Exchange bias in a ferrihydrite nanoparticle system: the role of a common integrated surface for a conglomerate (cluster) of nanoparticles

Magnetic hysteresis loops of two representative samples of synthetic ferrihydrite nanoparticles with the same sizes (the average size of $ \approx 2.7$ nm) and various interparticle distances have been studied under cooling conditions in the presence and absence of an external field. One initial sample is characterized by the aggregation of nanoparticles, and a shift in the magnetic hysteresis loop along the abscissa axis is observed after cooling from a temperature exceeding the superparamagnetic blocking temperature in the external field. The particles in another sample are spatially separated by coating their surface with an arabinogalactan layer, and the shift of the hysteresis loop after cooling in the external field is not observed in this sample. This experimental fact indicates that one of the important factors determining the shift of the hysteresis loop of nanoparticle systems is a pronounced subsystem of surface spins formed during the close contact of particles, which can be considered as a kind of the surface effect. Because of the exchange coupling between the subsystem of surface spins (common for a conglomerate of particles) and uncompensated moments of particles, an additional source of the unidirectional magnetic anisotropy arises during cooling in the external field, which is the origin of the observed exchange bias of the magnetic hysteresis loop.