The low-temperature magnetic properties of samples obtained by cold-compacting core-shell Fe/Fe oxide nanoparticles have been investigated, and their dependence on the structure, composition, and mean particle size D has been discussed. Samples with different D, varying from 6 to 15 nm, and different Fe to oxide ratio were analyzed by means of transmission electron microscopy, x-ray diffraction, and magnetization measurements in the 5–300-K temperature range. The results support the existence of a low-temperature (below $T_1\sim 20\mathrm{K})$ frozen, disordered magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-glass-like oxide matrix and the Fe nanocrystallites. Above $T_1,$ a different regime is distinguished, characterized by the coexistence of a quasi-static, ferromagnetic component, given by the Fe particles, and a relaxing component, represented by regions of exchange-interacting spins of the oxide matrix. As the temperature is increased above $T_1,$ the net moments of the oxide magnetic regions become able to thermally fluctuate and they tend to be polarized by the Fe particle moments. The above picture well accounts for the composition, particle size, and thermal dependence of the coercivity and of the exchange field, which strongly increase with reducing temperature in correspondence with the freezing of most of the moments of the oxide magnetic regions.

The exchange bias properties of a granular system composed of $\mathrm{Fe}$ nanoparticles (mean size $\sim 6\mathrm{nm}$) embedded in an $\mathrm{Fe}$ oxide matrix have been studied. The effect of the cooling field on the exchange field, coercivity and remanent magnetization at $T=5\mathrm{K}$ has been investigated. The observed variations are explained in terms of interfacial exchange coupling between a ferromagnetic phase (the $\mathrm{Fe}$ particles) and a disordered magnetic phase (the oxide matrix), whose spin configuration is strongly affected by the cooling field.

The low-temperature magnetic behavior of ball-milled nanocrystalline Fe is reported. The results of dc and ac susceptibility, remanence, and magnetic relaxation measurements indicate a transition from a high-temperature ferromagnetic state to a low-temperature disordered regime, where a collective freezing of the crystallites magnetic moments occurs. It is proposed that the behavior is determined by the presence of the structurally disordered grain boundaries, showing spin-glass-like properties.

Nanocrystalline Fe powder was synthesized by mechanical-energy high-transfer technique (ball milling). By varying the duration of the mechanical attrition, powder with grain sizes in the 8–25 nm range was produced. The nanocrystalline material exhibits a two-components Mössbauer spectrum due to the presence of two phases: the crystallites and the grain-boundary region among them. In particular, the grain boundaries give rise to a distribution of hyperfine magnetic fields typical of an amorphouslike configuration. The structural features of nanocrystalline iron affect strongly the magnetic behavior. A marked variation of the temperature dependence of magnetization above 580 K is observed, compared to that of coarse-grained iron. The effect has been connected with the order-disorder transition of the spins located at the grain boundary. Experiments performed on 8-nm-sized iron previously annealed at high temperature indicate an increase of the grain dimensions and an atomic rearrangement at the grain boundaries.

Hysteresis, thermal dependence of magnetization, and coercivity of oxide coated ultrafine Fe particles prepared by inert gas condensation and oxygen passivation have been studied in the 5–300 K range. The results are found to be consistent with a spin-glasslike state of the oxide layer inducing, through exchange interaction with the ferromagnetic core, a shift of the field cooled hysteresis loops at temperatures below the freezing at approximately 50 K.

The exchange bias effect has been studied in Ni/NiO nanogranular samples prepared by mechanical milling and partial hydrogen reduction of NiO; the Ni weight fraction varied between 4% and 69%. In this procedure, coarse-grained NiO powder has been ball-milled in air for 20 hours and subsequently subjected to annealing in H2 (at a temperature ranging between 200 °C and 300 °C) to induce the formation of metallic Ni. The structural properties of the samples have been studied by X-ray diffraction, electron microscopy and extended X-ray absorption fine structure. The magnetic properties have been extensively investigated by carrying out hysteresis loops and magnetization measurements in the 5-300 K temperature range, in zero-field-cooling and field-cooling conditions. The results indicate that both in the as-milled NiO powder and in the hydrogenated samples, the NiO phase is composed of nanocrystallites (having a mean size of ~ 20 nm, structurally and magnetically ordered) and of highly disordered regions. The samples with low Ni content (up to 15%) can be modeled as a collection of Ni nanoparticles (mean size of ~ 10 nm) dispersed in the NiO phase; with increasing the Ni content, the Ni nanoparticles slightly increase in size and tend to arrange in agglomerates. In the Ni/NiO samples, the exchange field depends on the Ni amount, being maximum (~ 600 Oe), at T = 5 K, in the sample with 15% Ni. However, exchange bias is observed also in the as-milled NiO powder, despite the absence of metallic Ni. In all the samples, the exchange bias effect vanishes at ~ 200 K. We propose a mechanism for the phenomenon based on the key role of the disordered NiO component, showing a glassy magnetic character. The exchange bias effect is originated by the exchange interaction between the Ni ferromagnetic moments and the spins of the disordered NiO component (in the as-milled NiO powder, the existence of ferromagnetic moments has been connected to chemical inhomogeneities of the NiO phase). The thermal dependence of the exchange bias effect reflects the variation of the anisotropy of the NiO disordered component with temperature