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<Ts, the trend reverses in that the line narrows and shifts to higher fields. In magnetic studies, the coercivity H_c is maximum at Ts (approaching zero at the blocking temperature TB≃65K) with the appearance of exchange bias H_E for T<Ts. The magnetic viscosity is also maximum near Ts, approaching zero above TB. These observations lead to the suggestion that Ts represents a phase transition to a new magnetically ordered state, with spin-glass-like ordering of the uncompensated Fe³⁺ spins.