Physics of Magnetism

152 CHAPTER 14. SOFT-MAGNETIC MATERIALS

annealing temperatures lead to poor results. That means that large values can only be

generated in alloys with sufficiently high Curie temperatures. This is the case for Ni–Fe

alloys with Ni content near 65% because at this composition the Curie temperature reaches

its maximum in this binary alloy system. Thermomagnetic treatments appear to be less

successful in ternary and quaternary alloys in which the Curie temperatures are lower.

Slip- or deformation-induced anisotropy is a second mechanism by means of which the

magnetic properties of soft-magnetic materials can be unproved (Chin and Wernick, 1980).

Also, this type of induced anisotropy depends on directional order of atom pairs, as already

discussed above. The difference with thermomagnetically induced anisotropy is that the

atomic order is brought about mechanically by means of plastic deformation. Figure 14.2.5

may serve to illustrate the mechanism of slip-induced anisotropy. The atoms are seen to be

perfectly ordered before slip (case a), each atom having only dissimilar neighbors. After

applying a horizontal sheer stress, the situation has changed (case b). The sheer stress has

caused the atoms to slip over one another and has led to the formation of crystallographic

defects known as antiphase domain boundaries. Pairs of similar atoms have been created

in the vertical direction across the antiphase domain boundaries, whereas the atoms have

kept their dissimilar neighbors in the horizontal direction. As in the thermomagnetic case,

The magnitude of the slip-induced-anisotropy constants are of the order of

which is about 50 times higher than the anisotropy constants obtained by magnetic anneal­

ing. The slip-induced-anisotropy constants increase with increasing Fe concentration, as

was also found with magnetic annealing. Furthermore, the larger the degree of atomic order

prior to deformation, the larger the ultimate anisotropy. This is true in particular for alloys

near the

this directional difference in pair ordering is the origin of the slip-induced anisotropy.

composition. The direction of the easy axis of the slip-induced anisotropy
depends on the type of order (long- or short-range), and on the crystal orientation (or texture
in the case of polycrystalline material).
Fe–Al and Fe–Al–Si alloys. This is an important group of soft-magnetic materials
that are primarily applied in recording heads, to be discussed in the next section. These
materials are characterized by high electrical resistivities, high hardness, high permeability,
and low magnetic losses. Optimal magnetic properties for the ternary alloys are obtained in
a fairly narrow concentration range around 9.6% Si, 5.4% Al, and 85% Fe. This material is
also known under the name Sendust.