Partially disordered antiferromagnetism and multiferroic behavior in a frustrated Ising system CoCl_2-2SC(NH_2)_2

摘要

We investigate partially disordered antiferromagnetism in CoCl_2-2SC(NH_2)_2, in which ab-plane hexagonal layers are staggered along the c axis rather than stacked. A robust 1β state forms in applied magnetic fields in which the spins are locked, varying as a function of neither temperature nor field. By contrast, in zero field and applied fields at higher temperatures, partial antiferromagnetic order occurs, in which free spins are available to create a Curie-like magnetic susceptibility. We report measurements of the crystallographic structure and the specific heat, magnetization, and electric polarization down to T = 50 mK and up to μ_0H = 60 T. The Co~(2+) S = 3/2 spins are Ising-like and form distorted hexagonal layers. The Ising energy scale is well separated from the magnetic exchange, and both energy scales are accessible to the measurements, allowing us to cleanly parametrize them. In transverse fields, a quantum Ising phase transition can be observed at 2 T. Finally, we find that magnetic exchange striction induces changes in the electric polarization up to 3 μC/m~2, and single-ion magnetic anisotropy effects induce a much larger electric polarization change of 300 μC/m~2.