We have found that the domain structure in the annealed state of a 40 percent cobalt-nickel single crystal is very fine and complicated as compared with those of ordinary ferromagnetic crystals but it becomes simpler and larger after quenching from above the Curie temperature. This can be explained as follows : Since the domain structure may in general be fine and complicated at temperatures just below the Curie temperature, domain walls must displace to establish a more stable domain configuration as the temperature lowers. But, this process is suppressed appreciably at relatively low temperatures, because, in solid solution, the ferromagnetic uniaxial anisotropy is induced, in compliance with the domain distribution, by the anisotropic distribution of atoms at high temperatures and the domain wall displacement can take place only by being accompanied by the redistribution of atoms which can not occur at low temperatures. While, in quenching, the specimen crystal is cooled down so rapid that the uniaxial anisotropy and hence the anisotropic distribution of atoms can not be induced, and thus the quenched specimen crystal behaves just as ordinary ferromagnetic crystals. It is shown that these findings together with the results of considerations reported previously lead us to the conclusion that the perminvar-type magnetic properties are due to the stabilization of domain walls by the induced uniaxial anisotropy in f. c. c. solid solutions with cubic anisotropy constants of any sign and b. c. c. solid solutions with negative cubic anisotropy constants
