Low temperature electron paramagnetic resonance anomalies in Fe-based nanoparticles

A study of the electron paramagnetic resonance of Fe-based nanoparticles em bedded in polyethylene matrix was performed as a function of temperature ra nging from 3.5 to 500 K. Nanoparticles with a narrow size distribution were prepared by the high-velocity thermodestruction of iron-containing compoun ds. A temperature-driven transition from superparamagnetic to ferromagnetic resonance was observed for samples with different Fe content. The unusual behavior of the spectra at about 25 K is considered evidence of a spin-glas s state in iron oxide nanoparticles. (C) 2000 American Institute of Physics . [S0021-8979(00)01213-5].