La NQR Study of Antiferromagnetic La₂NiO_4+δ

The magnetic properties in the La₂Ni0_4+δ system have been investigated by ¹³⁹La nuclear quadrupole resonance (NQR). The structure of the NQR spectrum at 1.4 K was found to depend sensitively on annealing procedures of the compounds, and therefore on δ. A well-characterized spectrum was obtained for samples oxygenated (δ≧ 0.066) and deoxygenated (δ~0.022) with different structures, exhibiting the existence of the internal magnetic fieldsH_mt of ~17.8 kOe nearly parallel to the basal Ni-O plane and ~19.3 kOe perpendicular to the plane. The strength ofH_m at the La site was significantly large when compared with that of ~ 1 kOe in La₂CuO₄ and could not be explained simply by the dipole field caused by Ni spin moments.