Surface and magnetic interaction effects in Mn3O4 nanoparticles

The magnetic properties of 15 nm Mn3O4 particles, in powder form and dispersed in a polymer at different concentrations (17.6%, 1.5%), have been investigated by magnetization and ESR measurements. Below the ferrimagnetic transition temperature (T-c=42 K), marked differences are observed among the three samples in the temperature dependence of the magnetization, hysteresis loop shape and type and intensity of ESR spectra. They are due to different strengths of interparticle interactions and different contributions from free spins (and free cluster spins) at the particle surface. With decreasing temperature their thermal fluctuations slow down and spins freeze in random directions, according to their local atomic environment. Their fraction is found to decrease with increasing inter-particle interactions, i.e. going from the more diluted particle dispersion to the powder sample.