Temperature evolution of superparamagnetic clusters in single-crystal La0.85Sr0.15CoO3 from nonlinear magnetic ac response and neutron depolarization

TL;DR

This study investigates the temperature-dependent behavior of superparamagnetic clusters in La0.85Sr0.15CoO3 using nonlinear ac magnetic response and neutron depolarization, revealing their growth, dynamics, and role in magnetic susceptibility.

Contribution

It introduces a novel combination of nonlinear ac response and neutron depolarization measurements with Fokker-Planck formalism to analyze superparamagnetic clusters in a single crystal.

Findings

Nanoscale ferromagnetic clusters are present below 230 K.

Cluster size and magnetic moment increase as temperature decreases below 140 K.

Clusters significantly influence magnetic susceptibility below 130 K.

Abstract

The representative measurements of the second harmonic in ac magnetization complemented by neutron depolarization have been performed for single-crystal La0.85Sr0.15CoO3 in the temperature range 97 K < T < 230 K, where occurrence of a small fraction (~ 0.001) 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 (FPE). 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 0.1 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. The employed technique supplemented with FPE-based data-treatment formalism is a novel method for studying superparamagnetic systems.