Magnetostrictive Anisotropy:

Another important form of anisotropy in magnetic materials is due to magnetostriction, a change of volume of an isotropic crystal due to magnetic order. Magnetostriction relates the stress in a magnetic material to an anisotropy created by that stress. The magnetostriction constant λ is the constant of proportionality. Figure 4.3 shows schematic views of bars with different applied stress conditions. If λ is positive, then application of a tensile stress to the bar creates an easy axis in the direction of the applied stress. If a compressive stress is applied, then the direction of the easy axis created will be perpendicular to the stress direction. On the other hand, if the magnetostriction constant for the material is negative, then this is all reversed: a tensile stress will create an easy axis perpendicular to the stress direction, and a compressive stress will create an easy axis in the direction of the applied stress. The value of λ is defined in the following way: Suppose one has a given length of a non-magnetic material. If the material has a positive value of λ , then causing the material to become magnetic will cause the material to lengthen or stretch in the direction of the magnetization. The fractional increase in length is defined as the magnetostriction constant, λ . If the material has a negative value of λ , then the material will shorten in the direction of the magnetization.

Quiz:

(1) What is anisotropy?

(2) What is the role of anisotropy in the magnetic materials?

(3) Which materials possess high magnetocrystalline anisotropy? Why?

(4) Summarize the different ways to induce the anisotropy in the magnetic materials.

(5) Which type of materials exhibit large shape anisotropy? Can the shape anisotropy play a crucial role in bulk materials?

(6) Is it possible to produce a permanent magnet of arbitrary shape using shape anisotropy alone?