Nanoparticles of Fe, Co, FeCo, SmCo, and NdFeB systems with sizes smaller than 30 nm and narrow size distribution have been successfully prepared by ball milling in the presence of surfactants and organic carrier liquid. It has been observed that the nanoparticles prepared by milling Fe and FeCo powders were close to spherical in their shapes, whereas those of Co, SmCo, and Nd-Fe-B showed elongated rod shapes. The nanoparticles showed superparamagnetic behavior at room temperature, except for the SmCo nanoparticles that were ferromagnetic. Nanoparticles of all types showed ferromagnetic behavior at low temperatures. The compositions of nanoparticles prepared by milling the SmCo, NdFeB, and FeCo powders were found to be deviated from the starting powders.
Transformation of the electrical transport from the Efros and Shklovskii ͓J. Phys. C 8, L49 ͑1975͔͒ variable range hopping to the "hard gap" resistance was experimentally observed in a low temperature range as the Fe compositions in Zn 1−x Fe x O 1−v ferromagnetic semiconductor films increase. A universal form of the resistance versus temperature, i.e., ϰ exp͓T H / T + ͑T ES / T͒ 1/2 ͔, was theoretically established to describe the experimental transport phenomena by taking into account the electron-electron Coulomb interaction, spin-spin exchange interaction, and hard gap energy. The spin polarization ratio, hard gap energy, and ratio of exchange interaction to Coulomb interaction were obtained by fitting the theoretical model to the experimental results. Moreover, the experimental magnetoresistance was also explained by the electrical transport model.
Nd 2 Fe 14 B and Sm 2 Co 17 particles of submicrometre sizes have been prepared by ball milling in a magnetic field. Structural and magnetic characterization reveal that these submicrometre particles milled in a magnetic field, consisting of nanosize grains, exhibit strong magnetic anisotropy compared with the particles milled without a magnetic field. Based on in situ observations of the field-ball milling in a transparent container, the mechanism of field-induced anisotropy in the nanostructured hard magnetic particles is discussed.
CoNi films were grown on thin Gd layers at room temperature using dc magnetron sputtering. A large increase in the coercivity of CoNi/Gd bilayer, compared to the coercivity of CoNi film, was observed. The increase in coercivity was not significant when the order of film growth was reversed for the same thicknesses and identical deposition conditions. The large increase in coercivity is attributed to the pinning of the CoNi layer by the magnetically compensated ferrimagnetic alloy forming at the CoNi (top)/Gd (bottom) interface due to the diffusion of Co and Ni into Gd. Due to the large coercivity in this system, it can be used to pin soft ferromagnetic layers in spin valve structures.
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