Origin of Perpendicular Magnetic Anisotropy in Co$_x$Fe$_{3-x}$O$_{4+\delta}$ Thin Films Studied by X-ray Magnetic Circular and Linear Dichroisms

TL;DR

This study uncovers that perpendicular magnetic anisotropy in CoFe2O4 thin films arises from enhanced out-of-plane orbital magnetic moments in Co2+ ions, driven by strain-induced charge distribution changes, elucidated through element-specific X-ray dichroism techniques.

Contribution

It provides a microscopic understanding of the origin of perpendicular magnetic anisotropy in Co-ferrite thin films using element-specific magnetic dichroism measurements.

Findings

PMA in low Co content films is linked to large perpendicular orbital moments in Co2+.

XMLD indicates oblate charge distribution in Co2+ sites under tensile strain.

Strain-induced charge redistribution enhances out-of-plane orbital magnetic moments.

Abstract

We investigate the element-specific spin and orbital states and their roles on magnetic anisotropy in the Co-ferrite (Co$_x$Fe$_{3-x}$O$_{4+\delta}$ (001)) thin films which exhibit perpendicular magnetic anisotropy (PMA). The origin of PMA in the low $x$ region ($x$ $<$ 1) can be mainly explained by the large perpendicular orbital magnetic moments in the Co$^{2+}$ (3$d^7$) states detected by X-ray magnetic circular and linear dichroisms (XMCD/XMLD). The XMLD for a PMA film ($x=0.2$) with square hysteresis curve shows the oblate charge distribution in the Co$^{2+}$ site, which is consistent with the change in local nearest neighbor distance in Co detected by extended X-ray absorption fine structure analysis. Our finding reveals that the microscopic origin of PMA in Co-ferrite comes from the enhanced orbital magnetic moments along out-of-plane [001] direction through in-plane charge distribution by tensile strain, which adds the material functionalities in spinel ferrite thin films from the viewpoint of strain and orbital magnetic moments.