Successive magnetic phase transitions in alpha-RuCl3: XY-like frustrated magnet on the honeycomb lattice

The layered compound alpha-RuCl3 is composed of a honeycomb lattice of magnetic Ru3+ ions with the 4d(5) electronic state. We have investigated the magnetic properties of a-RuCl3 via magnetization and specific heat measurements using single crystals. It was observed that a-RuCl3 undergoes a structural phase transition at T-t similar or equal to 150 K accompanied by fairly large hysteresis. This structural phase transition is expected to be similar to that observed in closely related CrCl3. The magnetizations and magnetic susceptibilities are strongly anisotropic, which mainly arise from the anisotropic g factors, i.e., gab similar or equal to 2.5 and g(c) similar or equal to 0.4 for magnetic fields parallel and perpendicular to the ab plane, respectively. These g factors and the obtained entropy indicate that the effective spin of Ru3+ is one-half, which results from the low-spin state. Specific heat data show that magnetic ordering occurs in four steps at zero magnetic field. The successive magnetic phase transitions should be ascribed to the competition among exchange interactions. The magnetic phase diagram for H parallel to ab is obtained. We discuss the strongly anisotropic g factors in alpha-RuCl3 and deduce that the exchange interaction is strongly XY-like. alpha-RuCl3 is magnetically described as a three-dimensionally coupled XY-like frustrated magnet on a honeycomb lattice.