Zero and finite temperature mean field study of magnetic field induced electric polarization in Ba2CoGe2O7

TL;DR

This study uses mean field theory to analyze magnetic field-induced electric polarization in Ba2CoGe2O7, revealing multiple magnetic phases and their properties at finite temperatures, aligning with experimental observations.

Contribution

It introduces a comprehensive mean field model incorporating Dzyaloshinskii-Moriya interaction and additional polarization coupling, explaining complex magnetic and electric behaviors in Ba2CoGe2O7.

Findings

Identification of three magnetic phases under varying magnetic fields.

Reproduction of experimental magnetization and polarization data.

Discovery of a canted ferrimagnetic phase with finite staggered polarization.

Abstract

We investigate the spin-induced polarization in the multiferroic compound Ba2CuGe2O7 using variarional and finite temperature mean field approaches. The compound is described by a spin-3/2 Heisenberg model extended with easy plane anisotropy and Dzyaloshinskii-Moriya (DM) interaction. Applying magnetic field parallel to the [110] axis, three phases can be distinguished: (i) At high magnetic field we find a partially magnetized phase with spins parallel to the fields and uniform polarization; (ii) Below a critical field the ground state is a twofold degenerate canted antiferromagnet, where the degeneracy can be lifted by a finite DM interaction; (iii) At zero field a U(1) symmetry breaking phase takes place, exhibiting a Goldstone-mode. To reproduce the magnetization and polarization measurements reported in Murakawa et al. [Phys. Rev. Lett. 105, 137202 (2010)], we introduce an additional term in the Hamiltonian that couples the polarizations on neighboring tetrahedra. This results in the appearance of a canted ferrimagnetic phase for h < 1 T, characterized by a finite staggered polarization, as well as by a finite magnetization along the [-1,1,0] axis that leads to torque anomalies.