Effect Of Grain Size On The Magnetic Property Of α-Fe/ND₂Fe₁₄B Nanocomposite Magnets

Nanocomposite magnets consisting of a fine mixture with nano-scale hard- and soft-magnetic phases have attracted much attention for their potential application as high-performance permanent magnets since they were first reported in 1989. However, up to now, although a substantial progress has been made in the preparation of nanocomposite magnets, there is still a discrepancy between the predicted and practical values for achieving the magnet with high-energy products, which is due to the difficulty in obtaining the optimum microstructures employed in the theoretical model. More recently, some studies indicate that a low coercivity is another problem that limits the realization of high-energy products in nanocomposite magnets. Exchange-coupling interaction as the main interaction between grains of nanocomposite magnets, is the origin of the remanence enhancement effect, and it has weakened the magnetocrystalline anisotropy of grains, which may be the cause of the coercivity decline. The inverse relationship between the exchange-coupling interaction and coercivity in nanocomposite magnets makes us realize that a thorough understanding of the coercivity mechanism and the exchange-coupling interaction between the hard- and soft-magnetic phases is necessary to the improvement of the magnetic properties in nanocomposite magnets.In this paper, α-Fe/Nd2Fe14 B nanocomposite magnets with different grain sizes were prepared by melt-spinning at different wheel speeds, to study the effect of grain size on the coercivity, domain-wall-pinning strength and the exchange-coupling interaction of α-Fe/Nd2Fe14 B melt-spun nanocomposite magnets. The magnetic properties of the α-Fe/Nd2Fe14 B nanocomposite magnets were measured using a vibrating sample magnetometer（VSM）, and the microstructure of magnets was studied by employing X-ray diffraction（XRD） measurements and transmission electron microscope（TEM） observations.The α-Fe/Nd₂Fe₁₄B-type nanocomposite ribbons were directly prepared by melt-spinning at wheel speeds of 14, 15 and 17 m/s, respectively, and a comparative experiment on thermally annealed ribbons were made from as-prepared amorphous ribbons, indicating that a high interface fraction that results from a small grain size in the magnets should result in a high coercivity in the magnets. It reveals that the coercivity mechanism of α-Fe/Nd2Fe14 B melt-spun nanocomposite magnets becomes “domain-wallpinning-controlled” mechanism. The relationship between domain-wall-pinning strength and coercivity of the magnets is Hp≈0.85 Hc, and the relationship between coercivity and grain sizes is Hc=-25（dNd Fe B/dα-Fe）+47.5. In addition, the smaller the grain sizes is, the more obvious exchange-coupling interaction. The interface engineering through varying grain size, is beneficial for keeping a high exchange-coupling interaction between the hard- and soft-magnetic phases, and it is a potential route to enhance the domain-wall-pinning strength and thus the coercivity of nanocomposite magnets.