Baoru Bian

Influence of H and Extra La on Magnetocaloric Effect of La

Magnetic properties and magnetocaloric effect were investigated on La0.5+xPr0.5Fe11.4Si1.6Hy (x=0, 0.1, 0.2) melt-spun ribbons. It was found that NaZn13-type phase could be obtained easily owing to that the segregation of α-Fe phase was suppressed obviously by adding appropriate amount of La after a short time annealing for melt-spun ribbons. For the La0.6Pr0.5Fe11.4Si1.6Hy ribbons, the Curie temperature was adjusted dramatically from low temperature of 193 K to room temperature of 321 K due to the interstitial hydrogen adsorption. With La content increasing, Curie temperature was slightly increased; the magnetic hysteresis was reduced, while a large magnetic entropy change remained. The maximum magnetic entropy change (- ΔSM) is 33.8 J/(kg·K) for La0.6Pr0.5Fe11.4Si1.6 while that of La0.6Pr0.5Fe11.4Si1.6Hy is 19.6 J/(kg·K) in a magnetic field change of 5 T. It is shown that rapid quenching to make LaFe13-xSix alloys is more cost effective than arc melting method by reducing annealing time substantially. High magnetic entropy change and low magnetic hysteresis were obtained by adding extra La and the Curie temperature was tuned to near room temperature by interstitial atom H. This work indicated that La0.6Pr0.5Fe11.4Si1.6Hy was a promising magnetic refrigeration material near room temperature.

_{0.5 + x}

Nanorod-like, cubic, and spherical FePt nanoparticles (NPs) are synthesized by adjusting the reaction temperature at 220 °C, 250 °C, and 298 °C, respectively. It has been found that the shapes of FePt NPs can be controlled by changing the preparing temperature while fixing the molar ratio of metal precursors, surfactants, and other parameters. Investigation shows that both the seed crystal structure and reaction temperature have an effect on the growth process and final shape of the particles. For nanorod particles prepared at a reaction temperature of 220 °C, it grows along with the axis direction of the twin-crystal seeds resulting in the nanorod shape. However, for cubic and spherical NPs prepared at higher temperatures, their shapes are dominantly determined by the competition of surface energy between {111} and {100} whaich is influenced by reaction temperature. The as-synthesized NPs were found to be superparamagnetic at room temperature and their blocking temperatures are size and shape dependent.

Pr

Nanoparticles covered with surfactants are often used to study particle motion patterns and self-assembly processes in solutions. Surfactants have influence on the interparticle interactions and therefore on the particle motion tracks and final patterns. In this study, CoPt nanoparticles are synthesized in aqueous solution without any surfactant. In situ transmission electron microscopy observation is performed to monitor the self-assemble process. Two types of magnetic nanoparticle superlattice arrays are formed: hexagonal equal distance superlattice arrays when particle size is 3 nm, and tight unequal distance superlattice arrays when particle size is 4.5 nm. It is interesting to observe that two small arrays merge into a large one through rotational and translational movements. A Monte Carlo simulation is carried out which successfully restores the whole process. It is identified that the underlying forces are van der Waals and magnetic dipolar interactions. The latter is responsible for orientation of each particle during the whole process. This investigation leads to a better understanding of the formation mechanism of magnetic nanoparticle superlattice arrays.