Hidden magnetoelectric multipoles
The concept of magnetic dipolar order -- for example ferro, ferri or antiferromagnetic -- is at the core of our understanding of the behavior of magnetic materials, and is invaluable in explaining their behavior and in selecting and tailoring them for applications. Sometimes, however, magnetic materials surprise us, and behave in ways that can not be captured within our established paradigms, suggesting additional kinds of order that are not yet identified. In this talk I will discuss the 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. First, I will describe how magnetoelectric multipoles underlie multiferroic behavior and in particular how they determine the magnetic response to applied electric fields. Then I will discuss signatures of hidden magnetoelectric multipolar order and possibilities for its direct measurement. I will close with an analysis of how antiferromagnets that contain ordered magnetoelectric multipoles are fundamentally different from those that do not, in particular in their surface magnetic properties.
Nicola Spaldin is the Professor of Materials Theory at ETH Zürich. She is best known for her work developing the class of materials known as multiferroics, which combine simultaneous ferromagnetism and ferroelectricity. She is a passionate science educator, coordinator of her department's curriculum revision project "The Materials Scientist 2030, Who is She?", and holder of the ETH Golden Owl award for excellence in teaching. When not trying to make a room-temperature superconductor she can be found playing her clarinet, or skiing or climbing in the Alps.
