Experimental Realization of Spin-1/2 Triangular-Lattice Heisenberg   Antiferromagnet
  We report the results of magnetization and specific heat measurements on Ba$_3$CoSb$_2$O$_9$, in which the magnetic Co$^{2+}$ ion has a fictitious spin-1/2, and show evidence that a spin-1/2 Heisenberg antiferromagnet on a regular triangular lattice is actually realized in Ba$_3$CoSb$_2$O$_9$. We found that the entire magnetization curve including the one-third quantum magnetization plateau is in excellent agreement with theoretical calculations at a quantitative level. 
Yutaka Shirata, Hidekazu Tanaka, Akira Matsuo, and Koichi Kindo
cond-mat.str-el