Anisotropy of exchange stiffness and its effect on the properties of magnets

TL;DR

This study calculates the anisotropic exchange stiffness tensor in hard magnets using a spin-spiral tight-binding method, revealing how anisotropy influences domain wall behavior and magnetization dynamics.

Contribution

It provides the first detailed analysis of exchange stiffness anisotropy in hard magnets and its impact on domain wall properties and magnetic microstructure.

Findings

Exchange stiffness is strongly anisotropic in studied magnets.

Anisotropy influences domain wall surface tension and reorientation.

Anisotropic exchange stiffness enables cell-boundary pinning mechanisms.

Abstract

Using the spin-spiral formulation of the tight-binding linear muffin-tin orbital method, the principal components of the exchange stiffness tensor are calculated for typical hard magnets including tetragonal CoPt-type and hexagonal YCo5 alloys. The exchange stiffness is strongly anisotropic in all studied alloys. This anisotropy makes the domain wall surface tension anisotropic. Competition between this anisotropic surface tension and magnetostatic energy controls the formation and dynamics of nanoscale domain structures in hard magnets. Anisotropic domain wall bending is described in detail from the general point of view and with application to cellular Sm-Co magnets. It is shown that the repulsive cell-boundary pinning mechanism in these magnets is feasible only due to the anisotropic exchange stiffness if suitably oriented initial pinning centers are available. In polytwinned CoPt-type magnets the exchange stiffness anisotropy controls the orientation of macrodomain wall segments. These segments may reorient both statically during microstructural coarsening and dynamically during the macrodomain wall splitting in external field. Reorientation of segments may facilitate their pinning at antiphase boundaries.