the epitaxial Ni film. 20–28 The MAE of Co/Cu~001!, on the other hand, favors an in-plane easy axis. 29–32 Although the gross magnetization direction is in the plane of the film in 3 ML Co/30 ML Ni/Cu~001!, it was possible by using XMCD to prove that the Ni film still maintains its perpendicular MAE. 4 The stronger in-plane an- isotropy contribution of the ~thinner! Co film, however, re- directs the magnetization direction of the entire film to the in-plane direction. From this competition of Ni perpendicular MAE, Co in- PHYSICAL REVIEW B 1 AUGUST 2000-II VOLUME 62, NUMBER 6 PRB 62 0163-1829/2000/62~6!/3824~10!/$15.00 3824 ©2000 The American Physical Society