A theory is developed for the contribution to the coercive force of iron of randomly distributed non-magnetic inclusions. Two contributing effects are examined: (1) the surface tension effect, and (2) the effect of internal magnetic poles. It is found that the coercive force depends both on the total volume fraction of inclusions and the state of dispersion. For a given composition of alloy, for particles both much larger and much smaller than the thickness of the domain wall, $\delta$, the net contribution to the coercive force is small. The largest effect occurs for particles whose diameter is about equal to $\delta$. Measurements have been made using a dispersion in iron of ${\mathrm{Fe}}_3$C in the shape of spheres, the diameters of which could be varied over the desired range. The maximum effect on coercive force is found to occur for particles of size about 1200 angstroms, indicating a value for $\delta$ of about this magnitude. For this value of d the measured value of the coercive force is found to agree well with the calculated value.

• Received 19 May 1950

DOI:https://doi.org/10.1103/PhysRev.79.979