Structure and thermomagnetic properties of new FeCo-based nanocrystalline ferromagnets

Abstract

FeCo-based ferromagnetic alloys (HIT-PERM variants) of composition Fe 44 5 Co 44 5 Zr 7 B 4 , Fe 44 Co 44 Zr 5 7 B 3 3 Ta 2 Cu 1 , Fe 44 Co 44 Zr 5 7 B 3 3 Mo 2 Cu 1 , and Fe 37 2 Co 30 8 Zr 7 B 4 Cu 1 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. Ta and Mo were observed to suppress the to a transition temperature of FeCo. The Fe : Co 65 : 35 alloy exhibits significant curvature in ( ) indicating proximity of the ferromagnetic to paramagnetic transition and the to transition. XRD phase analysis identified crystallized phases as -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 Ta and Mo substitutions decrease the activation energy. Index Terms-FeCo alloys, nanocrystals, soft magnetic alloys.

Magnetic and structural characterization of Mo-Hitperm alloys with different Fe/Co ratio

“…1a) and a linear dependence with two slopes can be established between them. The T c of the as-cast amorphous phase in these alloys is significantly lower than the observed for similar Hitperm [3] or Finemet [18] Mo-free alloys, as expected from the known effect of Mo addition on the Curie temperature of the alloy [8]. In fact the Fe 79 Mo 8 Cu 1 B 12 alloy is paramagnetic at room temperature [11].…”

“…Moreover, due to their small diffusivity in the amorphous phase, they accumulate at the crystal-matrix interface and constrain the growth of the crystalline phase to the nanocrystalline scale. Many studies on Hitperm FeCoZrB and FeCoNbB alloys have been reported [1][2][3][4][5] but, more recently, FeCoMoB alloys have been paid increasing attention [6]. Mo addition to Fe alloys delays the iron oxidation in this system and affects the thermal stability and magnetic properties of these amorphous alloys, with a significant lowering of the Curie temperature [7][8][9][10][11][12].…”

Soft Magnetic Nanocrystalline Alloys for High Temperature Applications

“…Another (Co 0.5 Fe 0.5 ) 88 Zr 7 B 4 Cu 1 alloy composition of interest has an Fe/Co ratio of two to one, with the maximum saturation magnetization for the nanocrystalline phase and smaller magnetostrictive losses. 13) That being said, the magnetostrictive contributions to the losses of (Co 0.5 Fe 0.5 ) 88 Zr 7 B 4 Cu 1 alloys are appreciable over the whole composition range due to the large values found in the nanocrystallites themselves.…”

“…Thermomagnetic measurements of the amorphous precursors to (Co 0.5 Fe 0.5 ) 88 Zr 7 B 4 Cu 1 show the magnetization of the amorphous phase persists until the primary crystallization temperature. 12,13) It is reasonable to assume that the orbital angular momentum of the transition metal atoms in the amorphous phase is effectively quenched and that only spin angular momentum (S) contributes to the moment, J . Hence, the thermomagnetic behavior of amorphous (Co 0.5 Fe 0.5 ) 88 Zr 7 B 4 Cu 1 can be fit to a spin only Brillouin function (J = ±S), yielding values for the internal field and magnetic moment.…”

“…(Note: These and other techniques presented in this paper are described in detail elsewhere.) [12][13][14][15] The endothermic peaks correspond to the crystallization of phases from the amorphous phase. Figure 1 shows three peaks for the (Co 0.95 Fe 0.05 ) 89−x Zr 7 B 4 Cu x alloys, with about 423 K between the primary crystallization (∼ 773 K) and the first secondary crystallization temperature.…”