Wing structure in the phase diagram of the Ising ferromagnet URhGe close to its tricritical point investigated by angle-resolved magnetization measurements

High-precision angle-resolved dc magnetization and magnetic torque studies were performed on a single-crystalline sample of URhGe, an orthorhombic Ising ferromagnet with the c axis being the magnetization easy axis, in order to investigate the phase diagram around the ferromagnetic (FM) reorientation transition in a magnetic field near the b axis. We have clearly detected a first-order transition in both the magnetization and the magnetic torque at low temperatures, and determined detailed profiles of the wing structure of the three-dimensional T-Hb-Hc phase diagram, where Hc and Hb denote the field components along the c and the b axes, respectively. The quantum wing critical points are located at μ0Hc∼±1.1 T and μ0Hb∼13.5 T. Two second-order transition lines at the boundaries of the wing planes rapidly tend to approach each other with increasing temperature up to ∼3 K. Just at the zero conjugate field (Hc=0), however, a signature of the first-order transition can still be seen in the field derivative of the magnetization at ∼4 K, indicating that the tricritical point exists in a rather high temperature region above 4 K. This feature of the wing plane structure is consistent with the theoretical expectation that three second-order transition lines merge tangentially at the tricritical point.