The B-H curve of a single crystal of ferromagnetic material depends on the crystal orientation relative to the field. Fig. 4.8 shows B-H curves for single crystal of iron and nickel, with the applied field along (111), (110), and (100) directions. In each case, one direction is more easily magnetized than the others. Saturation occurs at a lower field in this direction.
For BCC iron, the direction of easy magnetization is (100), and hard magnetization is (111). In FCC nickel, the reverse is true. In cobalt, which has an hcp structure the easy direction of magnetization is the crystallographic c-axis. This directionality of magnetic properties is termed anisotropy. On this account the different grains in a polycrystalline material approach saturation differently.
The grains whose directions of easy magnetization lie near the field direction saturate at low applied fields; those whose directions of hard magnetization lie near the applied field direction rotate their moment into the field direction only at higher fields.
The work required to rotate all the domains against the anisotropy is called magneto-crystalline energy or crystal anisotropy energy. It is generally recognized that anisotropy is due to the spin orbit interaction which makes the spin feel the anisotropy of the orbital charge distribution.
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The various crystals of some polycrystalline materials are oriented more or less at random and the properties in different directions are not greatly different. In several materials which undergo specific treatment such as cold rolling, some regularity in the distribution of orientations, makes the magnetic properties of the material markedly anisotropic. This is called induced anisotropy and is of considerable practical importance.
Thin films of Ni-Fe alloys deposited on to substrate by evaporation in vacuum with magnetic field applied in the plane of the substrate have subsequently easy direction for spontaneous magnetization as the original field direction. This is an example of induced anisotropy, and thin magnetic films are used as storage elements in computers.
In bulk materials, there are three basic methods by which the uniaxial anisotropy can be induced:
(i) Cold working, particularly cold rolling, uniaxial anisotropy is induced in the direction of rolling,
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(ii) Magnetic annealing, heat treatment of a material in a magnetic field produces a uniaxial anisotropy, related to field direction,
(iii) Magnetic quenching, material is cooled in the presence of a magnetic field through the Curie temperature, leading to a uniaxial anisotropy either parallel or perpendicular to the field direction.
