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Abstract
Magnetization measurements on NiFe$\rm\sb2O\sb4$ nanoparticles ($\rm\langle D\rangle$ = 65A), show behavior which is inconsistent with single domain behavior. We found that hysteresis loops remain "open" up to 16T, 400 times larger than the bulk magnetocrystalline anisotropy field. We measured the relaxation of remanent magnetization to be constant between 0.4-2.0K, similar to recent data which have been interpreted as quantum tunneling of magnetization. Additionally we have observed time dependent magnetization in fields as high as 7T. We proposed a model to explain the observed behavior, in which the surface spins are disordered, and the exchange coupling between the surface and core gives rise to a variety of magnetization distributions within a given particle. In this model, the open hysteresis loops and high field time dependence are due to irreversible reorientations of the surface spins. Modeling of the magnetization distribution within NiFe$\rm\sb2O\sb4$ particles, based on literature values of the exchange constants, indeed exhibits surface spin disorder. We show that the combination of surface spin disorder and surface anisotropy can account for the open hysteresis loops and anomalous relaxation behavior.
Antiferromagnetic nickel monoxide nanoparticles also exhibit anomalous magnetic properties such as large moments, and coercivities and loop shifts of up to 10kOe. This behavior is difficult to understand in terms of 2-sublattice antiferromagnetic ordering which is accepted for bulk NiO. Modeling of spin configurations in these nanoparticles yields 8, 6, or 4-sublattice configurations, a new finite size effect in which the reduced coordination of surface spins causes a fundamental change in the magnetic order. The relatively weak coupling between the sublattices allows a variety of reversal paths for the spins upon cycling the applied field, resulting in large coercivities and loop shifts when bulk and surface anisotropies are included.
Finally, we considered the phenomenon of "exchange anisotropy", in which a unidirectional anisotropy results from the coupling between antiferromagnetic and ferromagnetic films. We developed a model for the exchange field based on the density of interfacial uncompensated spins, accounting for grain size, orientation, and interfacial roughness. It predicts the inverse dependence on grain size, as observed experimentally, and the correct magnitude of the exchange field.
