Multiferroics
(T. Michael, Prof. S. Trimper)
Multiferroics are materials that reveal at least two different ferroic orders, e.g., a
ferromagnetic and a ferroelectric phase. Both can be described by order parameters which can be mutually dependent.
In order to understand the phenomenon of multiferroicity of type I, a many-body-model is investigated. The magnetic ordering is driven by the Heisenberg model with anisotropic exchange (DM) interaction. The ferroelectric ordering is carried out by the Ising model in a transverse field.
Further, the ferroelectric and the magnetic subsystem are coupled by a biquadratic term. The temperature dependence of the energy of the ferroelectric excitations in a multiferroic material with noncollinear spin structure is displayed in the figure below. As is visible the energy increases for stronger coupling in the multiferroic phase. Moreover, the inset illustrates absolute change in energy of excitations for two different strengths of anisotropy interaction (Dzyaloszinskii-Morya).
temperature dependence of the ferroelectric excitation energy