The inter-grain exchange-coupling interactions, effective anisotropy, and coercivity in nanocomposite Nd2Fe14B/a-Fe magnets were investigated. The effective anisotropy of nanocomposite magnets has been calculated starting from the statistics of boundaries between magnetically hard-hard, hard-soft, and soft-soft grains. The result shows that the effective anisotropy decreases with reduction in grain size and/or increase in soft phase components. When grain sizes reduce to 4-5nm, Keff decreases to 1/3-1/4 of the ordinary value of K. The coercivity in nanocomposite magnets demonstrates a similar behavior. The decrement of coercivity is mainly due to the reduction of effective anisotropy. Considering the opposite varying trend the remanence demonstrates with respect to the effective anisotropy and the coercivity, we conclude that the mean grain size should be in the range of 10-15nm and the volume fraction of soft phase should be less than 50% in order to achieve high energy product magnets.

The dependence of the hard magnetic properties on the alignment magnetic

Isotropic NdDyFeCoB bonded magnets with high coercivity of 1.59MA/m and low temperature coefficient of remanence of -0.056%/K (in the temperature range 298-428 K) were prepared successfully by controlling the HDDR process and adjusting the compositions. The influence of Co and Dy additions on the magnetic properties and the magnetization reversal process in magnet was investigated. The high coercivity in (Nd0.8Dy0.2) 13 (Fe0.875Co0.125) 81 B6 HDDR magnet can be attributed to its unique microstructure and the enhancement of anisotropy field of 2:14:1 phase by substitution of Nd by Dy.

The effects of the alignment degree of grain "easyaxes" on the hard magnetic properties of NdFeB sintered magnets have been investigated. The results show that the remanence polarisation Jr increases monotonously with an enhancement in the degree of grain alignment (i.e. decrease in the orientation coefficient

Two types of Gaussian distribution function ('θh type' and 'tan θ type') describing the degree of grain alignment in sintered NdFeB magnets have been compared in the distribution coe¦cient p (or óg), the distribution probability P (θ) and the grain alignment dependence of coercivity. The results show that when the grain alignment is good (the ratio of remanence-to-saturation polarization Jr/Js≥0.90), ó(óg) and P (θ) for the two types of Gaussian functions have similarvariation tendencies, the calculated values of normalized coercivity based on the starting field theory are basically the same and are consistent with experiments. When the grain alignment is not good (Jr/Js*0.80), the variation tendencies of ó and P (θ) are different. In addition, according to 'tan θ type' Gaussian function, the theoretical values of the normalized coercivity based on the starting field theory are still consistent with the experiments, but according to 'θ type' Gaussian function, the theoretical values seriously deviate from the experiments. This means that the 'tan θ type' Gaussian function is a better texture function for describing the grain alignment.

The hard magnetic properties and the interactions between the grains for sintered Nd16 Fe73 Co5 B6 magnets are investigated by using δM (H) plot technique. The results show that the δM (H) plot of NdFeB sintered magnet can explain the effects of the microstructure (size, shape and orientation of the grains) and the intergrain interactions on the hard magnetic properties of the magnet. However, the value of δM (H) is positive when the applied field is not strong enough, which means that the common δM (H) plot theory is not completely consistent with the sintered NdFeB magnet.

The alignment of Nd2Fe14B grains (T1) and the microstructure of (Nd,Dy)12.8 (Fe,M) 80.7 B6.5 strip cast ribbons are reported in this paper. In the strips there is a pronounced texture of the tetragonal T1 phase and the columnar grains exhibit apparent alignment along [O O L]. The alignment coefficient j changes with the wheel speed V and is highest at V=1.5-2m/s. The Nd-rich phase is well segregated and widely distributed enclosing the fine columnar grains of the main phase in the strips prepared at V=2m/s. The magnetic properties of sintered magnets prepared from this kind of (Nd,Dy)12.8 (Fe,M) 80.7B6.5 strip cast ribbons are as follows: Br=1.457 T (14.57 kG), Hcj=1048kA/m (BH) max=408kJ/m3 (51.3 MGOe).

The influence of exchange-coupling interaction on the effective anisotropyin nanocrystalline Nd2Fe14B permanent material has been investigated. The results show that the exchange-coupling interaction between grains causes adecrease of the effective anisotropyconstant, Keff; with decreasing grain size. The variation of Keff with grain size is basicallythe same as that of coercivity. The decrease of the effective anisotropyis the main reason for the reduction of the coercivity. In order to get high anisotropy or coercivity in nanocrystalline Nd2Fe14B material, the grain size should be larger than 20nm.

Effects of interactions between grains with different alignment degrees on the coercivity and its angular dependence for Nd16 (Fe0.8 Co0.2) 78 B6 sintered magnets have been studied. The experiments show that the intrinsic coercivity jHc decreases with enhancing grain alignment (decreasing alignment coefficient ó), the coercivity jHc (θ) increases with increasing angle θ between the applied field and the texture axis of the magnets and the variation ratio is larger for the magnets with better grain alignment. The coercivity of the magnets should be determined by the critical field making the moment of individual grains reverse and the interactions between the grains. For the sintered magnets composed of the grains with μm size, the magnetostatic interaction between the grains is stronger than the exchange coupling interaction and it makes the coercivity of magnet increase with increasing alignment coefficient ó. Taking into account the intergrain interactions, the starting field theory of coercivity is in good agreement with the experimental results for Nd16 (Fe0.8 Co0.2) 78 B6 sintered magnets.