Superparamagnetism occurs in ferromagnetic and ferrimagnetic nanoparticles which are single-domain, i.e. composed of a single magnetic domain.
The stable orientations define the magnetic easy axis of the nanoparticle.
The mean time between two flips is called the Néel relaxation time τN and is given by the Néel-Arrhenius equation:[1] where KV is the height of the energy barrier, a product of the magnetic anisotropy energy density K and volume V; kB is the Boltzmann constant, T the temperature and their product the thermal energy; and τ0 is a length of time, characteristic of the material, called the attempt time or attempt period (its reciprocal is called the attempt frequency).
In particular, it is an exponential function of the grain volume, which explains why the flipping probability becomes rapidly negligible for bulk materials or large nanoparticles.
Suppose that the magnetization of a single superparamagnetic nanoparticle is measured over a time τm.