SPIN DYNAMICS OF FRUSTRATED MAGNETIC SYSTEMS

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The studies of the frustrated magnetic systems are one of the current topics and have attracted much attention. In the present project, we have investigated spin dynamics of the triangular lattice antiferromagnets NaTiO_2 and LiNiO_2, and two-dimensional rerromagnetic-antirerromagnetic competing systems K_2Cu_xMn_<1-x>F_4 and K_2Cu_xCo_<1-x>F_4. Both NaTiO_2 and LiNiO_2 are expected having some quantum effects because of their S=1/2. We have found that ESR signals of them are quite different from those of ordinary antiferromagnets in their temperature dependence. The intensity of the EPR absorption line in NaTiO_2 makes a peak around 100K and decreases with further lowering temperature. This indicates that the singlet state is a ground state. LiNiO_2 shows two successive phase transitions at 200K and 20K, which means the partial ordering at 200K. On the other hand, the ferromagnetic-antiferromagnetic competing systems mentioned above show three phases with the change of x, namely, the ferromagnetic, spin-glass, and antiferromagnetic phases. We have clarified the spin structure of the system of the ferromagnetic phase. That is, the macroscopic easy direction of spins is parallel to [110], but Cu-spins around a Co-spin is not collinear. This results in a divergence of the ferromagnetic resonance linewidth.