Higher-order magnetic anisotropy in soft-hard magnetic materials

TL;DR

This study computationally investigates higher-order magnetic anisotropy in a FePt coreshell system, revealing unexpected anisotropy behavior and deviations from classical theories, with implications for magnetic material design.

Contribution

It uncovers the emergence of a significant fourth-order anisotropy constant induced by the coreshell structure and develops an analytic model explaining its origin and scaling behavior.

Findings

K2 anisotropy peaks at core-size ratio R=0.50

K2 scales with (M/Ms)^2.2 below Curie temperature

Deviation from Callen-Callen theory predictions

Abstract

We have computationally studied the properties of higher-order magnetic anisotropy constants in an L10/A1-FePt coreshell system which is characterized by a strong second-order 2-ion Fe-Pt anisotropy component. We show that the coreshell structure induces an unexpected fourth-order anisotropy constant K2 the magnitude of which varies non-monotonically with the core-size ratio R reaching a peak at R = 0.50. Furthermore, we find that K2 scales with the normalized magnetization by (M/Ms)^2.2 at temperatures below the Curie temperature - a remarkable deviation from the established Callen-Callen theory which instead predicts a scaling exponent of 10. We construct an analytic model which demonstrates K2 arises from the canting of the core and shell magnetization, and successfully reproduces and justifies the scaling exponent obtained from numerical simulation.