Magnetic viscosity has been measured in aged MnAlC alloys having an intrinsic coercive force of 1.44 kOe at 295 K rising to 1.97 kOe at 77 K. The measurements are used to estimate the derivative (∂E/∂H) of the activation energy E which must be supplied thermally to move segments of domain wall through coherent jumps. This derivative is found to vary linearly with the reciprocal of the effective applied field. These results are consistent with a domain wall pinning model which allows a range parameter 4b to be deduced, from the viscosity results, as 44 Å. This result is close to the estimated wall width of 49 Å.

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