Just as dielectrics change their length when they are polarized (electrostriction), so do most magnetic materials change their length when they are magnetized. This is because of the fact that when the magnetic electron spins dipole moments of the atoms in a solid are rotated into alignment, the length of the bonds between the atoms changes.
The fields of the dipoles themselves affect the atomic spacing, as they may attract or repel each other. Therefore, the shape and volume of ferromagnetic solid changes as it is magnetized. The principal change, called the magnetostriction, is a reversible strain along the axis of magnetization. Depending upon the material, the solid may expand or contract.
For example, Ni contracts in the direction of magnetization and expands in the transverse direction by about 40 parts per million at saturation magnetization. In spite of the small size of the effect, the change in length, called longitudinal or transverse magnetostriction, is important and it must be considered, because it plays an important role in domain geometry and in the practical use of transformer materials.
The dimensional changes accompanying magnetization are not always of the same sign. In low field, Fe expands in the direction of magnetization, whereas Ni and Co both contract. In high fields, all three contract. This behaviour is illustrated in Fig. 4.9 where λs, the coefficient of magnetostriction defined as λs = Δl/l0, is plotted as a function of H.
Not only is λs a function of field strength, but it is also a function of direction in the unit cube. For example, for single crystals of Fe magnetized in three particular crystal directions, the magnetostrictive coefficient even has opposite signs, as Fig. 4.10 shows.
The volume of ferromagnetic solid usually does not remain fixed as a solid is magnetized. Iron, for example, undergoes a small expansion in all fields, an expansion of about 1 part in 106 for the relatively strong field of 2,50,000 amp-turns/m. Up to this field strength the volume change, called the volume magnetostriction, is almost a linear function of field strength.
One of the important features of magnetostriction is that it has an inverse- since the dimensions of a magnetic material are changed when it becomes magnetized, its magnetic properties are changed when its dimensions are changed, i.e., when it is strained elastically. The solid does not necessarily acquire an overall magnetic moment when it is strained, for magnetostriction, like electrostriction, is insensitive to a change in direction of magnetization by 180°.
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However, through this inverse effect, internal strains in the materials influence the direction in which a given domain will become magnetized. Internal strains of random magnitude and direction, in fact, greatly affect the shape of the hysteresis loop, often broadening it considerably.
In general, material which extends on magnetization will have its permeability raised by a tensile strain, whereas for a negative λs material, an externally applied pull reduces the permeability. This converse of the magnetostriction effect is known as Villari effect which states that longitudinal deformation leads to a change in permeability in the direction of applied strain.
This is because of the fact that when a ferromagnetic material is strained the domains tend to realign themselves into positions of lower energy and accordingly, the permeability of the material is thereby changed, i.e., either material becomes easier or more difficult to magnetize.
The magnetostriction is anisotropic not only because the magnetization curve is anisotropic, but also because the elastic properties of the material are anisotropic. For any given crystal direction, the magnetostriction approaches a constant value at high magnetic fields, magnetostriction and magnetization usually structure at the same time. Fig. 4.9 shows magnetostriction versus applied field curve for Ni and Fe single crystal at [100], [110] and [111] field orientations.
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The magnetostriction has been largely of theoretical interest and of limited practical applications. There have been some applications of magnetostriction to high frequency oscillators and to generators of super sound (ultrasonic). Magnetostriction is also used for underwater sound projectors and sound detectors.
In the preparation of high permeability materials an effort is made to reduce both anisotropy and magnetostriction so that internal strains do not induce local anisotropy energy. This can be done by alloying two or more materials of opposite signs for λ100 and λ111.