Interplay between shape and magnetocrystalline anisotropies in patterned bcc Fe/Co(001) multilayers

Abstract

In this work the interplay between magnetocrystalline and shape anisotropies was studied in submicron size particles made of bcc Fe/Co(001) multilayers of thickness 20 nm and Co content in the range 27--75 at. %. Arrays of circular and elliptical particles with well-defined geometry and lateral sizes in the range 150--550 nm were prepared by electron-beam lithography and ion milling and investigated by magnetization measurements and magnetic force microscopy (MFM). The angular dependence of the magnetization of the initial films demonstrates the change of the easy direction of magnetization (in plane) from [100] to [110] as one goes from pure Fe to Fe/Co multilayers with increasing Co content. The first-order anisotropy constant ${K}_{1}$ is negative for the investigated Fe/Co films and depends linearly on the Co concentration. By extrapolation, the change of sign is obtained at about 23 at. % of Co. The high effective magnetic moment of the multilayers, up to $(2.8\ifmmode\pm\else\textpm\fi{}0.3){\ensuremath{\mu}}_{B}$ per atom for the Fe2/Co6 multilayer film, may be explained by an enhancement of the local moments both on Fe and Co and a spin dependent polarization of the electron gas, due to confinement in the individual (0.3--1.2 nm thick) Fe and Co layers. The domain structure and magnetization reversal processes of circular particles (diameter 550 nm) are governed by the shape anisotropy for the nearly isotropic Fe8/Co3 multilayers and by the high magnetocrystalline anisotropy in Fe2/Co6 multilayers. The MFM investigation of elliptical particles with aspect ratio 1:3 reveals that only in the case with cooperating shape and strong magnetocrystalline anisotropies a stable single domain state is formed in the majority (78%) of the elements. From magnetization measurements we deduce that the elliptical particles are in quasisingle domain states (so called C and S states) during magnetization reversal.