Co-59 zero-field NMR study of antiferromagnetism in low-temperature tetragonal phase of La2CoO3.86

In the low-temperature tetragonal phase of antiferromagnetic La2CoO4 (T-N = 270 K), we have observed a Co-59 zero-field nuclear-magnetic-resonance spe ctrum that consists of three lines: two dominant well-discriminative lines at 195 MHz (Co-I) and 227 MHz (Co-II) of nearly equal intensity, and a frac tional broad line in the 250 to 325 MHz range with a complicate structure, The former was assigned to the Co atoms with six nearest-neighbor oxygen at oms, and the latter to those with oxygen vacancies in the neighborhood gene rated by the oxygen nonstoichiometry. At low temperatures, the nuclear spin -lattice relaxation rate T-1(-1) of both Co-I and Co-II increases in propor tion to T-3, indicating that the system is in a long-range antiferromagneti c ordered state. With increasing temperature, T-1(-1) of Co-I shows a diver gence increase at T-C4 = 12K, and that of Co-II exhibits a cusp at T-C4 = 1 2K and the divergence increase at T-C3 = 50K, respectively, which are sugge stive of successive transitions of the spin structure. We conclude that the difference in the resonance frequency of Co-I and Co-II is caused by a dif ference in the transferred hyperfine and/or dipolar fields from the adjacen t Co spins. An orbital ordering of the d(xy) wave with q = 0.25 is proposed to be the most plausible ground state of Co2+ ions, which may give two ine quivalent Co sites in the hyperfine field.