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 . nH(2)O) nanoparticles of size similar or equal to 5 nm are reported at the frequencies of v = 190.62 and 285.93 GHz. The EMR line at 300 K, occurring near g similar or equal to 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 similar or equal to 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 Fe3+ spins. (C) 2004 Elsevier B.V. All rights reserved.