Neutron scattering from local magnetoelectric multipoles: a combined theoretical, computational, and experimental perspective

TL;DR

This paper explores how local magnetoelectric multipoles influence magnetic neutron scattering, combining theoretical, computational, and experimental approaches to detect and analyze these effects in materials like CuO.

Contribution

It introduces a comprehensive formalism for neutron scattering involving magnetoelectric multipoles and demonstrates how to identify their presence experimentally.

Findings

Magnetoelectric multipoles contribute significantly to neutron scattering signals.

First-principles calculations can predict magnetoelectric multipole effects in materials.

Experimental neutron polarimetry can detect magnetoelectric multipole order in CuO.

Abstract

We address magnetic neutron scattering in the presence of local non-centrosymmetric asymmetries of the magnetization density. Such inversion-symmetry breaking, combined with the absence of time-reversal symmetry, can be described in terms of magnetoelectric multipoles which form the second term after the magnetic dipole in the multipole expansion of the magnetization density. We provide a pedagogical review of the theoretical formalism of magnetic neutron diffraction in terms of the multipole expansion of the scattering cross-section. In particular, we show how to compute the contribution of magnetoelectric multipoles to the scattering amplitude starting from ab initio calculations. We also provide general guidelines on how to experimentally detect long-ranged order of magnetoelectric multipoles using either unpolarized or polarized neutron scattering. As a case study, we search for the presence of magnetoelectric multipoles in CuO by comparing theoretical first-principle predictions with experimental spherical neutron polarimetry measurements.