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Magnetostriction (cf. electrostriction) is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of iron.〔 Joule observed in this paper that he first reported the measurements in a "Conversazione" in Manchester, England, in 〕 This effect causes energy loss due to frictional heating in susceptible ferromagnetic cores. The effect is also responsible for the low-pitched humming sound that can be heard coming from transformers.〔(Questions & answers on everyday scientific phenomena ). Sctritonscience.com. Retrieved on 2012-08-11.〕 == Explanation == Internally, ferromagnetic materials have a structure that is divided into ''domains'', each of which is a region of uniform magnetic polarization. When a magnetic field is applied, the boundaries between the domains shift and the domains rotate; both of these effects cause a change in the material's dimensions. The reason that a change in the magnetic domains of a material results in a change in the materials dimensions is a consequence of magnetocrystalline anisotropy, that it takes more energy to magnetize a crystalline material in one direction than another. If a magnetic field is applied to the material at an angle to an easy axis of magnetization the material will tend to rearrange its structure so that an easy axis is aligned with the field to minimize the free energy of the system. Since different crystal directions are associated with different lengths this effect induces a strain in the material.〔James, R. D., & Wuttig, M. (1998). Magnetostriction of martensite. Philosophical Magazine A, 77(5), 1273-1299.〕 The reciprocal effect, the change of the susceptibility (response to an applied field) of a material when subjected to a mechanical stress, is called the Villari effect. Two other effects are thus related to magnetostriction: the Matteucci effect is the creation of a helical anisotropy of the susceptibility of a magnetostrictive material when subjected to a torque and the Wiedemann effect is the twisting of these materials when a helical magnetic field is applied to them. The Villari reversal is the change in sign of the magnetostriction of iron from positive to negative when exposed to magnetic fields of approximately 40,000 A/m. On magnetization, a magnetic material undergoes changes in volume which are small: of the order 10−6. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「magnetostriction」の詳細全文を読む スポンサード リンク
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