NEW IMPURITY ERRECTS IN ONE-DIMENSIONAL HEISENBERG ANTIFERROMANETS WITH A SPIN-SINGLET GROUND STATE

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NMR studies were performed to clarify magnetic properties of magnets with a spin-singlet ground state, particularly, impurity effects in one-dimensional Heisenberg antiferromagnets. Our research results are summarized in the following.1. NMR experiments on the Zn-doped spin-Peicris system Cu_<1-x>Zn_xGeO_3 were performed to study impurity effects in one-dimensional Heisenberg antiferromagnets. We compared experimental results of the Cu nuclear spin-lattice relaxation rate and the NMR line width with recent theoretical results and discussed the effects of the antiferromagnetic correlation on the spin dynamics in the Zn doped system.2. Magnetic properties of LaCoO_3 and RCoO_3 (R= Pr, Nd, Sm and Eu) with a singlet ground state were investigated. We found that LaCoO_3 undergoes the spin-state transition from the low-spin state to the intermediate-spin state at about 100 K, whereas the low-spin state remains in RCoO_3 up to about 500 K where the metal-insulator transition takes place. This finding shows no relation between the spin state and the metal-insulator transition in these cobalt oxides. We also found that La_<2-x>Sr_xCoO_4 undergoes the spin state transition at a Sr concentration.3. Based on V NMR results of vanadium oxides V_<2-y>O_3 and (V_<1-x>Ti_x)_2O_3, we found that a spin-gap behavior which leads to a non-magnetic state appears at low temperatures in the paramagnetic metallic state of these vanadium oxides.4. Magnetic properties of vanadium bronzes A_xV_2O_3 (A=Na, Ca and Ag) with one-dimensional zigzag chains and a ladder were studied. We found that the charge ordering of V_<4+> and V5_<5+> sites in AV_6O_<15> (A_<0.33>V_2O_5) commonly takes places at a low temperature in AV_6O_<15>. The mixed-valence state in the high-temperature region and the gradual charge ordering in the charge-order state are new interesting problems in quantum spin systems with a particular geometrical structure.