Random and exchange anisotropy in consolidated nanostructured Fe and Ni: Role of grain size and trace oxides on the magnetic properties

Results of magnetization measurements on nanocrystalline Fe and Ni produced by inert-gas condensation are presented. The grain size, which is about 10 to 20 nm in the as-prepared state, is increased by annealing the samples incrementally from 100 °C to 1000 °C. The coercive field shows a pronounced variation with grain size, with a maximum at around 30 nm and a steep decrease for smaller grain sizes. The coercivity is discussed on the basis of the random-anisotropy model that predicts that the effective anisotropy constant is reduced by averaging over magnetically coupled grains. This behavior is observed as long as the grain size is smaller than the effective bulk domain-wall width. The model also accounts for the approach to saturation in nanostructured Fe yielding values for the ferromagnetic correlation length and the anisotropy constant of the grains. The latter is about four times higher than the bulk value of Fe. Hysteresis measurements at 5 K after field cooling show a shift and broadening of the hysteresis loops for both Fe and Ni, which is attributed to an exchange coupling between the ferromagnetic grains and antiferromagnetic or ferrimagnetic (oxide) interfacial phases. The hysteresis shift decreases and finally vanishes with increasing grain size. This is indicative of a restructuring of the oxides, which is confirmed by the coercive field of the Fe samples showing a step at about 120 K caused by a phase transition of ${\mathrm{Fe}}_3{\mathrm{O}}_4.$ The step vanishes again with further increasing grain size. The saturation magnetization of the Ni samples increases with increasing annealing temperature, a fact that is attributed to the evolution of the oxides also.