Neutron scattering from local magnetoelectric multipoles: A combined theoretical, computational, and experimental perspective
Andrea Urru, Jian-Rui Soh, N. Qureshi, A. Stunault, B. Roessli, H. M. Rønnow, Nicola A. Spaldin
Abstract
We address magnetic neutron scattering in the presence of local noncentrosymmetric 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-range order of magnetoelectric multipoles using either unpolarized or polarized neutron scattering. We apply the formalism to CuO, and we discuss first-principles predictions and experimental spherical neutron polarimetry measurements that are consistent with the presence of magnetoelectric multipoles in CuO.