⁵⁹Co Zero-Field NMR Study of Antiferromagnetism in Low-Temperature Tetragonal Phase of La₂CoO_3.86

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In the low-temperature tetragonal phase of antiferromagnetic La₂CoO₄ (T_N=270, K), we have observed a ⁵⁹Co zero-field nuclear-magnetic-resonance spectrum 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 fractional 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 atoms, and the latter to those with oxygen vacancies in the neighborhood generated by the oxygen nonstoichiometry. At low temperatures, the nuclear spin-lattice relaxation rate T₁^-1 of both Co^I and Co^II increases in proportion to T³, indicating that the system is in a long-range antiferromagnetic ordered state. With increasing temperature, T₁^-1 of Co^I shows a divergence increase at T_C4=12, K, and that of Co^II exhibits a cusp at T_C4=12, K and the divergence increase at T_C3=50, K, respectively, which are suggestive 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 difference in the transferred hyperfine and/or dipolar fields from the adjacent 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 inequivalent Co sites in the hyperfine field.