In spite of the presence of the spontaneous magnetization, a block of ferromagnetic or ferrimagnetic substance is usually not spontaneously magnetized but exists rather in a demagnetized state. This is because the interior of the block is divided into many magnetic domains, each of which is spontaneously magnetized. Since only the direction of domain magnetization varies from domain to domain, the resultant magnetization can be changed from zero to the value of spontaneous magnetization. Actually if an external field id applied, the apparent magnetization of the block is changed and finally reaches the saturation magnetization which is equal to the spontaneous magnetization. If the field is reduced, the magnetization is again decreased, but does not come back to the original value. Such an irreversible process of magnetization is called hysteresis. The presence of saturation magnetization and hysteresis is an important feature of ferro and ferrimagnetism.
The ferro or ferrimagnetic substance consists of many magnetic domains each of which is magnetized in a different direction. The magnetization inside each domain is made up of many atomic moments, which are lined up, by the action of an exchange force. P.weiss first predicted the presence of the ferromagnetic domain structure in 1907 in his famous paper on the hypothesis of the molecular field. Barkhaussen made the first experimental verification of the presence of ferromagnetic domains in 1919 by an indirect method. The direct observation of ferromagnetic domain structure was attempted by Bitter in 1931. In 1932 Bloch showed theoretically that the boundary between domains is not sharp on an atomic scale but is spread over a certain thickness wherein the direction of spins changes gradually from one domain to the next. This layer is usually called a domain wall or Bloch wall.
The size and shape of the domain is dependant on the size and shape of the specimen in homogeneous materials. But in inhomogeneous magnetic materials, the size of the domain is determined exclusively by the inhomogeniety of the materials, independently of the shape of the specimen. The kinds of inhomogeineity, which can affect the domain structure, are voids, inclusions, precipitations, fluctuation in alloy compositions, internal stress, local directional order, crystal boundaries etc.
The ferro or ferrimagnetic substance consists of many magnetic domains each of which is magnetized in a different direction. The magnetization inside each domain is made up of many atomic moments, which are lined up, by the action of an exchange force. P.weiss first predicted the presence of the ferromagnetic domain structure in 1907 in his famous paper on the hypothesis of the molecular field. Barkhaussen made the first experimental verification of the presence of ferromagnetic domains in 1919 by an indirect method. The direct observation of ferromagnetic domain structure was attempted by Bitter in 1931. In 1932 Bloch showed theoretically that the boundary between domains is not sharp on an atomic scale but is spread over a certain thickness wherein the direction of spins changes gradually from one domain to the next. This layer is usually called a domain wall or Bloch wall.
The size and shape of the domain is dependant on the size and shape of the specimen in homogeneous materials. But in inhomogeneous magnetic materials, the size of the domain is determined exclusively by the inhomogeniety of the materials, independently of the shape of the specimen. The kinds of inhomogeineity, which can affect the domain structure, are voids, inclusions, precipitations, fluctuation in alloy compositions, internal stress, local directional order, crystal boundaries etc.
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