Magnetocaloric dependence of magnetic viscosity measurements in NdFeB

Measurements of the time dependence of magnetization may be analyzed by means of a constitutive equation to give information regarding the fundamental nature of magnetization reversal processes. The quantity Λ (≡ the fluctuation field H_f) may be measured and then related to the activation volumes involved in the magnetization reversal processes. 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 magnetocaloric effect (MCE) cannot be neglected at these temperatures. Magnetic viscosity measurements have been made in two ways which lead to different 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 Λ can be changed by as much as 10% when isothermal conditions do not hold. This effect is particularly evident near room temperature in NdFeB-type materials because of the strong dependence of coercivity on temperature and the proximity to the Curie point (T_c∼620 K) where reversible and irreversible components of MCE peak.