Structure and thermomagnetic properties of new FeCo-based nanocrystalline ferromagnets

FeCo-based ferromagnetic alloys (HIT-PERM variants) of composition Fe44.5Co44.5Zr7B4, Fe44Co44Zr5.7B3.3Ta2Cu1, Fe44Co44Zr5.7B3.3Mo2Cu1, and Fe37.2Co30.8Zr7B4Cu1 were prepared by melt-spinning amorphous precursors, followed by nanocrystallization. Samples were characterized using vibrating sample magnetometry (VSM), x-ray diffraction (XRD), and differential scanning calorimetry (DSC). VSM was used to measure magnetization vs. temperature from 370-1260 K. Magnetic data showed structural and magnetic phase transitions including primary and secondary crystallization events and the to a structural phase transition in FeCo nanocrystals. Th and Mo were observed to suppress the to a transition temperature of FeCo. The Fe : Co 65 : 35 alloy exhibits significant curvature in M(T) indicating proximity of the ferromagnetic to paramagnetic transition and the alpha to gamma transition. XRD phase analysis identified crystallized phases as alpha -FeCo, (Fe,Co)(3)Zr, in agreement with previous reports for unmodified HITPERM alloys. DSC data, analyzed using the Kissinger technique, indicated that the removal of Cu from the original HITPERM composition increased the activation energy for primary crystallization, whereas Th and Mo substitutions decrease the activation energy.