  Here, we report the synthesis and magnetic properties of a new triangular
lattice antiferromagnet Ba$_3$CoTa$_2$O$_9$. The effective spin of Co$^{2+}$ is
found to be $J=1/2$ at low temperatures due to the combined effect of crystal
field and spin-orbit coupling. Ba$_3$CoTa$_2$O$_9$ undergoes two successive
magnetic phase transitions at $T_{N1}\simeq0.70$~K and $T_{N2}\simeq0.57$~K in
zero applied field, which is typical for triangular antiferromagnets with the
easy-axis magnetic anisotropy. With increasing field, the transition anomalies
are found to shift toward low temperatures, confirming the antiferromagnetic
nature of the transitions. At higher fields, the transition peaks in the heat
capacity data disappear and give way to a broad maximum, which can be ascribed
to a Schottky anomaly due to the Zeeman splitting of spin levels. The $H-T$
phase diagram of the compound shows three distinct phases. The possible nature
of these phases is discussed.

Materials Science