Condensed Matter Physics Seminar: Sophie Weber

Abstract: 
Surfaces of crystalline solids exhibit reduced symmetry and atomic coordination compared to the bulk, enabling emergent phenomena like two-dimensional ferromagnetism in otherwise antiferromagnetic (AFM) materials1-3. Similarly, heterogeneous interfaces can host distinct magnetic properties due to orbital hybridization, magnetic proximity effects, and tunneling. This talk presents two cases where properties of a surface or interface drive counterintuitive magnetic phenomena.
I will first show the case of the (0001)Cr2O3-(111)Pt heterostructure, a prototypical magnetic-heavy metal system where spin-dependent transport in the heavy metal is used to probe magnetism at the interface. I discuss our first principles density functional theory (DFT) calculations indicating that interactions between Cr2O3 and Pt lead to a flipping of the Cr magnetic moments at the interface with respect to their preferred orientation in bulk and vacuum-terminated (0001) Cr2O3.4 Magnetotransport measurements by collaborators support this prediction. I discuss the possible origins of Pt-driven reorientation of interface magnetization in the context of analogous DFT calculations where the heavy metal element is varied.
As a second example, I will talk about the role of surface magnetic moments in magnetic field-based domain selection in antiferromagnets. Antiferromagnets are widely considered to be insensitive to moderate magnetic fields. I will use a combination of basic statistical mechanics arguments and Monte Carlo simulations to argue that this textbook understanding neglects the role of surface magnetic moments. In particular, because of the reduced magnetic exchange of surface atoms5, the pinning of the “loose” surface spins can lead to deterministic selection of a single AFM domain, with opposite domains selected by opposite field ramping directions. Finally, I discuss simulations of reflective circular dichroism (RCD) based on our magnetic configurations from Monte Carlo which are consistent with recently measured experimental RCD field dependence in the van-der Waals antiferromagnet MnBi2Te46, suggesting that domain selection via purely magnetic means may be a general and experimentally accessible phenomenon.
[1] SFW, A. Urru, S. Bhowal and N. A. Spaldin, Phys. Rev. X 14, 021033 (2024).
[2] O. Pylypovskyi, SFW, P. Makushko, I. Veremchuk, N. A. Spaldin, and D. Makarov, Phys. Rev. Lett. 132, 226702 (2024).
[3] SFW, A. Urru and N. A. Spaldin, Phys. Rev. X 15, 021094 (2025).
[4] M. Reher, N. A. Spaldin and SFW, Phys. Rev. Research 6, 033263 (2024).
[5] SFW and N. A. Spaldin, Phys. Rev. Lett. 130, 214439 (2023).
[6] V. Sunko, S. Ahsanullah, V. Jain, SFW, S. Kumaran, JQ Yan, J. Orenstein, D. Ovchinnikov, arXiv:2504.16167 (2025).