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

Temperature variations (300-4K) of the electron magnetic resonance (EMR) spectra of ferrihydrite (FeOOH.nH2O) nanoparticles of size ∼5nm are reported at the frequencies of ν=190.62 and 285.93GHz. The EMR line at 300K, occurring near g∼2, broadens and shifts to lower fields with decreasing T for T>Ts=30K. For TTs, the trend reverses in that the line narrows and shifts to higher fields. In magnetic studies, the coercivity Hc is maximum at Ts (approaching zero at the blocking temperature TB∼65K) with the appearance of exchange bias HE for TTs. 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