Temperature evolution of superparamagnetic clusters in single-crystalLa0.85Sr0.15CoO3characterized by nonlinear magnetic ac response and neutron depolarization

Abstract

The representative measurements of the second harmonic in ac magnetization complemented by neutron depolarization have been performed for single-crystal ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{CoO}}_{3}$ in the temperature range $97\phantom{\rule{4.pt}{0ex}}\text{K}<T<230\phantom{\rule{4.pt}{0ex}}\text{K}$, where occurrence of a small fraction $(\ensuremath{\lesssim}{10}^{\ensuremath{-}3})$ of nanoscale ferromagnetic clusters (FMC) has been found. Magnetic, geometrical, and dynamical parameters of the FMC system have been evaluated in the temperature range $T<140$ K, where superparamagnetic regime installs, by means of the formalism involving the Fokker-Planck equation. With lowering the temperature, the amount of clusters fraction, the cluster size, and magnetic moment along with its diffusion relaxation time strongly increase, each in its own temperature interval. Below 130 K, FMC contribute essentially to the total linear magnetic susceptibility. The damping factor of the order ${10}^{\ensuremath{-}1}$ proves the importance of precession in thermal relaxation of the cluster magnetic moment. The FMC are a precursor of long-range ferromagnetic correlations seen below 100 K with neutron-scattering techniques.