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Ferritin, an antiferromagnetic nanoparticle with a net magnetic moment, exhibits some unusual magnetic behavior. Above its blocking temperature, T B Ϸ12 K, the magnetization can be fit with the Langevin function only by including an additional linear term. The saturation moment decreases linearly with increasing temperature, and suggests an ordering temperature of about 460 K. Below T B , both zero-field-cooled and fieldcooled hysteresis loops exhibit large coercivity (ϳ1800 Oe͒ and irreversibility in the loops is found up to Hϭ35 kOe. An exchange field H e of few hundred Oe is observed at low temperatures. The dependencies of H e and H c on temperature were also studied. ͓S0163-1829͑97͒52022-4͔

As first noted by Néel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 Å were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Néel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 Å diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may be due to exchange coupling of the uncompensated spins with the antiferromagnetic core. However, the large coercive forces and high field irreversibility which accompany the shifted loops raise new questions about the nature of the uncompensated spins. The magnetization relaxation of these particles is also discussed.

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