Different Temperature Dependencies of Magnetic Interface and Volume Anisotropies in Gd / W(110)
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ftl Experimentalphysik, FU Berlin, Arnimallee 14, D-14195 Berlin, Germany
Abstract The magnetic anisotropy of epitaxial Gd(0001) films on W(1 10) is determined as a function of temperature (150 to 350 K) and film thickness (9 to 30 monolayers) by in situ ferromagnetic resonance. It is found that the usual analysis interms of a thickness independent part KV and a thickness dependent contribution 2KS/d must be performed at the same reduced temperature t = T/T(d). Kv shows qualitatively the same temperature dependence as the magnetocrystalline anisotropy of bulk Gd. It changes in sign near 0.7 Tc and does not vanish at T,. KS on the other hand decreases linearly from 1.2 meV/atom at 0.6.Tc to zero at Tc. It appears that the intrinsic origin for Kv and Ks is fundamentally different. The vanishing of Ks at T, indicates that two-ion anisotropy (spin-spin interaction) is dominating the interface anisotropy. The non- zero KV(TŽ_Tc) is likely due to a single ion magnetic anisotropy which is known for bulk Gd.
Introduction The orientation of the magnetization in magnetic ultrathin films and multilayers is determined by the magnetic anisotropy. Phenomenologically, the total anisotropy K&has been found to show a l/d dependence on magnetic layer thickness d [1-5].: Ku =Kv + 2KS/d
(1)
Here, KS is considered as an interface contribution, and Kv is a thickness independent volume coefficient composed of bulk magnetocrystalline anisotropy and a thickness independent magnetoelastic contribution arising from residual strain in the film [6]. The effective l/d dependence of Ku has been associated with NMel's surface anisotropy [1-3] due to the broken symmetry 2 at the interfaces and a thickness dependent relaxation of misfit strain [7,8]. Except for 27nM ,
215 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
which favors in-plane magnetization, 2KS/d and Kv may favor either in-plane or out-of-plane orientation of the magnetization. KV and KS are temperature dependent [3,9]. Interestingly, this fact has often been ignored in discussions of thin film anisotropies [10]. A measurement of KV(T) may be a good identification of the existence of an undisturbed magnetocrystalline volume anisotropy. Also the temperature dependence of the N6el surface anisotropy is unknown. Aside from the phenomenological approach of Eq.(1) the temperature dependence may give new insights on the intrinsic origin of the different coefficients of magnetic anisotropy. Two mechanisms based on spin-orbit interaction have been discussed in the bulk literature [11] to account for the different anisotropic behavior of ferromagnets: the single ion anisotropy and the two-ion model. In the two-ion or pair model the exchange interaction between neighbouring local magnetic moments causes differences in the free energy if the paired moments are aligned along different crystallographic directions. In the single ion model the anisotropy arises from the interaction of the local spin moment with its own non-spherical orbital momentum distribution. Consequ
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