Magnetic Structure and Microwave Properties of La0.7Sr0.3MnO3 Ultrafine Particles

Abstract

The effect of particle size on magnetic and microwave properties has been investigated on a series of La0.7Sr0.3MnO3 nanoparticles (NPs), with particles size 12 nm, 50 nm and 100-200 nm. Detailed studies of both 55Mn nuclear magnetic resonance and superparamagnetic resonance spectrum, completed by magnetic measurements, have been performed to obtain microscopic information on the local magnetic structure of the NP. The results on nuclear dynamics provide direct evidence of formation at the particle surface of a magnetically disordered layer of the thickness ap 1.5-2 nm. Temperature dependences of the magnetic resonance spectra have been measured for both 12 nm and 50-200 nm nanopowders. For an assembly of single domain 12 nm NPs electron paramagnetic resonance spectrum at 300 K shows a relatively narrow sharp line, while, as the temperature lowers to 5 K, the apparent resonance field decreases and the line width considerably increases. Computer simulations of the spectra have been carried out within the model when NPs are considered as superparamagnetic particles with randomly oriented magnetic moments and thermal fluctuations of anisotropy axes direction. The single set of parameters provides good fits to the spectra recoded in the whole temperature range (5 divide 300 K). Our results support the conclusion that particle of La0.7Sr0.3MnO3 of size ap 10 nm still keeps the important properties of the bulk material, that are of great relevance for spin-electronic devices based on an exploration of the nanoscale magnetism of manganites and their half-metallic properties.