M. A. López Quintela

Drop of magnetocaloric effect related to the change from first- to second-order magnetic phase transition in La2/3(Ca1−xSrx)1/3MnO3

From data of initial magnetization isotherms, the magnetic entropy change ΔSM of the series of ferromagnetic perovskites La2/3(Ca1−xSrx)1/3MnO3, with x=0, 0.05, 0.15, 0.25, 0.50, 0.75 and 1, has been measured near their Curie temperatures. The results go from 3.7 J kg−1 K−1 for La2/3Ca1/3MnO3 to 1.5 J kg−1 K−1 for La2/3Sr1/3MnO3. Nevertheless, the evolution of ΔSM with x is not monotonic, it shows a steep decrease between x=0.05 and x=0.15. This point corresponds to a tolerance factor t=0.92, at which the system changes from orthorhombic (Pbnm) to rhombohedral (R3c) structure, and the magnetic phase transition from first to second order. Provided that rhombohedral symmetry forbids static long-range cooperative Jahn–Teller distortions, present in orthorhombic samples, we interpret our results as the strong increase of lattice effects in them. The anomalous thermal expansion at the Curie temperature, due to these lattice effects, is thought to underlie the jump in ΔSM with x.

Preparation of colloidal Fe3O4 ultrafine particles in microemulsions

The preparation of magnetite particles is reported from the precipitation of Fe2+/Fe3+ with NH4OH in several kinds of microemulsions: AOT/n-heptane/water microemulsions and a new non-toxic microemulsion system whose phase behaviour and structure has been recently studied. It is shown that both the droplet size and the presence of interactions in the dispersed phase greatly influence the final size of the obtained particles. In addition, the size of the particles can be increased by successive repetition of the reaction inside the droplets of the microemulsion.

Structural and magnetic characterization of Co particles coated with Ag

Co fine particles coated with Ag have been synthesized through the microemulsion method in an inert atmosphere. The size of the particles is controlled by the water droplets of the microemulsions. Fine particles prepared by this method, consist of a magnetic core of Co covered by a layer of Ag. Samples containing from 3.3 to 40.5 vol % Co have been prepared. The average size of the particles obtained is in the nanometer range. The magnetic properties were studied by dc magnetization at 77 K and room temperature. The data show a strong dependence of the magnetic properties on the annealing temperature.

Production and characterization of FeB amorphous particles

Abstract Fine particles of FeB of different compositions were obtained by chemical reactions in solutions. Diverse factors, such as initial reactant concentrations, the speed at which the reactants were added, etc., influence the homogeneity of the final particles obtained. Mossbauer spectroscopy and X-ray diffraction studies evidenced the presence of different phases. The results are discussed as a function of the initial nominal concentrations. It is concluded that only an iron content close to the eutectic in the FeB phase diagram can lead to rather pure amorphous particles.

THERMAL TREATMENT DEPENDENCE OF THE DYNAMIC MAGNETIC BEHAVIOR OF GD2CUO4

dc and ac magnetization measurements in Gd2CuO4 ceramic samples synthesized at different annealing temperatures show that their magnetic properties are influenced by the thermal treatment. The changes consist of variations of the onset point for differences between zero field cooled and field cooled dc magnetizations, in the coercive fields, and in the position and frequency dependence of a maximum in the real part of ac susceptibility. An explanation suggesting a temperature‐dependent homogeneity of the oxygen distortions causing weak ferromagnetism in the CuO2 planes is given.

Size and structural effects on the magnetic behaviour of Gd2CuO4 particles

Abstract The magnetic properties of Gd 2 CuO 4 , synthesized by ceramic and sol-gel methods, have been investigated by dc and ac susceptibility measurements. Both static and dynamic properties have been found to depend on the synthesis temperature for ceramic samples, and on the particle size for sol-gel ones. The role of structural and size effects on the observed changes is discussed.

Quantum Effects in Ultrafine Nd-Fe-B Particles

The production of ultrafine particles, that is, particles in the 10–100 A interval, is a topic of great interest because of their peculiar behavior which differentiates them from the bulk material1. One of the main differences is the existence of quantum size effects in the optical properties of the particles2.