Magnetic anisotropy and thermal stability study on FePt nanoparticle assembly

Abstract

The temperature dependence of the first-order magnetic anisotropy constant has been investigated for off-stoichiometric chemically ordered L1 0 , Fe 45 Pt 55 nanoparticle samples. A representative sample shows an anisotropy energy density of K u =0.7×10 7 erg/cm 3 and a thermal stability factor of K u V/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.