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MA: Fachverband Magnetismus

MA 21: Phd Focus Session: Altermagnets: Foundations and Experimental Evidence

MA 21.7: Invited Talk

Wednesday, March 20, 2024, 11:45–12:15, H 1058

Is my altermagnet ferromagneto-octupolar or ferromagneto-triakontadipolar (and does it matter)? — •Nicola Spaldin — Materials Theory, ETH Zurich, Switzerland

The non-relativistic spin splitting characteristic of altermagnets is usually understood in terms of local antiferromagnetically ordered spin magnetic dipole moments and their associated symmetries. Sometimes it can be helpful, however, to work with a ferroic ordering of local entities, all of which have the same size and orientation, rather than an antiferroic arrangement. In particular, ferroic orders often carry a readily identifiable associated macroscopic thermodynamic measurable quantity, such as the magnetization in ferromagnets or the polarization in ferroelectrics. In this talk I will show that d-wave altermagnetism results from the ferroic ordering of local non-relativistic magnetic octupoles. Using MnF₂ as an example, we’ll see that this ferromagneto-octupolarization provides a convenient framework for understanding or predicting properties such as time-reversal symmetry breaking, piezomagnetism, neutron-scattering asymmetry, surface magnetization and second-order magnetoelectric response, as well as the usual non-relativistic spin splitting. In g-wave altermagnets, the corresponding ferroically ordered quantity is the magnetic triakontadipole, which gives us yet more interesting physics as well as a spectacularly good name.