Magnetic anisotropy and thermal stability study on FePt nanoparticle assembly

The temperature dependence of the first-order magnetic anisotropy constant has been investigated for off-stoichiometric chemically ordered L1(0), Fe45Pt55 nanoparticle samples. A representative sample shows an anisotropy energy density of K-u=0.7x10(7) erg/cm(3) and a thermal stability factor of KuV/k(B)T=77 at room temperature. With decreasing temperature, from room temperature to 5 K, a 50% increase of K-u, accompanied by a 25% increase of the saturation magnetization M-S is observed. The temperature dependence of K-u is explained on the basis of the spin-Hamiltonian approach with an Fe effective spin. The estimated median magnetic grain size, obtained from time-dependent measurements, decreases by about 15% with decreasing temperature from 298 to 100 K. This magnetic grain size reduction is attributed to the decrease of the superparamagnetic fraction in the FePt nanoparticle sample. (C) 2003 American Institute of Physics.