Professor Paolo G. Radaelli OSI

Abstract:

I present a tensorial approach to the description of k/-k-symmetric, time-reversal-odd splitting of electronic bands in magnetic materials, which can be of nonrelativistic origin and was recently given the name of "altermagnetism". I demonstrate that tensors provide a general framework to discuss magnetic symmetry using both spin groups and magnetic point groups, which have often been contrasted in recent literature. I also provide a natural classification of altermagnets in terms of the lowest-order tensorial forms that are permitted in each of the 69 altermagnetic point groups. This approach clarifies the connection between altermagnetism and well-known bulk properties, establishing that the vast majority of altermagnetic materials must also be piezomagnetic and MOKE-active, and provides a rational criterion to search for potential altermagnets among known materials and to test them when the magnetic structure is unknown or ambiguous.

Abstract:

We report a detailed study of the magnetic field switching of ferrotoroidal/multiferroic domains in the Y-type hexaferrite compound Ba_0.5Sr_1.5Mg₂Fe₁₂O₂₂. By combining data from superconducting quantum interference device (SQUID) magnetometry, magnetocurrent measurements, and resonant x-ray scattering experiments, we arrive at a complete description of the deterministic switching, which involves the formation of a temperaturedependent mixed state in low magnetic fields. This mechanism is likely to be shared by other members of the hexaferrite family, and presents a challenge for the development of high-speed read-write memory devices based on these materials.