Magnetic phase diagram in the three-dimensional triangular-lattice antiferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">Sr</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">CoTa</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>9</mml:mn></mml:msub></mml:math> with small easy-axis anisotropy

We report the results of low-temperature magnetization and specific heat measurements Sr$_3$CoTa$_2$O$_9$ powder, in which Co$^{2+}$ ions with effective spin-1/2 form a uniform triangular lattice $ab$ plane. It was found that undergoes successive antiferromagnetic transitions at $T_{\rm N1}\,{=}\,0.97~{\rm K}$ N2}\,{=}\,0.79~{\rm zero magnetic field. As field increases, both N1}$ N2}$ decrease monotonically. The obtained vs temperature phase diagram together sharp anomaly saturation ${\mu_0}H_{\rm s}\,{=}\,2.3~{\rm T}$ indicates is described as three-dimensional triangular-lattice antiferromagnet weak easy-axis anisotropy. discuss characteristics diagram, approximates for perpendicular to $c$ axis.