Non-Langevin behaviour of the uncompensated magnetization in nanoparticlesof artificial ferritin

The magnetic behaviour of nanoparticles of antiferromagnetic artificial fer ritin has been investigated by Fe-57 Mossbauer absorption spectroscopy and magnetization measurements, in the temperature range 2.5-250 K and with mag netic fields up to 7 T. Samples containing nanoparticles with an average nu mber of Fe-57 atoms ranging from 400 to 2500 were studied. By analysing the magnetic susceptibility and zero field Mossbauer data, the anisotropy ener gy per unit volume is found, in agreement with some of the earlier studies, to have a value typical for ferric oxides, i.e. a few 10(5) ergs/cm(3). By comparing the results of the two experimental methods at higher fields, we show that, contrary to what is currently assumed, the uncompensated magnet ization of the ferri tin cores in the superparamagnetic regime does not fol low a Langevin law. For magnetic fields below the spin-flop field, we propo se an approximate law for the field and temperature variation of the uncomp ensated magnetization, which was early evoked by Neel but has so far sever been applied to real antiferromagnetic systems. More generally, this approa ch should apply to randomly oriented antiferromagnetic nanoparticles system s with weak uncompensated moments.