Investigations on the magnetic properties of the Fe5-xCoxSiB2 alloys by experimental and band structure calculation methods

Abstract

We present theoretical and experimental investigations on the electronic and magnetic properties of the Fe5-xCoxSiB2 alloys (x = 0, 0.5, 1.0, 1.5, 2.5 and 5). Theoretical calculations show the dependence of the magnetic properties (magnetic moments, magneto-crystalline anisotropy, exchange-coupling parameters, Curie temperature) on the Fe/Co ratio and preferential occupation of Co atoms for 16l crystallographic site. The spontaneous magnetization as a function of temperature was measured and fitted using the Kuz'min function and modified Arrott-Belov plots. The first procedure shows the decrease of the spontaneous magnetization at 0 K with Co content, with obtained values between 8.79 µB/f.u. (x = 0) and 5.97 µB/f.u. (x = 1.5), in agreement with the theoretical calculations. Also, by thermo-magnetic measurements fitted using Kuz’min function Curie temperatures (Tc) have been obtained, which decrease with the Co content, from 845 K (x = 0) to 667 K (x = 1.5). According to the theoretical exchange-coupling parameters calculated for both Fe spins, the decrease of the Tc is mainly related to the decrease of the Fe 16l–Fe 16l exchange interaction. Moreover, the evolution of the magneto-crystalline anisotropy energy (MAE) with Co content shows good agreement between theoretical and experimental investigations. The 3d magnetism character is found to evolve from well localized behavior in the Fe5SiB2 compound, featured by critical exponents typical of Ising systems, to more delocalized upon Co for Fe substitution with larger range interactions and modified critical exponents.This study provides an insight regarding the influence of the Co doping on the stoichiometry and on the magnetic properties of the Fe5-xCoxSiB2 alloys, which have been previously suggested as good candidates for obtaining rare-earth free permanent magnets with enhanced MAE and higher coercivity.