⁵⁹Co-NMR Studies on La₄Co₃O_10+δ

Abstract

Samples of La₄Co₃O_10+δ with δ∼ 0.1, 0.3 and 0.6 have been prepared. For δ∼ 0.1, the crossover-like changes of the spin state at ∼100 K and ∼550 K have been confirmed by the measurements of the magnetic susceptibility. At ∼550 K, we have observed for the sample the anomalous T-dependence of the electrical resistivity and the thermoelectric power. Its NMR spectra exhibit the very large broadening at low temperatures due to the antiferromagnetic ordering. NMR spectra of the sample with δ∼ 0.6 show that there exist two kinds of Co atoms: The NMR signal intensity of one of the two with the larger relaxation rate 1/T₂ decreases, while the intensity of the other with the smaller 1/T₂ increases, with decreasing T below 100 K, which indicates that the spectra of the latter are from the Co³⁺ in the low spin state. The T-dependence of 1/T₁ shows the existence of the energy difference between the low spin (spin S=0) and intermediate spin (S=1) states of the order of 100 K.