Spin-state change of Co atoms of La4Co3O10+δ

Samples of La4Co3O10+δ with δ∼0.1 and 0.6 have been prepared and studied mainly by NMR. The NMR spectra for δ∼0.1 exhibit the very large broadening at low temperatures due to the antiferromagnetic ordering. The spectra for δ∼0.6 consist of two components. The intensity of one of the two with the larger relaxation rate 1/T2 decreases, while that of the other with the smaller 1/T2 increases, with decreasing T below 100 K. The result indicates that the spectra of the latter are from the Co3+ in the low spin state. Detailed studies of the longitudinal relaxation rate 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