The crystal and magnetic structures of orthorhombic ε-Fe2O3 have been studied by simultaneous Rietveld refinement of X-ray and neutron powder-diffraction data in combination with Mössbauer spectroscopy, as well as magnetization and heat-capacity measurements. It has been found that above 150 K, the ε-Fe2O3 polymorph is a collinear ferrimagnet with magnetic moments directed along the a axis, whereas the magnetic ordering below 80 K is characterized by a square-wave incommensurate structure. The transformation between these two states is a second-order phase transition and involves subtle structural changes mostly affecting the coordination of the tetrahedral and one of the octahedral Fe sites. The temperature dependence of the ε-Fe2O3 magnetic properties is discussed in light of these results.

The structural evolution and magnetic properties of nanostructured copper ferrite, CuFe2O4, have been investigated by x-ray diffraction, Mössbauer spectroscopy, and magnetization measurements. Nanometre-sized CuFe2O4 particles with a partially inverted spinel structure were synthesized by high-energy ball milling in an open container with grain sizes ranging from 9 to 61 nm. Superparamagnetic relaxation effects have been observed in milled samples at room temperature by Mössbauer and magnetization measurements. At 15 K, the average hyperfine field of CuFe2O4 decreases with decreasing average grain size while the coercive force, shift of the hysteresis loop, magnetic hardness, and saturation magnetization at 4.2 K increase with decreasing average grain size. At 295 K the coercive-field dependence on the average grain size is described, with particles showing superparamagnetic relaxation effects. At 4.2 K the relationship between the coercive field and average grain size can be attributed to the change of the effective anisotropy constant of the particles. The interface anisotropy of nanostructured CuFe2O4 is found to be about 1.8(1) × 105 erg cm-3. Although spin canting was present, approximately 20% enhancement of the saturation magnetization in CuFe2O4 nanoparticles was observed, which could be explained by a cation redistribution induced by milling. The high-field magnetization irreversibility and shift of the hysteresis loop detected in our samples have been assigned to a spin-disordered phase, which has a spin-freezing temperature of approximately 50 K.

The structural and magnetic evolution in copper ferrite (CuFe 2 O 4 ͒ caused by high-energy ball milling are investigated by x-ray diffraction, Mössbauer spectroscopy, and magnetization measurements. Initially, the milling process reduces the average grain size of CuFe 2 O 4 to about 6 nm and induces cation redistribution between A and B sites. These nanometer-sized particles show superparamagnetic relaxation effects at room temperature. It is found that the magnetization is not saturated even with an applied field of 9 T, possibly as the result of spin canting in the partially inverted CuFe 2 O 4 . The canted spin configuration is also suggested by the observed reduction in magnetization of particles in the blocked state. Upon increasing the milling time, nanometer-sized CuFe 2 O 4 particles decompose, forming ␣-Fe 2 O 3 and other phases, causing a further decrease of magnetization. After a milling time of 98 h, ␣-Fe 2 O 3 is reduced to Fe 3 O 4 , and magnetization increases accordingly to the higher saturation magnetization value of magnetite. Three sequential processes during high-energy ball milling are established: ͑a͒ the synthesis of partially inverted CuFe 2 O 4 particles with a noncollinear spin structure, ͑b͒ the decomposition of the starting CuFe 2 O 4 onto several related Fe-Cu-O phases, and ͑c͒ the reduction of ␣-Fe 2 O 3 to Fe 3 O 4 .

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We have studied the magnetic and power absorption properties of a series of magnetic nanoparticles (MNPs) of Fe 3 O 4 with average sizes ranging from 3 to 26 nm. Heating experiments as a function of particle size revealed a strong increase in the specific power absorption (SPA) values for particles with = 25-30 nm. On the other side saturation magnetization M S values of these MNPs remain essentially constant for particles with above 10 nm, suggesting that the absorption mechanism is not determined by M S . The largest SPA value obtained was 130 W/g, corresponding to a bimodal particle distribution with average size values of 17 and 26 nm.

Pure ultrafine ZnFe 2 O 4 particles have been obtained from mechanosynthesis of the ZnO and Fe 2 O 3 oxides. The average grain diameter was estimated from x-ray diffraction to be = 36(6) nm. Refinement of neutron diffraction (ND) data showed that the resulting cubic spinel structure is oxygen-deficient, with ~7% of Fe 3+ ions occupying the tetrahedral A sites.Magnetization curves taken at 4.2 K showed absence of saturation up to fields H = 9 Tesla, associated to a spin-canted produced by the milling process. Field-cooled (FC) and zero-fieldcooled (ZFC) curves showed irreversible behavior extending well above room temperature, which is associated to spin disorder. Annealing samples at 300 °C yields an average grain size = 50(6) nm, and ~16% of Fe 3+ ions at A sites.

We report on the magnetic and power absorption properties of a series of iron oxide nanoparticles with average sizes ranging from 3 to 23 nm, prepared by thermal decomposition of Iron(III) acetylacetonate in organic media. From the careful characterization of the magnetic and physicochemical properties of these samples, we were able to reproduce the specific power absorption (SPA) values experimentally found, as well as their dependence with particle size, using a simple model of Brownian and Néel Relaxation at room temperature. SPA experiments in ac magnetic fields (0 = 13 kA/m and f = 250 kHz) 0.25. The observed SPA dependence with particle diameter and their magnetic parameters indicated that, for the size range and experimental conditions of f and H studied in this work, both Néel and Brown relaxation mechanisms are important to the heat generation observed.