Magnetic Properties of Nd-Fe-B Nanocrystalline Composite Magnets

Abstract

Using 57Fe Mössbauer spectroscopy, X-ray diffraction and magnetization measurements, the magnetic properties of the melt-spun Nd4.5Fe77B18.5 ribbon have been investigated as a function of annealing treatments which make nanocrystalline composite of various phases like Fe3B, Nd2Fe23B3, Nd2Fe14B and α-Fe. These phases have been formed by decomposition of the as-prepared amorphous phase and make a nanocrystalline composite. Magnetic properties of the nanocrystalline composite depend on the heat-treatments applied subsequently to the ribbon. By heating up to 873 K, Fe3B and Nd2Fe23B3 phases have been formed in the ribbon. Mössbauer spectra of these two phases show magnetically split-patterns with large hyperfine magnetic fields suggesting that the ribbon has a large saturation magnetization. Coercivity of this ribbon is determined to be nearly zero because of the low magnetic anisotropy of these phases. The ribbon annealed up to 943 K shows a coercivity and the Mössbauer spectrum obtained shows clearly the existence of the Nd2Fe14B component. Results obtained from the Mössbauer measurements suggest that the coercivity of the ribbon rises from the formation of hard magnetic phase, Nd2Fe14B. The ribbon which has the suitable magnetic properties for a permanent magnet consists of the mixture of Fe3B and Nd2Fe14B and the fractional amounts of these two phases have been determined to be 90%Fe3B and 10%Nd2Fe14B.