Effect of temperature of long annealing on the structure and magnetic properties of nanocrystalline FeSiNbCuB alloy

A dependence of the soft magnetic properties of the Fe$_{73.5}$Si$_{13.5}$Nb$_{3}$Cu$_{1}$B$_{9}$ alloy on the temperature of annealing $(T_{an})$ carried out in air for 2 hours at temperatures from 520 to 620$^{\circ}$C was investigated. It was shown that with Tan increasing, the magnetic hysteresis loop broadens significantly and becomes more inclined, and the Curie temperature of the amorphous matrix surrounding the $\alpha$-FeSi nanocrystals decreases. The atomic structure and phase composition of the alloy samples were investigated by X-ray diffraction in transmission geometry. After annealing at temperatures of up to 580$^\circ$C, nanocrystals contain predominantly D03 phase (Fe$_3$Si stoichiometry) and have average size of about 7 nm. Their relative fraction in the alloy increases as the temperature increases due to the additional diffusion of iron from the matrix into the nanocrystals. After annealing at $T_{an}\ge 600^\circ$C, the average size of the nanocrystals increases, and reflections of iron boride crystals appear in the diffractograms. The deterioration of the soft magnetic properties of the Fe$_{73.5}$Si$_{13.5}$Nb$_{3}$Cu$_{1}$B$_{9}$ nanocrystalline alloy, when the annealing temperature rises from 520 to 580$^{\circ}$C, is explained by a decrease in the silicon concentration in FeSi nanocrystals, which leads to a growth of the constant of the magnetocrystalline anisotropy.