Effects of heat treatment on the structure and magnetic properties of Al-Ge addedFe73.5−xSi13.5B9Nb3Cualloys

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In this work, we have studied the effects of the coaddition of $\mathrm{Al}(1.5\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%)∕\mathrm{Ge}(1\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%)$ and heat-treatment temperature on the structure and magnetic properties of rapidly quenched pure ${\mathrm{Fe}}_{73.5\ensuremath{-}x}{\mathrm{Si}}_{13.5}{\mathrm{B}}_{9}{\mathrm{Nb}}_{3}\mathrm{Cu}$ (FINEMET) alloys prepared by the single roller melt spinning process. Two series of as-spun and heat-treated alloys, 400, 480, 560, 640, and $730\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ for $1\phantom{\rule{0.3em}{0ex}}\mathrm{h}$ in vacuum $({10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}\text{Torr})$, were studied using various techniques such as x-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), M\ossbauer spectroscopy, and $B\text{\ensuremath{-}}H$ loop tracer. M\ossbauer analysis carried out on both series of alloys heat treated at $560\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ revealed the presence of two phases, ${\mathrm{Fe}}_{3}\mathrm{Si}$ phase with $D{0}_{3}$ structure and a residual amorphous phase with reduced hyperfine field $(16.9\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ compared to those of $\mathrm{Al}∕\mathrm{Ge}$ added $(22.5\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ and pure FINEMET $(\ensuremath{\sim}23.4\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ alloys, in close agreement with XRD and TEM results. The highest maximum permeability, saturation magnetization, and the lowest coercivity were obtained for both series of alloys when heat treated at $560\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. However, the magnitude of coercivity was lower for the $\mathrm{Al}∕\mathrm{Ge}$ substituted alloy (heat treated at $560\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$) compared to that of the pure FINEMET alloy. This is due to the substitution of Al atoms for Fe as confirmed by our XRD and M\ossbauer results. Further, a higher ferromagnetic amorphous phase Curie temperature $({T}_{C,\mathrm{am}})$ was detected for the $\mathrm{Al}∕\mathrm{Ge}$ added alloy $(322\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ compared to that of the pure FINEMET alloy $(317\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, which is believed to be mainly related to the presence of Ge atoms in the amorphous matrix. Furthermore, the removal of stress and the setup of nanocrystallization for both series of alloys gave rise to a higher collinear magnetic moment angle. However, this was not the case for the samples heat treated at temperatures above $560\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, for which the formation of the Fe-B hard magnetic phase was confirmed by our XRD, TEM, and M\ossbauer results. Finally, the structural-magnetic properties relationship is discussed for various samples studied in this work.