Unusual ferrimagnetism in CaFe2O4

TL;DR

This study uses resonant x-ray techniques to demonstrate that CaFe2O4 exhibits ferrimagnetism caused by crystallographic inequivalence of Fe3+ sites, revealing both intrinsic and extrinsic origins of its weak magnetization.

Contribution

It provides the first direct experimental evidence that crystallographic inequivalence alone can induce ferrimagnetism in CaFe2O4.

Findings

CaFe2O4 has one antiferromagnetic and one ferrimagnetic phase.

Ferrimagnetism arises from inequivalent Fe3+ sites with similar local environments.

Weak magnetization has intrinsic and extrinsic origins, with different coercive fields.

Abstract

Incomplete cancellation of collinear antiparallel spins gives rise to ferrimagnetism. Even if the oppositely polarized spins are owing to the equal number of a single magnetic element having the same valence state, in principle, a ferrimagnetic state can still arise from the crystallographic inequivalence of the host ions. However, experimental identification of such a state as ferrimagnetic is not straightforward because of the tiny magnitude expected for M and the requirement for a sophisticated technique to differentiate similar magnetic sites. We report a synchrotron-based resonant x-ray investigation at the Fe L2,3 edges on an epitaxial film of CaFe2O4, which exhibits two magnetic phases with similar energies. We find that while one phase of CaFe2O4 is antiferromagnetic, the other one is ferrimagnetic with an antiparallel arrangement of an equal number of spins between two distinct crystallographic sites with very similar local coordination environments. Our results further indicate two distinct origins of an overall minute M; one is intrinsic, from distinct Fe3+ sites, and the other one is extrinsic, arising from defective Fe2+ likely forming weakly-coupled ferrimagnetic clusters. These two origins are uncorrelated and have very different coercive fields. Hence, this work provides a direct experimental demonstration of ferrimagnetism solely due to crystallographic inequivalence of the Fe3+ as the origin of the weak M of CaFe2O4.