Study On The Structrue And Magnetic Properties Of NdFeB/α-Fe Nanocomposites Containing Intergranular Amorphous Phase

In the present study, an intergranular amorphous phase was successfully induced in the Nd₂Fe₁₄B/α-Fe nanocomposite magnets by controlling the process of the melt solidification by changing the wheel speed. X-ray diffraction (XRD) and differential scaning calorimeter (DSC) were employed to study the volume fraction, microstructure and the wheel speed dependence of the intergranular amorphous phase. The magnetic properties of the melt-spinning ribbons were measured by means of a vibrating sample magnetometer (VSM). Results show that there is no intergranular amorphous phase in the ribbon prepared at the wheel speed of 20 m/s. The volume fraction of the intergranular amorphous phase is 20% in the ribbons prepared at 25 m/s, which increases to 33% at the wheel speed of 30 m/s. There is almost complete amorphous phase at 40m/s. The grain size dNd₂Fe₁₄B =35 nm and dα-Fe=22 nm decrease to 24 nm and 14 nm when the wheel speed increase from 20 m/s to 30 m/s. The microstructure of the intergranular amorphous phase is different with that of the complete amorphous. The value of short-range order of the intergranular amorphous phase is largerer while the nearest atomic distance of the intergranular amorphous phase is smaller than that of the complete amorphous. The magnetic properties of the ribbons with no intergranular amorphous phase prepared at the wheel speed of 20 m/s are not good and the hysteresis loop has a characteristic of single magnetic phase, B_r=10.34 T,H_ci =503.2 kA/m,(BH)_max =122.7 kJ/m³. For the ribbons with 20% intergranular amorphous phase prepared at the wheel speed of 25 m/s, the hysteresis loop has a characteristic of single magnetic phase, B_r =11.161 T,H_ci =583.5 kA/m,(BH)_max =179.9 kJ/m³. When the wheel speed increases to 30 m/s the volume fraction of the amorphous phase increases to 33%, but the hysteresis loop has a characteristic of two magnetic phases。 The existence of the intergranular amorphous phase can enhance the magnetic properties of the nanocomposites. This is attributed to the enhanced exchanged-coupling between the hard and soft magnetic grains by inducing the intergranular amorphous phase and weakened exchanged-coupling effect between the hard magnetic nanocrystals.