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

Abstract Temperature variations (300–4 K) of the electron magnetic resonance (EMR) spectra of ferrihydrite (FeOOH· n H 2 O) 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 > T s = 30 K . For T T s , the trend reverses in that the line narrows and shifts to higher fields. In magnetic studies, the coercivity H c is maximum at T s (approaching zero at the blocking temperature T B ≃ 65 K ) with the appearance of exchange bias H E for T T s . The magnetic viscosity is also maximum near T s , approaching zero above T B . These observations lead to the suggestion that T s represents a phase transition to a new magnetically ordered state, with spin-glass-like ordering of the uncompensated Fe 3+ spins.