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

The magnetic behaviour of nanoparticles of antiferromagnetic artificial fer ritin, with a mean Fe loading of 410 atoms per core, has been investigated by Fe-57 Mossbauer absorption spectroscopy down to very low temperature (34 mK) and by magnetometry down to 2.5 K. In previous experiments of frequenc y-dependent magnetic susceptibility chi(omega) and magnetic noise S(omega) performed at 25 mK in similar artificial ferritin particles, a resonance ob served at a frequency of about 10(8) Hz was claimed to be due to a macrosco pic quantum coherent state. In this work, we measured on the one hand the p arameters which enter the theoretical expression for the tunnel splitting a nd for the incoherent tunnel fluctuation rate, namely the anisotropy energy density in ferritin, the transverse antiferromagnetic susceptibility and t he excess magnetic moment due to uncompensated spins. We show that, using t hese measured values, the tunnel splitting is expected to be orders of magn itude lower than claimed in the above mentioned chi(omega) experiment. On t he other hand. assuming that the tunnel splitting has the value claimed in the above mentioned chi(omega) experiments, we performed a search for incoh erent tunnel fluctuations in these artificial ferritin particles by perform ing a Fe-57 Mossbauer spectrum at 34 mK. Mossbauer spectroscopy on Fe-57 ha s a "window" of measurement of fluctuation frequencies centered around 10(8 ) Hz, and we show that the expected signature of incoherent tunnel fluctuat ions is absent from the 34 mK Mossbauer spectrum. These experiments cast a doubt about the previous observation of a macroscopic quantum coherent stat e in ferritin. (C) 2001 Elsevier Science B.V. All rights reserved.