MAGNETOCALORIC DEPENDENCE OF MAGNETIC VISCOSITY MEASUREMENTS IN NDFEB

Measurements of the time dependence of magnetization may be analyzed b y means of a constitutive equation to give information regarding the f undamental nature of magnetization reversal processes. The quantity LA MBDA (= the fluctuation field H(f)) may be measured and then related t o the activation volumes involved in the magnetization reversal proces ses. These measurements are commonly made at or near room temperature for hard permanent magnet materials, since this is the usual operating temperature, and the system is assumed to be isothermal. However, the experimental work described shows for these materials the magnetocalo ric effect (MCE) cannot be neglected at these temperatures. Magnetic v iscosity measurements have been made in two ways which lead to differe nt thermal behavior in the sample. Thermometric measurements were made concurrently with the magnetic viscosity measurements in a sample of sintered NdFeB. The measurements show that the calculated value of A c an be changed by as much as 10% when isothermal conditions do not hold . This effect is particularly evident near room temperature in NdFeB-t ype materials because of the strong dependence of coercivity on temper ature and the proximity to the Curie point (T(c)-626 K) where reversib le and irreversible components of MCE peak.