Magnetic viscosity and domain wall pinning in an MnAlC permanent magnet

Magnetic viscosity in a hot extruded MnAlC permanent magnet has been measured over a temperature range from 80 to 295 K. The results are fitted to a recent theory of “strong” and “weak” domain wall pinning which allows values of the thermal activation free (“absolute zero”) coercive field to be determined from the data. If the pinning is due to strong pins, randomly distributed throughout the material, a pin density of ≈3×10¹⁵ cm^-3 is predicted. There is no obvious evidence of pin sites of this density within the grains visible in electron micrographs.If the pinning is localized (possibly at grain boundaries) then weak pinning by a similar, but localized, density of pins is predicted. Lorentz electron microscopy domain contrast in such regions might however be overwhelmed by strain contrast effects.