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Nanoparticles of the ferrimagnetic epsilon-Fe2O3 oxide have been synthesized by sol-gel method. Here, we report on the measurements of the dielectric permittivity as a function of temperature, frequency and magnetic field. It is found that, coinciding with the transition from collinear ferrimagnetic ordering to an incommensurate magnetic state occurring at about 100 K, there is an abrupt change (about 30 %) of permittivity suggesting the existence of a magnetoelectric coupling in this material. Indeed, magnetic field dependent measurements at 100 K have revealed an increase of the permittivity by about 0.3 % in 6 T. Prospective advantages of epsilon-Fe2O3 as multiferroic material are discussed.

We report on the magnetic, magnetotransport and structural characterization of (Ca1?yNdy)2Fe1+xMo1?xO6 (x 0) of Fe ions in the metallic sublattice produces a remarkable increase, by more than 90?K, of the Curie temperature. M?ssbauer spectroscopy data indicate a reinforcement of the magnetic interactions. We argue that this dramatic enhancement of the ferromagnetic order is due to the strong antiferromagnetic superexchange coupling between near-neighbour Fe?Fe occupying regular and antisite positions in the structure. Moreover, the results indicate that the excess of magnetic ions (Fe) is essential to overcome the dilution effects caused by antisite defects.

We present high-resolution neutron and synchrotron powder diffraction, magnetotransport, and magnetization measurements on Bi0.5Sr0.5MnO3. We report that, for the whole range of temperature 1.5 K≤T≤300 K, the sample studied exhibits coexistence of two phases. The majority phase presents charge and orbital order at least below 350 K. The existence and stability of charge order up to this high temperature agrees with the fact that magnetic fields higher than 45 T are not sufficient to melt the charge-ordered state at 4 K. This is in great contrast with the behaviour of non-Bi-based Ln1/2Sr1/2MnO3 isostructural oxides.

The value of TC and the magnetic behavior of La2MnCoO6 depend on preparation conditions, in particular on the cooling rate after the last calcinations process. The valence state and the degree of order of cobalt and manganese ions vary according. We expose here two samples of La2MnCoO6 sintered by solid state reaction with different cooling rates. The Rietveld refinement of neutron powder diffraction (NPD) data showed that our samples crystallize in a monoclinic structure with symmetry P21/n up room temperature and ferromagnetic transition around 230K. Sample with lower cooling rate presents a saturation magnetization of 5.58μB/f.u. besides the value of 4.41μB/f.u. for that with higher cooling rate.

We present a plausible explanation for the intriguing phase separation phenomena displayed by Pr0.50Ca0.50Mn1−xMxO3 compounds. This explanation is based on the simulations that authors have recently published (2008 Europhys. Lett. 84 67011). The results obtained there are compared with those found for M = Ti by means of neutron and synchrotron X-ray powder diffraction. Phase separation can be explained from very local effects of M ions together with their random distribution in Mn sites.

We have studied the role of the intrinsic disorder, introduced by the presence of two different cations at the same crystallographic position, for promoting phase separation in highly correlated oxides. We focus on perovskites with strong phase competition and dilute chemical doping (X'), in which the presence of X' instead of X is able to produce a local modification of the ground state (for instance, from insulating charge order to ferromagnetic metal). Our results prove that in the dilute case, a low density of X' cations promoting a local change of state in a volume as small as the first coordination sphere, generates large phase separated regions instead of a diluted distribution of ferromagnetic nanoclusters. Our results demonstrate that diluted chemical disorder can promote phase separation at the microscopic scale in a large variety of oxides with competing states.

Oxygen vacancy disorder is very common in RBaCo2O5+δ cobaltites with large rare earths (R). This disorder affects the magnetic interactions and, obviously, the magnetic properties of these compounds. In this work we report the preparation and characterization of a well ordered PrBaCo2O5.50 sample that has allowed us to study their magnetic properties and (by means of neutron powder diffraction) their low temperature magnetic structure in the absence of disorder. By comparison with a sample presenting 10% of oxygen vacancy disorder its effect on the studied properties has been determined.