  A typical coercivity versus particle size curve for magnetic nanoparticles
has been explained by using the Gilbert equation followed by the corresponding
Fokker Plank equation. Kramer's treatment has been employed to explain the
increase in coercivity in the single domain region. The single to multi-domain
transformation has been assumed to explain the decrease in coercive field
beyond a certain particle size. The justification for using Langevin theory of
paramagnetism (including anisotropy energy) to fit the M vs H curve is
discussed. The super-symmetric Hamiltonian approach is used to find out the
relaxation time for the spins (making an angle greater than $90^0$ with applied
field) at domain wall. The main advantage of our technique is that we can
easily take into account the time of measurement as we usually do in realistic
measurement.
