Two-step transition in a magnetoelectric ferrimagnet Cu2OSeO3

Abstract

We report a detailed single-crystal investigation of a magnetoelectric ferrimagnet ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ using dc magnetization and ac susceptibility along the three principal directions [100], [110], and [111]. We have observed that in small magnetic fields two magnetic transitions occur, one at $T_c=57$ K and the second one at $T_N=58$ K. At $T_c$ the nonlinear susceptibility reveals the emergence of the ferromagnetic component and below $T_c$ the magnetization measurements show the splitting between field-cooled and zero-field-cooled regimes. Above 1000 Oe the magnetization saturates and the system is in a single domain state. The temperature dependence of the saturation below $T_c$ can be well described by $\mu \left(T\right)=\mu \left(0\right){[1-{(T/T_c)}^2]}^{\beta }$, with $\mu \left(0\right)=0.56{\mu }_B$/Cu, corresponding to the 3-up-1-down configuration. The dielectric constant measured on a thin single crystal shows a systematic deviation below the transition, indicating an intrinsic magnetoelectric effect. DOI: http://dx.doi.org/10.1103/PhysRevB.85.224402 ©2012 American Physical Society