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

Here we report on the impact of reducing the crystalline size on the structural and magnetic properties of γ-Fe₂O₃ maghemite nanoparticles. A set of polycrystalline specimens with crystallite size ranging from ~2 to ~50 nm was obtained combining microwave plasma synthesis and commercial samples. Crystallite size was derived by electron microscopy and synchrotron powder diffraction, which was used also to investigate the crystallographic structure. The local atomic structure was inquired combining pair distribution function (PDF) and X-ray absorption spectroscopy (XAS). PDF revealed that reducing the crystal dimension induces the depletion of the amount of Fe tetrahedral sites. XAS confirmed significant bond distance expansion and a loose Fe-Fe connectivity between octahedral and tetrahedral sites. Molecular dynamics revealed important surface effects, whose implementation in PDF reproduces the first shells of experimental curves. The structural disorder affects the magnetic properties more and more with decreasing the nanoparticle size. In particular, the saturation magnetization reduces, revealing a spin canting effect. Moreover, a large effective magnetic anisotropy is measured at low temperature together with an exchange bias effect, a behavior that we related to the existence of a highly disordered glassy magnetic phase.

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.

The heating efficiency of an assembly of Mn-doped magnetite nanoparticles can be tuned so as to depend linearly on the non-superparamagnetic fraction.