Thermal limits on field alignment of nanoparticle FePt media

We derive a simple expression for the average angular orientation distribution of ferromagnetic FePt particles in an applied field in thermal equilibrium. This system is closely related to the Langevin expression for paramagnetic susceptibility, which computes the average orientation of particles in this configuration. We note that practical fields are unlikely to be able to provide well-aligned arrays of the smallest thermally stable FePt particles at room temperature and that this has significant consequences for self-organized nanoparticle magnetic media. It suggests that it may be difficult to obtain an acceptable degree of perfection in orientation for the smallest particles. (c) 2006 American Institute of Physics.