Field-Induced Phenomena and Its Frustration Effect in Quantum Spin System

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Various new quantum spin systems have been developed and investigated in the light of frustration effect, both experimentally and theoretically. The most remarkable achievement is that the origin of magnetization plateau and the magnetic superstructure at 1/8 plateau in the orthogonal dimer system, SrCu_2(BO_3)_2 with the Shastry-Sutherland lattice have been elucidated from various experiments and theoretical analyses. This phenomenon of quantized magnetization plateau can be regarded as a Wigner crystallization of triplets (magnons). We also found phenomena of magnetization plateaus in other various quantum spin systems, for examples, 1/4 and 3/4 plateaus in NH_4CnCl_3, 1/3 and 2/3 plateaus in Cs_2CuBr_4, 1/2 plateau in Ba_3Mn_2O_8, 1/4 plateau in BIP-TENO, 1/3 and 2/3 plateaus in BIPNNBNO. Any magnetic frustration can be considered to play an important role for these phenomena of magnetization plateau. The pressure-and impurity-induced antiferromagnetic transitions have been observed in spin-dimer system TlCuCl3 and the field-induced antiferromagnetic transition, so-called Bose-Einstein condensation of magnons, has been found in F_2PNNNO as well as 1/2 magnetization plateau. In the three dimensional frustrated spinel systems, a spin-1/2 (d^1) compound, MgTi_3O_4 was successfully synthesized and a Peierls like transition has been found. The orbital degree of freedom plays an important role not only in the transition of. MgTi_2O_4 but also in the structural transition of spin-1 ZnV_2O_4. These results present how to lift the frustration in the magnetically frustrated systems. The Cr compounds such as ZnCr_2O_4, CdCr_2O_4 and HgCr_2O_4 without orbital degree of freedom have been found to exhibit metamagnetic transitions accompanied by the structural changes, showing the magnetization plateau.