A series of Fe-Ni alloy/Ni-ferrite composite nanofibers with average diameters of 60～70 nm were successfully fabricated using electrospinning combined with the hydrogen-thermal reduction method. The thermal decomposition behavior of electrospun precursor nanofibers and crystal structures, phase compositions, morphologies and magnetic properties of the resultant products were characterized by means of thermogravimetric and differential thermal analysis, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and vibrating sample magnetometer. It is found that both the preparation temperature of the pristine Ni-ferrite nanofibers and the reduction temperature have considerable influences on the phase compositions and magnetic properties of the corresponding reduction products. With increasing reduction temperature from 270 to 400 ℃ for the Ni-ferrite nanofibers prepared at 550 ℃, the content of Fe-Ni alloy in the reduction products increases gradually, the saturation magnetizations, remanences and coercivities increase initially, respectively reaching a maximum value of 203.0 emu·g^-1, 54.9 emu·g^-1 and 42.7 kA·m^-1 for the composite obtained after reduction at 320 ℃, with a mass fraction of about 82% for Fe-Ni alloy, and then decrease with a further increase in reduction temperature. As the preparation temperature of the pristine Ni-ferrite nanofibers decreases from 700 to 400 ℃, the saturation magnetizations of the corresponding products obtained at the same reduction condition (i.e., reduction at 300 ℃ for 1 h) increase gradually, while their remanences and coercivities show a trend of first increase and then decrease, and the reduction product of Ni-ferrite nanofibers synthesized at 500 ℃ has a maximum remanence and coercivity of 62.2 emu·g^-1 and 47.8 kA·m^-1, respectively. Compared to the pristine Ni-ferrite nanofibers, the prepared Fe-Ni alloy/Ni-ferrite composite nanofibers exhibit more excellent soft magnetic properties with much enhanced saturation magnetization due to introduction of Fe-Ni alloy phase. Furthermore, the composite nanofibers composed of three magnetic phases in crystallography show a good single-phase magnetic behavior, implying that these magnetic phases in composites are well exchange- coupled with each other. The observed changes in magnetic properties for the prepared composite nanofibers can be explained on the basis of the differences in intrinsic magnetic properties between Fe-Ni alloy and Ni-ferrite phases and the changes in grain size, phase composition and magnetic interactions. These novel magnetic composite nanofibers have potential application prospects in flexible magnets, sensing devices, catalysis, microwave absorption and electromagnetic interference (EMI) shielding.

Key words: Ni-ferrite, Fe-Ni alloy, composite nanofibers, magnetic properties, electrospinning, hydrogen-thermal reduction