Recent advances in the development of (Fe,Co)88M7B4Cu1 magnets (invited)

Abstract

Annealing of amorphous precursor alloys, with compositions (Fe,Co)88M7B4Cu1 (M=Zr, Nb, Hf), above their primary crystallization temperature results in the nanocrystallization of the ferromagnetic α′-FeCo phase. This work describes results of the characterization of these alloys, including morphological and chemical stability of the α′-FeCo phase, examination of alloy compositions, and development of a pseudo-Slater–Pauling curve for the amorphous precursor alloys. Samples with the composition Fe44Co44Zr7B4Cu1 were annealed at 600 °C for 10, 31, 100, 308, 1000, and 3072 h in Ar and examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Scherrer analysis of x-ray peak breadths was used to infer only a slight increase in the grain size of the sample annealed for 3072 h (∼60 nm) compared to the samples annealed for short times (∼40 nm). TEM studies revealed a distribution of grain sizes in the material with an average grain size of 42 nm for the 3072 h annealed sample. Samples annealed at higher temperatures exhibited the additional minority phases (Fe,Co)3Zr and ZrO. XRD analysis of the samples annealed for extended times at 600 °C do not indicate any formation of these phases, even in the sample annealed for 3072 h. TEM indicates the formation of minority phases, probably resulting from the devitrification of the amorphous grain-boundary phase and/or oxidation upon extended annealing. Differential scanning calorimetry results on alloys of composition (Fe0.5Co0.5)89Zr7B4 and (Fe0.65Co0.35)88Zr7B4Cu1 show primary crystallization temperatures of 495 and 480 °C, respectively.