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Viscous remanent magnetization (Abbreviated VRM), also known as viscous magnetization, is remanence that is acquired by ferromagnetic materials by sitting in a magnetic field for some time. The natural remanent magnetization of an igneous rock can be altered by this process. This is generally an unwanted component and some form of stepwise demagnetization must be used to remove it. == Origin of VRM == Viscous remanent magnetization is the result of jumps between magnetic states driven by thermal fluctuations. In the simplest case, there is a single characteristic time called the ''thermal relaxation time''. If the starting magnetization is and the magnetization in equilibrium is , the magnetization after a time is : If is a natural remanent magnetization (NRM) acquired in the Earth's field at one time, and is the equilibrium magnetization in a different field at some later time, the change in remanence is the VRM.〔 The above equation is applicable to the simplest single domain magnets, those described by the Stoner–Wohlfarth model. The relaxation time depends on factors such as the size of the magnet and its magnetic anisotropy. Rocks have collections of magnetic minerals with varying size and anisotropy, and therefore a broad spectrum of relaxation times. The magnetization tends to have a logarithmic dependence on time, so the rate of change is often represented by a ''viscosity coefficient''〔 : 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「viscous remanent magnetization」の詳細全文を読む スポンサード リンク
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