Magnetic phase diagram in the three-dimensional triangular-lattice antiferromagnet Sr3CoTa2O9 with small easy-axis anisotropy

We report the results of low-temperature magnetization and specific heat measurements of Sr3CoTa2O9 powder, in which Co2+ ions with effective spin-1/2 form a uniform triangular lattice in the ab plane. It was found that Sr3CoTa2O9 undergoes successive antiferromagnetic transitions at TN1 = 0.97 K and TN2 = 0.79 K at zero magnetic field. As the magnetic field increases, both TN1 and TN2 decrease monotonically. The obtained magnetic field vs temperature phase diagram together with a sharp magnetization anomaly at a saturation field of mu 0Hs = 2.3 T indicates that Sr3CoTa2O9 is described as a spin-1/2 three-dimensional triangular-lattice antiferromagnet with a weak easy-axis anisotropy. We discuss the characteristics of the magnetic phase diagram, which approximates the phase diagram for the magnetic field perpendicular to the c axis.