Morphology And Magentic Properties Of Magnetic Nanofibers Prepared By Electrospinning Method

Recently, with the rapid development of nanoscale science and technology, one-dimensional nanomaterials have been paid much attention because of their distinctive physical and chemical properties in basic scientific research and potential technology applications. Magnetic nanofibers-like or nanowires-like materials have attracted a great deal of interest due to their advantages of high aspect ratio, large surface area and remarkable shape anisotropy. There are several methods to prepare nanofibers, such as AAO template, hydrothermal method, phase separation, etc. Although these methods have been able to prepare magnetic nanofibers with good morphology and magnetic properties, there are some disadvantages, such as manipulation complex, high cost of raw materials, difficult of industrial mass production. Electrospinning is a simple and versatile technique for generating a rich variety of nanofibers made of polymers and composites, the diameter of nanofibers can be controlled form micro-to nano scales.The electrospinning method has been applied in many fields, such as tissue engineering, biomedical, spinning, energy, environmental protection. However, only a little attention is paid to the application of magnetic nanofibers prepared by electrospinning. In this paper, spinel ferrite, hexagonal ferrite, metal Fe and Ni nanofibers were fabricated by electrospinning. The phase strucure, morphology, composition, magnetic properties of static and high field, high frequency characteristics were investigated by XRD, SEM, TEM, VSM and vector network analyzer, and the effect of process parameters on crystal structure and magnetic properties was studied.1. The effect of heating rate on morphology and crystal structure of CoFe2O4naofibers was systematically studied. The results show that low heating rate is a key condition for maintaining good fibrous morphology, however, a destroyed ones is attributed to higher heating rate. All heating rate retain at1-2℃/min in this article.2. Sintering temperature can affect the crystal structure, morphology and magnetic properties of magnetic nanofibers. For CoFe2O4naofibers, the morphology changes from a smooth surface and continuous straight fibers when sintering temperature is lower than900℃to a rough surface and winding ones when the sintering temperature increases to900and1000℃; the nanofibers calcined at500℃has the largest coercivity value, Hc=772.8Oe. For permanent magnets SrFe12O19nanofibers, the ones calcined at700℃has the largest coercivity value, Hc=5508Oe. This is attributed to single domain for magnetic nanofibers calcined at lower temperature. With the sintering temperature increasing, the magnetic crystal will grow and the domain structure will transform from a single domain to a multi domain state.3. The effects of Zn2+and Cu2+ions substitution on crystal structure, morphology and magnetic properties of CuFe2O4and NiFe2O4nanofiber were systematically studied, and the lattice structure changes from inverse spinel structure to normal spinel structure, or changes from inverse spinel structure to the other inverse ones. Zn2+ions substitution significantly improves the morphology of CuFe2O4nanofibers. The surface of Cu1-xZnxFe2O4with x greater than0clearly becomes smoother and denser. The effects of Zn2+and Cu2+ions substitution on magnetic properties are due to the change of species and quantity of metal ions on A and B sites, which results in the change of super-exchange interaction between A and B sites. For Cu0.6Zn0.4Fe2O4nanofibers, hystersis loops of the nanofibers measured with magnetic field parallel and perpendicular to sample plane exhibit that the magnetic easy axis along the long axis of the nanofibers. However, it is found that the shape anisotropy of nanofibers is not domination the effective anisotropy according to results of calculation for the demagnetization energy and effective anisotropy field. In other words, the easy magnetization direction is not perfectly along the long axis of the nanofibers, which is evidenced by Mossbauer spectra of Cu1-xZnxFe2O4samples. This is due to the dipolar interaction between nanoparticles of which the nanofibers are composed.4. Fe3O4nanofbers were fabricated by electrospinning, and the effect of reduction conditions on the crystal structure and morphology of Fe3O4nanofbers is systematically studied. The obtained Fe3O4nanofbers have a high coercivity,Hc=188.4Oe, which is attributed to the shape anisotropy come from the high aspect ratio of nanofibers samples. The magnetoresistances (MR) at room and low temperatures of Fe3O4nanofbers were measured by interdigitated electrodes, and the conductive mechanism is explained in terms of the tunneling of adjacent grain boundary.5. High magnetocrystalline anisotropy FePt nanofibers were fabricated by electrospinning, and the high coercivity is attributed to the combined interaction of magnetocrystalline anisotropy and shape anisotropy, Hc=10.27kOe. The double-resonance behavior of microwave magnetic permeabiligy is observed,4.0GHz and12.5GHz. The natural resonance peak happens at4.0GHz for the contribution of shape anisotropy according to Kittel formula projections, and the second resonance peak originates from exchange resonance effect. A minimum RL reaches-35.4dB at the matching thickness of8.4mm and the matching frequency of1.3GHz.