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Experimental Realization of Spin-1/2 Triangular-Lattice Heisenberg Antiferromagnet

H. Tanaka and K. Kindo

It is one of the main subjects of condensed matter physics that revealing the ground state of a frustrated quantum magnet. A two-dimensional spin-1/2 Triangular-Lattice Heisenberg Antiferromagnet (TLHAF), which is one of the well-known frustrated quantum magnets, has an ordered ground state of the 120° spin structure. Although the ground state at zero-field is the same as that for the classical spin, the ground state of a small spin TLHAF in a magnetic field cannot be defined with the classical spin model. For example, when an up-up-down spin state in the TLHAF is stabilized in a magnetic field, one-third magnetization plateau must appear in the magnetization process as a quantum effect.

Fig. 1. (a) The magnetization process (M) and the derivative magnetization (dM/dH) for Ba3CoSb2O9 at 1.3 K. The saturation field (HS) is 31.9 T. The increase in magnetization above HS arises from the large temperature-independent Van Vleck paramagnetism. (b) The magnetization curves corrected for the Van Vleck paramagnetism. The quantum magnetization plateau is clearly observed at MS/3. Thick dashed and solid lines denote fits by the higher order coupled cluster method (CCM) and exact diagonalization (ED) for a 39-site rhombic cluster, respectively. Thins dotted lines show the classical magnetization curves.

Here, we report the result of magnetization on Ba3CoSb2O9 and present verification that this substance is almost the ideal spin-1/2 TLHAF [1]. Ba3CoSb2O9 crystallizes in a highly symmetric hexagonal structure P63/mmc, and is composed of a single CoO6 octahedron and a face-sharing Sb2O9 double octahedron. Magnetic Co2+ ions frame regular triangular lattice layers on the ab plane and are separated by the non-magnetic layer of the Sb2O9 double octahedron and Ba2+ ions. Thus, Ba3CoSb2O9 is expected to be categorized as a good two-dimensional magnet.

Figure 1 (a) shows the magnetization process (M) and the derivative magnetization (dM/dH) for Ba3CoSb2O9 measured at 1.3 K. The magnetic field was applied up to 53 T and the whole magnetization process on Ba3CoSb2O9 was observed. The saturation of the Co2+ spin occurs at the saturation field (HS) of 31.9 T. The increase in magnetization above HS arises from the large temperature-independent Van Vleck paramagnetism characteristic of Co2+ in the octahedral environment [2]. Figure 1 (b) shows the magnetization curves corrected for the Van Vleck paramagnetism. The quantum magnetization plateau is clearly observed at MS/3. Thick dashed and solid lines denote fits by the higher order coupled cluster method [3] and exact diagonalization for a 39-site rhombic cluster [4], respectively. Both theoretical curves include the one-third magnetization plateau but the classical magnetization curves (thin dotted lines) show no one-third magnetization plateau. Although the experimental magnetization curve is smeared around the critical fields due to the finite temperature effect and the small anisotropies of the g factor and the interaction, the agreement between the experimental and theoretical results is excellent. This work verifies recent theory on the magnetization process for the spin-1/2 TLHAF


References
  • [1] Y. Shirata, H. Tanaka, A. Matsuo, and K. Kindo, Phys. Rev. Lett. 108, 057205 (2012).
  • [2] M. E. Lines, Phys. Rev. 131, 546 (1963).
  • [3] D. J. J. Farnell, R. Zinke, J. Schulenburg, and J. Richter, J. Phys. Condens.: Matter 21, 406002 (2009).
  • [4] T. Sakai and H. Nakano, Phys. Rev. B 83, 100405(R) (2011).
Authors
  • Y. Shirataa, H. Tanakaa, A. Matsuo, and K. Kindo
  • aTokyo Institute of Technolog