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

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 1T , characterized by a finite staggered polarization, as well as by a finite magnetization along the [1, 1, 0] axis that leads to torque anomalies.

Evolution of two-dimensional antiferromagnetism with temperature and magnetic field in multiferroicBa2CoGe2O7

“…[27]). This field value is much smaller than the critical field of abut 1 T where the field induced electric polarization disappears [1,11], supporting the presence of an antiferromagnetic polarization-polarization coupling present in the spin Hamiltonian [28].…”

Direct observation of spin-quadrupolar excitations in Sr2CoGe2O7 by high-field electron spin resonance

“…The symmetry properties of the crystal reflected by the local environment of the magnetic ions determine the explicit form of tensor c α βγ resulting in the following spin-induced polarization characteristic for theåkermanite family [11,18,19]:…”

“…Therefore we consider the following 2D spin Hamiltonian as a minimal microscopic model for SCGO [15,18],…”

“…Without the J pz term the Hamiltonian represents an easy plane antiferromagnet with a Goldstone mode (∆ 1 = 0). A finite J pz was introduced for Ba 2 CoGe 2 O 7 to explain the vanishing of electric polarization in zero field [18] and the absence of Goldstone modes in the neutron scattering study [19]. At zero magnetic field, the electric polarization is also zero in SCGO (Fig.…”

“…Of course, including the polarization-polarization term would give a better agreement with the experimentally measured polarization and would reproduce the drop of P z to zero at small field. [18] (a) For fields lying in the xy-plane the ground state energy has the following form…”