Search for incoherent tunnel fluctuations of the magnetisation in nanoparticles of artificial ferritin

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 108 Hz, due to a macroscopic coherent state, had been observed. However, our search of incoherent tunnel fluctuations around 108 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.