Surface Magnetization in Antiferromagnets: Classification, example materials, and relation to magnetoelectric responsesSophie F. Weber*

Theoretical arguments [1,2] and experimental measurements [3,4] have definitively shown that antiferromagnets (AFMs) with particular bulk symmetries posses a finite magnetic dipole moment per unit area, referred to as ``surface magnetization", on certain surfaces. Surface magnetization greatly enhances the utility of spintronics-based devices that use bulk AFM domains as logical bits. Surface magnetization is also connected to exchange bias, that is, the pinning of magnetization in a ferromagnet via an interfaced AFM. However, a universal description of surface magnetization has been lacking. 

In this talk, I will discuss our recent results [5] in which we use symmetry analysis and ab-initio density functional theory to determine and characterize surface magnetization in AFM materials. We first introduce a classification system based on whether the surface magnetization is sensitive or robust to roughness, and on whether the surface of interest is magnetically compensated or uncompensated in the bulk magnetic ground state. We then show that each category of surface magnetization can be described in terms of bulk magnetic multipoles, and in terms of corresponding bulk magnetoelectric responses arising from the effective electric field at the surface. We use density functional calculations to show that compensated surfaces of $mathrm{Cr_2O_3}$ and $mathrm{FeF_2}$ develop a finite surface magnetization, in agreement with our predictions based on group theory and the ordering of the bulk multipoles. Our analysis provides a comprehensive basis for understanding AFM surface magnetization, and has important implications for phenomena such as exchange bias. 

[1] A. F. Andreev, JETP Lett. 63, 756 (1996)

[2] K. D. Belashchenko, Phys. Rev. Lett. 105, 147204 (2010)

[3] P. Appel et al, Nano Lett. 19, 1682 (2019)

[4] M. S. Wornle et al., Phys. Rev. B 103, 094426 (2021)

[5] S. F. Weber, A. Urru, S. Bhowal, C. Ederer, and N. A. Spaldin, arXiv:2306.06631(2023)