where τ eff
N and τ eff
B are the effective relaxation times,
f (V) is the distribution of physical volumes and f (Vh)
is the distribution of hydrodynamic volumes, Vp(0) is the
volume of a particle having a diameter Dp(0) and Vh(0) is
the hydrodynamic volume of a particle having a physical
diameter Dp(0).
In the case of hysteresis heating the amount of heat
generated is proportional to the frequency multiplied by the
area of the loop. However, it is only the irreversible part of
the magnetisation that contributes to this effect. Since the
particles are randomly oriented and, hence, their remanence
is equal to 0.5Ms, and only particles with a diameter D
[Dp(0) < D < Dp(H)] are blocked and can be switched
in the field H, Phys can be written as