Previous work on Alnico single crystals (permanent magnet alloy) showed that magnetic torque reversals are associated with an inhomogeneous structure. More recently, it has been found that alloys of iron, nickel, and cobalt in the Perminvar region also exhibit these torque reversals although the alloys are considered to be solid solutions. An inhomogeneous structure occurs simultaneously with the torque reversals in some of these alloys. The inhomogeneous structure has been observed by means of electron diffraction, and the observations have been interpreted to mean that there are faults in the lattice normal to [111] directions. The faults form in the temperature region where the magnetic field heat treatment is effective and their density is roughly proportional to the response of the magnetic field treatment. When the crystals are not heat treated in a field, the faults are arranged to have cubic symmetry and their magnetic anisotropy has cubic symmetry. When the crystals are heat treated in a magnetic field, electron diffraction indicates that the faults are usually affected also.Evidence has been obtained that small amounts of oxygen as an impurity in Perminvar cause both the faulting and the heat treatment in a magnetic field. An extremely small amount of oxygen (0.0014%) is sufficient to cause such effects.In a more detailed sense we feel that the magnetic anisotropy introduced by heat treatment in a magnetic field probably arises either from short range order within the faults or from the arrangements of the faults each of which is parallel to a (111) into regions whose shape anisotropy is along the direction of the magnetic field.