The effect of magnetostatic coupling on spin configurations in ultrathin multilayers

Abstract

(Co/Au)N ultrathin multilayers with perpendicular and in-plane magnetic anisotropy were studied experimentally using a combination of ferromagnetic resonance, magneto-optical magnetometry and microscopy (with both in-plane and out-of-plane magnetization-sensitive longitudinal and polar Kerr effects), and magnetic force microscopy (MFM). Three-dimensional magnetization distributions were reconstructed from micromagnetic simulations complemented by the measured magnetic parameters of the multilayers and observations by Kerr microscopy and MFM. It is shown that, in the reorientation phase transition (RPT) zone – the range of anisotropy characterized by 0 < Q < 1 (the ratio of the anisotropy energy to be gained by magnetization along the easy axis perpendicular to the sample surface, and the magnetostatic energy of a uniformly magnetized layer along the surface normal) – the three-dimensional magnetization distributions consist of alternating pairs of vortices and half-antivortices, both with in-plane magnetized cores and elliptical cross-sections. It is shown that an increase in N leads to a significant increase in the saturation field as well as changes in the hysteresis loop shape, domain structure size, and geometry. We demonstrate by simulations and prove by experimentation that, in multilayers with Q < 1, an increase in N induces the nucleation of out-of-plane magnetized domains, which then triggers magnetostatic interlayer coupling and RPT onset to out-of-plane magnetization states. We report on the observations of large micrometer-sized metastable domains (connected with the vortex cores) with the in-plane magnetization modulated by submicrometer-sized out-of-plane domains with sizes defined mainly by magnetostatics.