Magnetic anisotropy from dipolar interactions in amorphous ferrimagnetic alloys

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Abstract
Classical magnetic dipolar interactions can make significant contributions to magnetic anisotropy in amorphous ferrimagnetic alloys with slight structural anisotropies on either atomic or microstructural scales. Simple expressions have been derived for this magnetic anisotropy in terms of structural anisotropy parameters, p, V, and Δx. For an alloy with atomic‐scale structural anisotropy (pair ordering), p is a measure of alignment of unlike nearest‐neighbor pairs. For an alloy with aligned elongated composition inhomogeneities, V is the volume fraction of inhomogeneities and Δx is the composition difference (atom fraction) between the inhomogeneities and their surroundings. Accounting for magnetic anisotropy observed experimentally in bias‐sputtered GdCo‐based alloy films solely in terms of pair ordering and classical dipolar interactions requires p∼0.015. Accounting for the anisotropy solely in terms of composition inhomogeneities requires Δx∼0.1 for V=0.5. Although such atomic‐scale structural anisotropy will be very difficult to detect with currently available experimental techniques, composition inhomogeneities of this magnitude should be easily detectible in x‐ray, electron, or neutron small‐angle scattering experiments. Results of small‐angle x‐ray scattering experiments on micron‐thick sputtered GdCo‐based films indicate that occurrence and characteristics of microstructural inhomogeneities depend critically on substrate bias. Although intense anisotropic small‐angle scattering is present for nonbiased films, scattering from biased magnetically anisotropic films is weak and isotropic and can be attributed to formation of small Gd‐oxide clusters during film growth, apparently unrelated to magnetic anisotropy. Although no evidence for anisotropic composition inhomogeneities is seen in x‐ray scattering experiments on micron‐thick bias‐sputtered GdCo‐based films, such inhomogeneities may be present in oxygen‐doped evaporated Gd‐Co films and in some annealed Gd‐Co films prepared by evaporation and by zero‐bias sputtering. Magnetic anisotropy in GdCo‐based films probably involves spin‐orbit interactions as well as classical magnetic dipolar interactions.