Temperature and doping-level dependence of magnetic order in La_2-xSr_xNiO_4+δ studied by muon spin rotation

We report muon-spin-rotation (mu SR) measurements on a series of compounds with composition La2-xSrxNiO4+delta with strontium doping level x between 0 and 1. A magnetic transition is found in all the samples studied, which occurs at a composition-dependent temperature, T-M. Below T-M clear precession signals are observed in zero applied magnetic field for all x, indicating the existence of at least short-range magnetic order on a time scale longer than 10(-8) s. Above T-M the magnetic correlation times decrease by several orders of magnitude. At x approximate to 0.33, we find peaks in both T-M and the zero-temperature staggered magnetization, an observation which we attribute to the higher degree of localization of the holes at this doping level. The measurements of T-M as a function of x extend the determination of Neel temperatures by previous neutron diffraction and mu SR measurements. [S0163-1829(99)09505-3].