Temperature Dependence of Magnetic Parameters in FePt Nanoparticles

Abstract

Here, particular focus is placed on the atomic alignment and the order–disorder phase transition of the FePt alloy with the aid of a model that can describe realistic phenomena. Here, we present a method to study the order–disorder phenomena of FePt alloys. We will discuss the increase in the coercivity of FePt nanoparticles by an increasing annealing temperature for annealed FePt nanoparticles. According to the experimental evidence, we will present a model for explaining spin-glass-like behavior of these particles during the annealing procedure. In the phase transition from disordered FCC to ordered FCT in FePt nano-particles can be treated by first-order phase transitions. So, the mean field approach can be used in order to model this kind of phase transition. In nanoparticles, which are synthetized by sol–gel methods, the short range parameter is predominant because in these kinds of preparations, the long range order takes a lot of time to happen. By using a short range parameter, we are able to use the mean field approach, which considers the diffusion of atoms to the near neighborhood sites. The effects of random exchange and random magnetocrystalline anisotropy are known as the main results of coercivity reduction in magnetic nanoparticles. These effects are strongly dependent to the annealing temperature of nanoparticles. During disorder–order transition, these two effects are max. As transition continues, these effects vanish and coercivity of nanoparticles increases. Here, we will add these two effects to the Hamiltonian of the FePt system. With increasing the annealing temperature, the fraction of the FePt nanoparticles with a FCC disordered phase vanishes, which leads to decrease in the amount of 〈j〉. During the annealing process, the $\frac{c}{a}$ ratio varies as a function of the annealing temperature. The relation between the $\frac{c}{a}$ ratio and the annealing temperature is derived. The derived $\frac{c}{a}$ ratio and coercivity formulas are compared with experimental results. The results are in a good agreement with experimental data.