Magnetic hysteresis and superantiferromagnetism in ferritin nanoparticles

The magnetic behaviour of nanoparticles of antiferromagnetic ferritin has been investigated by means of magnetic measurements. First. using a combination of low-field susceptibility and magnetisation measurements (up to 5.5 x 10(4) Oe) in the superparamagnetic regime (30-250 K). we propose a method to estimate the Neel temperature of ferritin by which we obtained T-N approximate to 500 K. The hysteresis loop at 2.5 K has been measured, and interpreted with a simple model, by which the irreversibilities are due to the switching of the uncompensated moments. The magnetisation curve at 2.5 K up to 30 x 10(4) has also been measured, and we show that it can be interpreted in terms of superantiferromagnetism, which is a finite size effect predicted by Neel in 1961, but which has not been previously experimentally evidenced. Superantiferromagnetism occurs in small antiferromagnetic particles with an even number of ferromagnetic reticular planes, and results in an enhancement of the antiferromagnetic susceptibility and in a non-linear field dependence of tire magnetisation Lit higher fields. (C) 2002 Elsevier Science B.V. All rights reserved.