Takuya Susuki, Nobuyuki Kurita, Takuya Tanaka, Hiroyuki Nojiri, Akira Matsuo, Koichi Kindo, Hidekazu Tanaka
We have performed high-field magnetization and ESR measurements on Ba$_3$CoSb$_2$O$_9$ single crystals, as a representative two-dimensional spin-1/2 Heisenberg antiferromagnet on a uniform triangular lattice, and have determined all the magnetic parameters. For an applied magnetic field $H$ parallel to the $ab$-plane, the entire magnetization curve including the plateau at one-third of the saturation magnetization ($M_\mathrm{s}$) is in excellent agreement with the results of theoretical calculations except an anomaly near $(3/5)M_\mathrm{s}$, indicative of an additional field-induced transition. However, for $H\,{\parallel}\,c$, the magnetization curve exhibits a cusp near $M_\mathrm{s}/3$. Paramagnetic resonance signals above the N\'eel temperature indicate the nearly isotropic $g$-factor. A detailed analysis of the collective ESR modes observed in the ordered state, combined with the magnetization process, provides evidence of the weak easy-plane anisotropy.
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http://arxiv.org/abs/1303.5271
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