We report the results of low-temperature magnetization and specific heat measurements of Sr$_3$CoTa$_2$O$_9$ powder, in which Co$^{ 2+}$ ions with effective spin-1/2 form a uniform triangular lattice in the $ab$ plane. It was found that Sr$_3$CoTa$_2$O$_9$ undergoes successive antiferromagnetic transitions at $T_{ \rm N1}\,{ =}\,0.97~{ \rm K}$ and $T_{ \rm N2}\,{ =}\,0.79~{ \rm K}$ at zero magnetic field. As the magnetic field increases, both $T_{ \rm N1}$ and $T_{ \rm N2}$ decrease monotonically. The obtained magnetic field vs temperature phase diagram together with a sharp magnetization anomaly at a saturation field of ${ \mu_0}H_{ \rm s}\,{ =}\,2.3~{ \rm T}$ indicates that Sr$_3$CoTa$_2$O$_9$ 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.
Categories: cond-mat.str-el