Ferritin: a model superparamagnet

Signatures of spin-glass freezing in NiO nanoparticles

“…In 1961 Néel suggested that small particles of an antiferromagnetic material should exhibit superparamagnetism and weak ferromagnetism 5 . Antiferromagnetic nanoparticles are known to show more interesting behavior compared to ferro and ferrimagnetic nanoparticles one of which is that the magnetic moment of tiny antiferromagnetic particles increases with increasing temperature 6,7,8,9 . This is quite unlike what is seen in ferro and ferrimagnetic particles.…”

“…For many canonical spin glasses it lies between 0.0045 and 0.06 whereas for a known superparamagnet a-(Ho 2 O 3 )(B 2 O 3 ) a value of 0.28 has been reported 17 . Ferritin, an antiferromagnetic nanoparticle, which has been found to be superparamagnetic, has a value of ≈ 0.13 for this quantity 8 . We note that our value of 0.018 falls in the spin glass range.…”

Thermoinduced Magnetization in Nanoparticles of Antiferromagnetic Materials

“…1, together with a number of similar experimental observations, give experimental evidence for a temperature dependent magnetic moment that is in accordance with the model for thermoinduced magnetization. Thus the thermoinduced magnetization can explain anomalous magnetic properties of nanoparticles of antiferromagnetic materials, which have been discussed in the literature [4,[8][9][10][11][12][13][14][15].…”

“…Still, the dependence of the magnetization on particle size and temperature has shown features that are not fully understood. Several studies have revealed that the temperature dependence is not in accordance with the Langevin behavior, i.e., the magnetization does not decrease with increasing temperature in the expected way [8][9][10][11][12][13][14][15]. These results have been observed in several synthetic samples, in the iron storage protein ferritin, and in ferrihydrite, which can be found in sediments in nature.…”

“…Harris et al [9] found in a study of ferritin samples with different particle sizes that the magnetic moment of the particles increases with temperature, and the authors remarked that ''the reason for this temperature dependence is not clear.'' In another study of ferritin particles with about 4000 iron atoms, Kilcoyne and Cywinski [10] noted that ''above 120 K, is surprisingly found to increase slowly with increasing temperature.'' Makhlouf et al [11] found that the magnetic moment of ferritin particles at finite temperatures was considerably larger than expected from the low-temperature value.…”