Magnetic and electronic structures of antiferromagnetic La2NiO4+ delta and La2-xSrxNiO4+ delta : 139La nuclear quadrupole resonance study

Abstract

The authors carried out a nuclear quadrupole resonance study of 139La 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(x3-y3) and 3d(z2) orbitals with parallel spins (S=1) causes some characteristic properties. (1) The large internal magnetic field (Hint=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 2pz orbital of apex oxygen. (2) Discontinuities in the magnitude of Hint and electric field gradient observed at delta approximately=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 approximately=1.0 from antiferromagnetic to paramagnetic), Hint 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 Hint 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 localised spins. (5) The dependence of T1-1 proportional to T2.6 indicates that the 139La relaxation can be attributed to the short-wavelength antiferromagnetic fluctuations notwithstanding the geometrical cancellation at the La site.