TEMPERATURE-DEPENDENCE OF COERCIVITY IN MELT-SPUN AND DIE UPSET NEODYMIUM-IRON-BORON

We compare the temperature dependence of the intrinsic coercivity H ci between 5 and 600 K in rapidly quenched Nd-Fe-B ribbons and a die upset Nd-Fe-B magnet. At room temperature, ribbons have higher coercivity than the die upset magnet, but have a smaller technical temperature coefficient of coercivity β between 25 and 125°C: Hci=15.1 kOe and β=-0.38%/°C for the ribbons, while Hci=10.4 kOe and β=-0.64%/°C for the die upset magnet. Because the die upset magnet has a stronger temperature dependence, the Hci(T) curves converge at low temperature. The ribbons can be phenomenologically modeled using H ci(T)=cHA(T)-Neff4πMs(T), the two terms representing the strength of domain wall pinning at grain boundaries and the effective local demagnetizing field, respectively. The model parameters c=0.25 and Neff=0.26 are significantly smaller than the corresponding values obtained in sintered Nd-Fe-B magnets. This model is less successful in describing Hci(T) for the die upset magnet. Their qualitatively different behaviors indicate that different mechanisms of domain wall pinning are responsible for coercivity in ribbons and die upset magnets.