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

The effect of particle size on magnetic and microwave properties has been investigated on a series of La
_0.7
Sr
_0.3
MnO
₃
nanoparticles (NPs), with particles size 12nm, 50nm and 100-200nm. Detailed studies of both
⁵⁵
Mn 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 ≈ 1.5-2nm. Temperature dependences of the magnetic resonance spectra have been measured for both 12nm and 50-200nm nanopowders. For an assembly of single domain 12nm NPs electron paramagnetic resonance spectrum at 300K shows a relatively narrow sharp line, while, as the temperature lowers to 5K, 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÷300K). Our results support the conclusion that particle of La
_0.7
Sr
_0.3
MnO
₃
of size ≈ 10nm 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.