Hidden magnetoelectric multipoles and their detection via neutron diffraction

by Dr Andrea Urru (1Department of Physics and Astronomy, Center for Materials Theory, Rutgers University)

Monday 13 Jan 2025, 14:30 → 15:30 Europe/Berlin

PH HS 3 (Physics Department)

Magnetoelectric multipoles represent the first terms beyond the magnetic dipole moment in the expansion of a non-uniform magnetization density. They break both time-reversal and space-inversion symmetries, a fact that links them to the linear magnetoelectric effect [1], whereby an applied electric field induces a net magnetization or, vice versa, an applied magnetic field induces a net polarization. Magnetoelectric multipoles are often significantly smaller than magnetic dipoles: as a consequence, the concurrent presence of a magnetic and, possibly, a magnetoelectric multipolar order makes their detection challenging. Here, we address the possible detection of multipolar hidden orders with neutron diffraction measurements [2]. Neutrons interact with the full electronic magnetization density, however measurements are usually interpreted in the localized magnetic dipole moments picture. Thus, to include contributions from higherorder multipoles, the theoretical formulation of the neutron scattering amplitude needs to be extended accordingly [2, 3].
In this talk, I will briefly review the theory and I will discuss how spherical neutron polarimetry (SNP) can be used to detect magnetoelectric multipoles. As a relevant case study, I will consider CuO in the AF1 phase [4]. I will present SNP experimental data for selected magnetic reflections and discuss how the difference between the xx, yy, and zz entries of the polarization matrix can be explained by magnetoelectric multipoles on the Cu and O sites. I will finally support the experimental findings with theoretical predictions based on ab initio calculations of the magnetoelectric multipoles in CuO [2].

[1] N. A. Spaldin, M. Fechner, E. Bousquet, A. Balatsky, and L. Nordstr¨om, Phys. Rev. B 88, 094429 (2013).
[2] A. Urru, J.-R. Soh, N. Qureshi, A. Stunault, B. Roessli, H. M. Rønnow, and N. A. Spaldin, Phys. Rev. Res. 5, 033147 (2023).
[3] S. W. Lovesey, Journal of Physics: Condensed Matter 26, 356001 (2014).
[4] V. Scagnoli, U. Staub, Y. Bodenthin, R. A. De Souza, M. Garc´ıa-Fern´andez, M. Garganourakis, A. T. Boothroyd, D. Prabhakaran, and S. W. Lovesey, Science 332, 696 (2011).