Anisotropy, coercivity and thermal activation in $L$1$_{0}$-FePt Nanoparticles.

$L$1$_{0}$ FePt materials have attracted tremendous attention for their potential applications due to their high anisotropy and excellent mechanical properties. It is well known that the magnetization behavior of small particles is strongly affected by thermal fluctuation especially when the particle size is close to the critical size of superparamagnetic. In this work, the size effect on anisotropy constant K$_{u}$, magnetic viscosity parameter S, activation volume V$_{ac}$ and coercivity H$_{c}$ of the $L$1$_{0}$ FePt nanoparticles obtained by the salt-matrix annealing have been studied systematically. It was found that K$_{u}$ increases with increasing particles size. The maximum field-dependent viscosity parameter S$_{max}$ increases monotonously with temperature for the particles with size of 3 nm to 8 nm. Moreover, S$_{max}$ of small particles is more sensitive to temperature than that of large particles. However, the temperature dependence of both S$_{max}$ and V$_{ac}$ of the 15 nm particles are different from those for 3-8 nm particles. Further analysis of relation between H$_{c}$ and V$_{ac}$ suggested that the 3-8 nm particles are ideal single-domain particles, while the 15 nm particles can not be well described as single-domain particles even the particle size is much smaller than the single-domain size.