  In the present chapter, we focus on the switching of magnetisation, or the
metastable lifetime of a ferromagnetic system. In this regard, particularly the
Ising model and the Blume-Capel model, have been simulated in the presence of
an externally applied magnetic field by the Monte-Carlo simulation technique
based on the Metropolis algorithm. Magnetisation switching is found to be
faster in the presence of disorder, modelled here by a quenched random field.
The strength of the random field is observed to play a similar role to that
played by temperature. Becker-D\"oring theory of classical nucleation
(originally proposed for the spin-1/2 Ising system) has been verified in the
random field Ising model. However, a stronger random field affects the
nucleation regime. In a cubic Ising lattice, surface reversal time is found to
be different from the bulk reversal time. That distinct behaviour of the
surface in contrast to the bulk has been studied here by introducing a relative
interfacial interaction strength ($R$). Depending on $R$, temperature, and
applied field, a competitive switching of magnetisation of surface and bulk is
noticed. The effect of anisotropy ($D$) on the metastable lifetime has been
investigated. We report a linear dependency of the mean macroscopic reversal
time on a suitably defined microscopic reversal time. The saturated
magnetisation $M_f$, after the reversal, is noticed to be strongly dependent on
$D$. $M_f$, $D$, and $h$ (field) are found to follow a proposed scaling
relation. Finally, Becker-D\"oring theory as well as Avrami's law are verified
in spin-$s$ Ising and Blume-Capel models. The switching time depends on the
number of accessible spin states.
