Field-driven successive phase transitions in the quasi-two-dimensional frustrated antiferromagnet Ba2CoTeO6 and highly degenerate classical ground states

We report the results of magnetization and specific heat measurements of Ba2CoTeO6 composed of two subsystems A and B, which are magnetically described as an S = 1/2 triangular-lattice Heisenberg-like antiferromagnet and a J(1)-J(2) honeycomb-lattice Ising-like antiferromagnet, respectively. These two subsystems were found to be approximately decoupled. Ba2CoTeO6 undergoes magnetic phase transitions at T-N1 = 12.0 K and T-N2 = 3.0 K, which can be interpreted as the orderings of subsystems B and A, respectively. Subsystem A exhibits a magnetization plateau at one-third of the saturation magnetization for the magnetic field H perpendicular to the c axis owing to the quantum order-by-disorder, whereas for H parallel to c, subsystem B shows three-step metamagnetic transitions with magnetization plateaus at zero, one-third, and one-half of the saturation magnetization. The analysis of the magnetization process for subsystem B shows that the classical ground states at these plateaus are infinitely degenerate within the Ising model.