  The magnetic behaviour of nanoparticles of antiferromagnetic ferritin, with a
mean Fe loading of 410 atoms per core, has been investigated by 57Fe Mossbauer
absorption spectroscopy down to very low temperature (34mK). In previous
experi- ments of frequency dependent magnetic susceptibility and magnetic noise
performed at 25mK in similar samples, it was claimed that a resonance at a
frequency of about 10**8 Hz, due to a macroscopic coherent state, had been
observed. However, our search of incoherent tunnel fluctuations around 10**8 Hz
using 57Fe Mossbauer spectroscopy, whose "window" of measurement of fluctuation
frequencies lies in this frequency range, was unsuccessful. This casts a doubt
about the previous observa- tion of macroscopic quantum coherence in ferritin.
