Effect of Post-Sintering Annealing Treament on Magnetic Properties of some Nd-Fe-Based Magnets

Effects of post-sintering annealing treatment between 300 and 1000ºC for up to 8 hrs time periods followed by either furnace cooling (FC) or air-cooling (AC) on the hard magnetic properties of some Nd-Fe-B based sintered magnets was investigated. It was shown that the intrinsic coercivity (Hci), second quadrant demagnetization loop shape and thus the energy density (BH)max varied considerably as a result of the annealing treatment. However, no significant changes occurred in the remanence (J) values of the magnets. Annealing at 500ºC for 1 hr followed by FC resulted in a considerable deterioration in the demagnetization loop shapes and thus in (BH)max values in all the magnets. These changes were almost recoverable. By annealing at 500ºC, followed by AC, no deterioration in the demagnetization loop shape occurred for any of the magnets and thus there were some increases in (BH)max values of the magnets due to increase in their Hci values. The highest Hci in magnets A and C was obtained by a standard annealing treatment for 1 hr at 600ºC, while in magnet B, the highest Hci occurred after an annealing treatment for 2 hrs at 500ºC, both followed by AC. No deterioration in the demagnetization loop shape was identified for any of the magnets either in FC or AC from 600ºC. However, the coercivity enhancement in magnet A and in particular, in magnet B, was found to be much higher after AC than FC. In all the annealing treatments, the improvement in Hci and/or (BH)max values occurred in the first 1 or 2 hrs of annealing. Any further annealing beyond these times resulted in significant falls in the magnetic properties. Thus, long term annealing for up to 300 hrs at 400ºC resulted in dramatic reductions in the hard magnetic properties, which could only be partially recovered. This behavior has been related to the oxidation of the magnets during long term annealing in a roughing pump vacuum. In this paper, the changes in Hci and (BH)max values during these annealing treatments are related to possible microstructural changes reported elsewhere.