High-press. NMR　　　　　 Frustration　　　　 Mott-Insulator 　　　 Spin liquid　　　　 Orbital order

■ Orbital-induced molecule formation under frustration

In presence of the orbital degrees of freedom, geometric frustration can be relieved by orbital ordering. Still, long-range magnetic order and spin singlet formation compete with each other. When the lattice can easily couple to the spin degrees of freedom, the spin-dimerized state is favored. The three-dimensional network of dimerization looks like a ‘molecule’ formation of transition metals.
In spinel oxides, a variety of molecules have been found, e.g., octamer, heptamer, spiral, etc. NMR observed the heptamer in AlV2O4, coexisting with the incommensurate magnetic order [11]. In triangular lattice, the molecule can be both trimer and tetramer, as proposed theoretically. The orbital ordering texture was observed by the electric field gradient around the V nucleus in BaV10O15 [12]. We found the microscopic evidence for the spin-singlet V trimer with the direct d-d covalent bonds and the magnetic tetramer.

■ Anisotropic slow dynamics in low-dimensional frustrated system

Ising triangular lattice antiferromagnets provide a textbook example of geometric frustration. This is realized in Ca3Co2O6 having the ferromagnetic spin S = 2 chain coupled antiferromagnetically on triangular lattice. A phase transition at 25 K and a field-induced stepwise transition have not been understood. We have microscopically observed a ferromagnetic-ferromagnetic transition by 59Co NMR on a single crystal. The development of spin correlation is highly anisotropic and has the critical exponents consistent with the Ising model [13].

References

  • [11] Y. Shimizu, M. Takana, M. Itoh, T. Katsufuji
    "Spin-singlet formation in the geometrically frustrated spinel oxide AlV2O4: 51V and 27Al NMR measurements"
    Physical Review B 78, 14423 (2008).

  • [12] J. Miyazaki, K. Matsudaira, Y. Shimizu, M. Itoh, Y. Nagamine, S. Mori, J. E. Kim, K. Kato, M. Takata, and T. Katsufuji,
    Formation of a three-dimensional network of V trimers in A2V13O22 (A =Ba, Sr),
    Phys. Rev. Lett. 104, 207201 (2010).


  • [13] Y. Shimizu, M. Horibe, H. Nanba, T. Takami, and M. Itoh,
    Anisotropic spin dynamics in the frustrated chain Ca3Co2O6 detected by single-crystal 59Co NMR,
    Physical Review B 82, 094430 (2010).


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