High-frequency electron magnetic resonance and magnetic studies of ferrihydrite nanoparticles and evidence of a phase transition

Temperature variations (300-4 K) of the electron magnetic resonance (EMR) spectra of ferrihydrite (FeOOH·nH2O) nanoparticles of size ≃5 nm are reported at the frequencies of ν=190.62 and 285.93 GHz. The EMR line at 300 K, occurring near g≃2, broadens and shifts to lower fields with decreasing T for T>Ts=30K. For T