ルテニウム系ペロブスカイト型酸化物の異常な遍歴電子物性

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Recently, the 4d transition metal (especially, Ru) oxide systems with perovskite type structure have been extensively investigated. Among them, in this research work, we focus on the Sr_<1-x>Ca_xRuO_3 system, which shows metallic conductivity. The purpose of this work is to elucidate the mechanism of magnetic properties of this metallic system from the standing point of spin fluctuations. In order to get systematic and microscopic understandings we mainly conducted the nuclear magnetic resonances (NMR) of Ru and Oxygen. Nuclei, former of which belongs to the magnetic atom itself and the latter locates in the chemical bonds between Ru atoms. The specific heat and high-field magnetization have also been measured and analyzed from a view point of itinerant magnetism.First, from the results of the high-field magnetization measurements up to 40 T, a typical property of itinerant system was observed : the magnetization does not saturate even at 40 T for all the sample of this system. The res … More ults of the specific heat experiments revealed characteristic property of strongly correlated electron system : the electronic specific heat coefficient gamma is largely enhanced by exchange interaction and increases with increasing x from 0, and takes a maximum value of about 100 mJ/K^2mol at x=0.8. These values of gamma and their temperature variations were found to be understood by the self-consistent renormalization (SCR) theory of spin fluctuations for itinerant-electron ferromagnetic system. As a result of their analyses, the spin fluctuation parameters T_0 and TA can be obtained for every x. Here, T_0 is the parameter indicating the energy width of spin-fluctuation spectrum, and T_A the parameter which gives the wave number dependence of spin-fluctuation spectrum.From 170 NMR the spin-lattice relaxation rates 1/T_1 for x=l .0 and 0.6 were found to be well understood by the SCR.However, 1/T_1 was not explained by the SCR theory, leading to that SrRuO_3 locates in the intermediate regime between weak itinerant and localized moment ferromagnetic systems, which is consistent with the discovery of the invar effect in this compound. Furthermore, these results indicate that CaRuO_3, which loses the long-range magnetic order, can be classified not as the antiferromagnetic compound but as the nearly ferromagnetic compound. Therefore, we conclude every result can be understood as itinerant ferromagnetic properties and there is evidence for the existence of antiferromagnetic interaction. Ru NMR is now in progress and can be expected to give us a direct information of magnetic atom itself. Less