MAGNETIC AND ELECTRONIC-STRUCTURES OF ANTIFERROMAGNETIC LA2NIO4+DELTA AND LA2-XSRXNIO4+DELTA - LA-139 NUCLEAR-QUADRUPOLE RESONANCE STUDY

We carried out a nuclear quadrupole resonance study of La-139 in La2-xSrxNiO4+delta (delta = 0.00-0.10; x = 0.0-1.3) in the temperature range T = 1.4-200 K, paying particular attention to its relationship to the superconducting cuprates. In lanthanum nickel oxide, the occupation of holes on both Ni 3d(x2-y2) and 3d(z2) orbitals with parallel spins (S = 1) causes some characteristic properties. (1) The large internal magnetic field (H(int) = 26-18 kOe) at the La site indicates the existence of a strong transferred exchange interaction between Ni 3d(z2) and La 6s orbitals through the 2p(z) orbital of apex oxygen. (2) Discontinuities in the magnitude of H(int) and electric field gradient observed at delta congruent-to 0.02 are attributed to the crystal phase transition from a low-T orthorhombic to a high-T tetragonal phase. (3) Up to high Sr doping (close to the phase transition at x congruent-to 1.0 from antiferromagnetic to paramagnetic), H(int) remains almost constant, suggesting that the doped holes first go into the 3d(x2-y2) orbital without producing Ni-Ni spin frustrations. (4) The T dependences of H(int) and relaxation rate T1(-1) at low T are explained basically by the spin-wave theory for three-dimensional antiferromagnets, and the sublattice magnetic moments of Ni2+ are close to the localized spins. (5) The dependence of T1(-1) proportional to T2.6 indicates that the La-139 relaxation can be attributed to the short-wavelength antiferromagnetic fluctuations notwithstanding the geometrical cancellation at the La site.