Viscous Remanent Magnetization - Origin of VRM

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 τ called the thermal relaxation time. If the starting magnetization is M₀ and the magnetization in equilibrium is M_eq, 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 M_eq 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