Agenda

Résumé : Most magnetic materials, phenomena and devices are well described in terms of their constituent magnetic dipoles. There is mounting evidence, however, that higher-order magnetic multipoles can lead to intriguing magnetic behaviors, which are often attributed to « hidden order » since they are difficult to characterize with conventional probes. In this talk I will focus on the existence and relevance of the so-called magnetoelectric multipoles, which form the next-order term, after the magnetic dipole, in the multipolar expansion of the energy of a magnetization density in a magnetic field. I will describe how magnetoelectric multipoles underlie multiferroic behavior and dominate the magnetic response to applied electric fields, then discuss signatures of hidden magnetoelectric multipolar order and possibilities for its direct measurement. I will argue that all is not lost if your material lacks magnetoelectric multipoles, and that hidden magnetic octupoles and even triakontadipoles also cause fascinating physics, including the currently rather fashionable « altermagnetism ». Finally, I will show that ferroic ordering of these higher-order magnetic multipoles results in a magnetization at the surface of a sample, even in materials with no net magnetization in their bulk and with apparently compensated surface dipoles.