Magnetization has been measured for polycrystalline samples of LaCo 1-x Rh x O 3 (0 ≤ x ≤ 0.9) in order to investigate magnetism induced in the solid solution of two nonmagnetic end phases of LaCoO 3 and LaRhO 3 . It is found that a ferromagnetic transition is observed below 15 K in the range of x from 0.1 to 0.4. The effective Bohr magnetic moment evaluated from the temperature dependence of magnetic susceptibility at around room temperature is independent of x for 0 < x < 0.5 (approximately 3 µ B per formula unit), and rapidly decreases above x = 0.5. On the basis of detailed magnetization measurements, the spin state and magnetic ordering of LaCo 1-x Rh x O 3 are discussed.

Magnetization and x-ray diffraction have been measured on polycrystalline samples of LaCo 0.8−y Rh 0.2 Ga y O 3 for 0 y 0.15 in order to understand the spin state of Co 3+ through the Ga 3+ substitution effect. The ferromagnetic order in LaCo 0.8 Rh 0.2 O 3 below 15 K is dramatically suppressed by the Ga 3+ substitution as the ferromagnetic volume fraction is linearly decreased. The normal state susceptibility also systematically decreases with the Ga content from which we find that one Ga 3+ ion reduces 4.6 μ B per formula unit. We have evaluated how the concentration of the high-spin-state Co 3+ changes with temperature by using an extended Curie-Weiss law and have found that the substituted Rh 3+ ion stabilizes the high-spin-state Co 3+ ion down to low temperatures. We find that Ga 3+ preferentially replaces the high-spin-state Co 3+ , making a remarkable contrast to our previous finding that Rh 3+ preferentially replaces the low-spin-state Co 3+ . These results strongly suggest that the magnetically excited state of LaCoO 3 at room temperature is a mixed state of the high-spin-state Co 3+ and the low-spin-state Co 3+ .

We have prepared polycrystalline samples of LaSrRh1−xGaxO4 and LaSr1−xCaxRhO4, and have measured the x-ray diffraction, resistivity, Seebeck coefficient, magnetization and electron spin resonance in order to evaluate their electronic states. The energy gap evaluated from the resistivity and the Seebeck coefficient systematically changes with the Ga concentration, and suggests that the system changes from a small polaron insulator to a band insulator. We find that all the samples show Curie-Weiss-like susceptibility with a small Weiss temperature of the order of 1 K, which is seriously incompatible with the collective wisdom that a trivalent rhodium ion is nonmagnetic. We have determined the g factor to be g=2.3 from the electron spin resonance, and the spin number to be S=1 from the magnetization-field curves by fitting with a modified Brillouin function. The fraction of the S=1 spins is 2-5%, which depends on the degree of disorder in the La/Sr/Ca-site, which implies that disorder near the apical oxygen is related to the magnetism of this system. A possible origin for the magnetic Rh 3+ ions is discussed.

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