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We have investigated the magnetostriction and the magnetization process of 3%Si-Fe single crystal with (110) parallel to the surface, when the direction of magnetization is deviated from [001]. It was found that large contraction and expansion occurred as a result of magnetostriction in the magnetization process, even if the deviation angle was small. Complicated magnetic domain structures were observed with external field being applied. The behavior of magnetostriction corresponds to changes of magnetic domain structure. On the basis of the principle that no magnetic pole appears at domain walls, a model of domain structure is proposed. Using the model, the deformation behavior was explained well.

The effect of microstructures on the magnetic properties in the 6.5% Si-Fe alloy was studied. The alloy was subjected to two patterns of heat treatments, and the microstructures were observed by means of TEM. The alloy after an appropriate treatment has shown excellent soft magnetic properties, with the maximum permeability of 7.25x[10.sup.4] and the core loss [W.sub.10/50] of 0.39 W/kg. The corresponding microstructure was characterized by the extremely fine domains of D[O.sub.3] order phase existing in B2 order phase. However, the coarsening of the D[O.sub.3] domains was found to reduce the maximum permeability and increase the core loss of the alloy. It is suggested that the coarsening of the D[O.sub.3] domains be accompanied with the formation of magnetic contrast around the boundary of the D[O.sub.3] and B2 phases, Therefore, it is assumed that the phase boundary tends to oppose the displacement of magnetic domain walls due to the existence of the magnetic contrast.