Origin of VRM
See also: SuperparamagnetismViscous remanent magnetization is the result of jumps between magnetic states driven by thermal fluctuations. In the simplest case, there is a single characteristic time τ called the thermal relaxation time. If the starting magnetization is M0 and the magnetization in equilibrium is Meq, the magnetization after a time t is
If M0 is a natural remanent magnetization (NRM) acquired in the Earth's field at one time, and Meq is the equilibrium magnetization in a different field at some later time, the change in remanence M(t) -M0 is the VRM.
The above equation is applicable to the simplest single domain magnets, those described by the Stoner–Wohlfarth model. The relaxation time τ 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
Read more about this topic: Viscous Remanent Magnetization
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