Multiferroic Thermodynamics

TL;DR

This paper investigates the thermodynamic behavior of multiferroic materials, focusing on phase transitions, susceptibilities, and the effects of fluctuations and inhomogeneities, providing theoretical insights into their coupled electric and magnetic properties.

Contribution

It offers a theoretical analysis of multiferroic thermodynamics, including phase transition characteristics, susceptibility temperature dependence, and the impact of fluctuations and inhomogeneities.

Findings

All phase transitions are second order.

The phase boundary T_M(E) follows the Ehrenfest relation.

Magnetoelectric susceptibility is nonzero only below the lower transition temperature.

Abstract

We have studied the thermodynamic properties of a multiferroic that couples ferromagnetic and ferroelectric order. Some of the results are independent of the form of the free energy. We calculate the temperature dependence of the electric, magnetic, and magnetoelectric susceptibilities. The cross susceptibility has a temperature dependence related to the mixed (with respect to E and B) derivatives of the specific heat. The phase transitions are all second order. In particular, the phase boundary T$_M$(E), where T$_M$ is the lower magnetic transition as a function of electric field, is described by the Ehrenfest relation. The magnetoelectric susceptibility is nonzero only below the lower of the two transition temperatures. We study the properties of the specific heat, with and without the inclusion of gaussian fluctuations. The perturbative renormalization group is used to understand the fixed points of the theory, and we include a discussion of the effect inhomogeneities have for this model.