Spontaneous Magnetoelastic Deformation of the Type-II Multiferroic CuBr2

Abstract

Using neutron and X-ray powder diffraction measurements, we have determined the temperature dependence of the crystal structure of the low-dimensional antiferromagnet CuBr2. Below TN ≈ 74 K, CuBr2 orders with an incommensurate antiferromagnetic magnetic structure, which constitutes a type-II improper multiferroic. The analysis of the temperature dependence of the crystal structure parameters reveals spontaneous magnetostriction below TN which expands primarily the monoclinic b - axis and leads to a negative thermal expansion below TN. The monoclinic angle β exhibits a ramp at TN and falls off again below. Thermal expansion measurements up to room temperature on a pressed pellet using a capacitance expansion cell indicate a linear thermal expansion coefficient of 3.52 × 10–5 K–1. Temperature dependent X-ray measurements of the crystal structure parameters up to room temperature reveal a volume thermal expansion coefficient of αV = 9.71(15) × 10–5 K–1. Temperature, magnetic field, and frequency dependent dielectric and pyroelectric polarization measurements on crystals of CuBr2 are presented.