Inclusions in soft ferromagnetic materials generally cause an increase in the observed coercive force Hc. The present objective was to determine whether the mechanism of this increase is the magnetostatic interaction of inclusions with moving domain walls, as originally proposed by Néel. Information on this point can be obtained by measuring H c as a function of temperature in two-phase alloys which consist of a ferromagnetic matrix phase plus a distribution of ferromagnetic inclusions. For such alloys, Néel's theory predicts a peak in Hc at the Curie temperature of the inclusions. Previous experiments showed that a peak was sometimes present in samples of 0.8% C steel (α-iron plus cementite), depending on microstructure. The role of microstructure has now been clarified: the peak is always present if the inclusion size is about 0.1 μ, independent of the details of the inclusion morphology. The effect is not peculiar to FeSingle Bond signC alloys, since it has also been observed in Fe-25%Ni. It is noteworthy that a substantial volume fraction of larger inclusions (diam. ≃1 μ) raises Hc, but apparently not via the magnetostatic mechanism. © 1969 The American Institute of Physics.
