Phase transitions in BaMnF4

Abstract

BaMnF₄ is a pyroelectric ferromagnet which displays a number of unusual physical characteristics, some of which are unique. It has the only known continuous antiferroelectric phase transition. It has two- and three-dimensional antiferromagnetic ordering temperatures. It is a weak ferromagnet with ferromagnetism caused by the linear magnetoelectric effect-the only case yet known. It exhibits dielectric anomalies at its Neel temperature. At high temperatures its dielectric constant diverges with increasing temperature; a ferroelectric phase transition would occur if the crystals did not melt first. At high temperatures BaMnF₄ is also an anisotropic ionic conductor. The antiferroelectric phase is incommensurate. The incommensurate phase appears unusual in that its translation vector is temperature-independent. Sound velocity measurements made near the antiferroelectric phase transition temperature demonstrate the presence of and characteristics for a relaxational mode; this mode couples strongly to transverse acoustic phonons and is probably the 'phason' predicted theoretically for incommensurate lattices. This article reviews theory and experiment for this unusual material, including neutron, Raman and Brillouin scattering; X-ray, dielectric and conductivity measurements; magnetic resonance and susceptibility studies.