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 ϳ20 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.

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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 h and subsequently subjected to annealing in H 2 ͑at a temperature ranging between 200 and 300°C͒ 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 fieldcooling 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 ϳ20 nm, structurally and magnetically ordered͒ 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 ϳ10 nm͒ dispersed in the NiO phase; with increasing 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 ͑ϳ600 Oe͒, 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 ϳ200 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 phase͒. The thermal dependence of the exchange bias effect reflects the variation of the anisotropy of the NiO disordered component with temperature.

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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.

Ferrofluids are nanomaterials consisting of magnetic nanoparticles that are dispersed in a carrier fluid. Their physical properties, and hence their field of application are determined by intertwined compositional, structural, and magnetic characteristics, including interparticle magnetic interactions. Magnetic nanoparticles were prepared by thermal decomposition of iron(III) chloride hexahydrate (FeCl3·6H2O) in 2-pyrrolidone, and were then dispersed in two different fluids, water and polyethylene glycol 400 (PEG). A number of experimental techniques (especially, transmission electron microscopy, Mössbauer spectroscopy and superconducting quantum interference device (SQUID) magnetometry) were employed to study both the as-prepared nanoparticles and the ferrofluids. We show that, with the adopted synthesis parameters of temperature and FeCl3 relative concentration, nanoparticles are obtained that mainly consist of maghemite and present a high degree of structural disorder and strong spin canting, resulting in a low saturation magnetization (~45 emu/g). A remarkable feature is that the nanoparticles, ultimately due to the presence of 2-pyrrolidone at their surface, are arranged in nanoflower-shape structures, which are substantially stable in water and tend to disaggregate in PEG. The different arrangement of the nanoparticles in the two fluids implies a different strength of dipolar magnetic interactions, as revealed by the analysis of their magnetothermal behavior. The comparison between the magnetic heating capacities of the two ferrofluids demonstrates the possibility of tailoring the performances of the produced nanoparticles by exploiting the interplay with the carrier fluid.

Nanocrystalline Fe3O4 and a composite system constituted by nanocrystalline Fe and Fe3O4 have been synthesized by ball-milling commercial magnetite and an equimolar mixture of iron and magnetite powders. The physical parameters governing the milling process have been strictly controlled so as to achieve the nanocrystalline state of the precursor material and to avoid chemical reactions. X-ray diffraction and Mössbauer spectroscopy measurements have been carried out both on as-milled powders and on samples previously subjected to annealing treatments in the 100–600 °C temperature range. The results, providing information on the structural and compositional features of the produced samples, are discussed in terms of structural disorder which is healed by subsequent annealing. In the case of the composite system, this analysis indicates that a high mixing degree between the constituent phases has been reached. In particular, the presence of a sextet with anomalous hyperfine parameters in the Mössbauer spectrum of as-milled Fe+Fe3O4 has been associated with an alteration of the magnetite structure at the interface with bcc Fe. For both sets of samples, the influence of the structural features on the macroscopic magnetic behavior has been investigated by performing magnetic hysteresis loop measurements at room temperature.