On ferroelectricity of magnets

PhD ceremony: Mr. A. Scaramucci, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen

Thesis: On ferroelectricity of magnets

Promotor(s): prof. M.V. Mostovoy, prof. J. Knoester

Faculty: Mathematics and Natural Sciences

Ferroelectric compounds possess a spontaneous and reversible electric polarization, P.Magnetic materials, on the other hand, display a spontaneous order of magnetic dipoles induced by electron spins. The cross-coupling between P and magnetism, which can potentially lead to novel memory storage devices, is found in magnetoelectric compounds, where magentization can be induced by an appliedelectric field (or vice versa), and in so-called magnetic ferroelectrics, where a spontaneous polarization is induced by spin ordering. Understanding the physics underlying the rich variety of observed phenomena in these compounds is both of fundamental and technological interest.

In this thesis we study model Hamiltonians for compounds in which magnetic ordering and electric polarization are coupled. Using symmetry considerations, the form of microscopic couplings between spins and electric polarization are obtained for different systems. These expressions are used to reveal the mechanism behind the magnetoelectric effect in Cr2O3 and to find the origin of so-called electromagnon excitations in YMn2O5. Furthermore, the theoretical analysis of materials with so-called magnetic spiral states allows us to explain a number of recently observed phenomena, in which electric polarization is controlled by an applied magnetic field. These include the rotation of electric polarization by rotating an applied magnetic field, and the clamping of ferromagnetic and ferroelectric domain walls. Finally, novel effects, such as the high-temperature induction of electric polarization by arrays of ferromagnetic domain walls, are predicted.