Synthesis of ultrastrong nanocomposite-permanent magnet with magnetic anisotropy

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Nanocomposite permanent magnets have become more attractive as new magnetic materials. Recently, mechanical alloying (MA) and the melt-spinning method have been applied to the production of nanocomposite magnets such as Nd_2Fe_<14>B+α-Fe(Fe_3B),Sm_2Fe_<17>Nx+α-Fe,SmCo_5+α-Fe, etc. These nonocomposite magnets, however, are magnetically isotropic, and a magnetically anisotropic composite magnet is significantly required. We have tried to synthesize magnetically anisotropic powders in the Sm_2Fe_<17>Nx+α-Fe. Generally, an anisotropic Sm_2Fe_<17>Nx single hard phase was produced by the mechanical grinding (MG) of coarse Sm-Fe mother alloy powders into about 1 μm and subsequent nitriding. Mixtures of the Sm-Fe mother alloy powder and the commercial α-Fe powder were mechanically ground and subsequently nitrided. An exchange coupling between the hard and soft magnetic phases could not be obtained, because it was difficult to pulverize the α-Fe powder into nanoscale particles. According to the … More Sm-Fe binary diagram, the Fe phase and Sm_2Fe_<17> phase coexist on the Fe-rich side from the composition of Sm_2Fe_<17>. Based on this fact, we tried to prepare the Sm_2Fe_<17>Nx+α-Fe composite alloy powders by Sm evaporation during heat treatment at 1273 K in a vacuum from the mechanically ground Sm_2Fe_<17> alloy powder and subsequent nitriding. However, it was found that the properties as a permanent magnet were hardly obtained because of the significant oxidation of these fine powders during sample preparation. On the other hand, we have prepared the anisotropic powder of the Sm_2Fe_<17>Nx single hard phase, which has a grain size of about 300 nm, by MG in NH_3 for a short time using a rotary ball mill with subsequent nitriding. Based on these findings, the synthesis of magnetically anisotropic Sm_2Fe_<17>Nx+α-Fe composite alloy powders was tried by Sm evaporation from the Sm-Fe mother alloy at 1227 K in a vacuum, MG in NH_3 and nitriding. The powders were composed of the Sm_2Fe_<17>Nx+α-Fe phase possessed a magnetic anisotropy, and showed a reversible magnetization reaching 66.8 %. This result indicates that a satisfactory exchange coupling between the soft and hard phases. These powders also showed a magnetic anisotropy, which can align the directions of the c-axis of each hard particle in an applied magnetic field. Including the above mentioned permanent nanoccomposite magnets, all of the strong permanent magnets are produced by using rare-earth alloys. However, the rare-earth magnets are notorious for their poor corrosion resistance. In this study, the Nd-Fe-B magnet produced by HDDR (hydrogen decomposition desorption recombination) process was selected as the representative rare-earth magnet, and its corrosion behavior was investigated through the potentiodynamic polarization curve and Nyquist plot by electrochemical impedance spectroscopy. As the result, it was confirmed that the excellent corrosion resistant film was formed on the specimen surface polarized strongly to a cathodic region. Less