Determination of spin chirality using x-ray magnetic circular dichroism

TL;DR

This paper reveals how x-ray magnetic circular dichroism (XMCD) can detect negative spin chirality in kagome lattices with zero net spin moment, through symmetry rules and anisotropic signals.

Contribution

It introduces a new symmetry-based rule for interpreting XMCD signals in complex magnetic systems, highlighting the role of local anisotropy and geometry.

Findings

Non-zero XMCD signals can occur in zero net spin moment systems due to symmetry.

The angular dependence of XMCD is influenced by atomic orientation, not just spin direction.

Sum rules relate XMCD signals to orbital and dipole moments, excluding isotropic spin contributions.

Abstract

A 3-fold symmetric kagome lattice that has negative spin chirality can give a non-zero x-ray magnetic circular dichroism (XMCD) signal, despite that the total spin moment amounts to zero. This is explained by a hitherto unnoticed rule for the rotational symmetry invariance of the XMCD signal. A necessary condition is the existence of an anisotropic XMCD signal for the local magnetic atom, which can arise from a spin anisotropy either in the ground state or the final state. The angular dependence of the XMCD as a function of the beam direction has an unusual behavior. The maximum dichroism is not aligned along the spin direction, but depends on the relative orientation of the spin with respect to the atomic orientation. Therefore, different geometries can result in the same angular dependence, and the spin direction can only be determined if the atomic orientation is known. The consequences for the x-ray magneto-optical sum rules are given. The integrated XMCD signals are proportional to the anisotropy in the orbital moment and the magnetic dipole term, where the isotropic spin moment drops out.