Magnetic properties, exchange bias, and memory effects in core-shell superparamagnetic nanoparticles of La0.67Sr0.33MnO3

Abstract

The static magnetic properties and memory and exchange bias effects have been studied in sol-gel prepared La0.67Sr0.33MnO3 (LSMO) nanoparticles. Transmission electron microscopy (TEM) micrographs and static magnetization show log-normal particle and magnetic size distributions with a core-shell structure. Analysis of the magnetization measurements indicates the presence of a magnetic structure with a 7.8 nm core radius and a magnetic dead layer of thickness 1.6 nm in the LSMO nanoparticles, which comprises about 40% of the volume. The disordered spins in the shell freeze at lower temperatures than the core and produce a surface spin glass state exhibiting a weak exchange bias effect. Field cooled and zero-field cooled magnetization measurements have been carried out to study the slow dynamics of the sample and associated magnetic memory effects; the results reveal the superparamagnetic behavior of LSMO nanoparticles described in terms of the magnetic size distribution rather than a superspin glass state.