Small-field Thermal Relaxation Rates for Single-domain Magnets

Abstract:

Any magnetic process involving hysteresis is affected by thermally driven transitions between magnetic states. The transition rates depend on the energy barriers, which are generally difficult to calculate. However, some simple results can be obtained for small magnetic fields. In a single-domain magnet with uniaxial anisotropy, a magnetic field at an angle to the easy axis makes the moments of the stable states and energy barrier states rotate; yet the effects of these rotations cancel out, leaving an expression that depends only on the zero-field states. Some of the cancellation occurs because, in general, there are two energy barriers, a fact that (to the presenter's knowledge) has been ignored in all previous calculations.

In a small static field, the relaxation rate is the same as the zero-field rate to first order; and also to first order, the blocking temperature is equal to the unblocking temperature.