Improvement in Structural and Magnetic Properties of Electrospun Ni 1-xCuxFe 2 O 4 Nanofibers

A series of Ni1-xCuxFe2O4 (0.0 ≤ x ≤ 1.0) nanofibers have been synthesized employing electrospinning method at 650 ̊C. The effect of Cu substitution on structural, morphology and magnetic properties of NiFe2O4 nanofibers is reported. The XRD analysis showed the formation of single-phase cubic spinel Ni-Cu ferrite and an increasing behavior of lattice constant. The surface morphology is characterized by SEM, it is investigated that nanofibers have uniform and continuous morphology. The VSM results showed Cu substitution played an important role in magnetic properties of Ni1-xCuxFe2O4. The saturation magnetization (Ms) decreases linearly with increasing Cu content, while coercivity (Hc) has slowly decreased before x ≤ 0.5, and then sharply increased to 723.9 Oe for x = 1.0. The magnetic properties of Ni1-xCuxFe2O4 can be explained in Neel’s model, cation distribution and shape anisotropy.

In earlier work, with increasing copper content the saturation magnetization of Ni 1-x Cu x Fe 2 O 4 microparticles prepared by double-sintering method decreases linearly, whereas coercivity decreases up to x = 0.6 and then increases [10].
The effect of Cu substitution on chemical states of surface ions and surface composition in Ni 1-x Cu x Fe 2 O 4 spherical nanoparticles prepared by sol-gel combustion method [11], and the effect of Cu 2+ substitution on electromagnetic properties of Ni 1-x Cu x Fe 2 O 4 nanoparticles is well studied [12]. Similar structure and magnetic properties are obtained for Ni 1-x Cu x Fe 2 O 4 nanostructures prepared by citrate-gel auto combustion technique [13], microwave-induced combustion [14], co-precipitation method [15] [16], and ceramic method [17] [18]. Compared to commercial mechanical process, electrospinning represents a simple, effective and convenient method for generating 1D nanofibers [19]. One of the most important advantages of electrospinning is the ability to control the component of composites, morphology and diameter of nanofibers. Electrospun nanofibers have been applied in a broad range of applications owing to their large specific surface area, high aspect ration, and good dimensional stability [20].
In this paper, a series of Ni

Characterization
The calcined nanofibers were characterized by X-ray diffraction (XRD) pattern

Structural Studies
The effect of Cu substitution on structural and morphology of NiFe 2 O 4 nanofi- where d is the interplanar distance and h, k, l is the Miller indices of plane [21]. The average crystallite size D is calculated using Debye-Scherrer's formula with respect to peak plane (311). The values of a and D are extracted and listed in Ta

Morphological Studies
The morphology of Ni 1-x Cu x Fe 2 O 4 nanofibers were investigated by SEM and TEM. Figure 2 shows the SEM images of Ni 1-x Cu x Fe 2 O 4 nanofibers calcined at 650˚C. It can be seen that all samples remained as continuous and randomly oriented morphology, the diameter of nanofibers is about 50 -60 nm. The surface of nanofibers is smooth when x less than 0.3, rough surface were observed after x increasing to 0.5 and 0.7, the surface of CuFe 2 O 4 nanofibers(x = 1.0) consists of small open porosity. A similar result was also observed in Ni 0.5-x Cu x Zn 0.5 Fe 2 O 4 nanofibers with x = 0.0 -0.5 prepared by electrospinning [22]. The Cu 2+ content has some influences on morphology of Ni 1-x Cu x Fe 2 O 4 nanofibers. Figure 3 shows the typical TEM (a-b) and HRTEM (c-d) images of Ni 0.5 Cu 0.5 Fe 2 O 4 nanofibers, respectively. From Figure 3(a) and Figure 3(b), it can be seen that these nanofibers exhibited a fibrous, continuous and good dispersity morphology, and a nanofiber is composed of randomly aligned nanoparticles. This is well consistent with that observed from SEM ( Figure 2). In HRTEM image of Ni 0.5 Cu 0.5 Fe 2 O 4 nanofibers (Figure 3(c)), the crystalline phase has well-resolved lattice fringes. The value of distance between the adjacent

FT-IR Studies
The ideal spinel structure consists of two sub-lattices, namely tetrahedral sites

Magnetic Studies
The magnetic structure of spinel ferrite is ferrimagnetic, the magnetic moments of A and B sites are coupled antiparallel to each other. There are twice as many B sites filled, so there is a net magnetic moment equal to the difference between the two sites. The magnetization behavior of spinel ferrite can be understood in Neel's model. In Ni 1-x Cu x Fe 2 O 4 nanofibers, the composition and cation distribution among the A and B sites will influence the magnetic properties of samples.  According to Neel's model, the magnetic moment per formula is expressed as: The variation of H c with Cu 2+ contents can be understood on basis of domain structure, anisotropy and critical diameter [26]. The initial decrease trend of H c (x ≤ 0.5) is due to the increase in crystallite size, which is observed in XRD results. This may be attributed to the magnetization mechanism which is a domain rotation process. The H c value of 723.9 Oe obtained for CuFe 2 O 4 nanofibers in present work is higher than the value of 93.7 Oe and 151.0 Oe of CuFe 2 O 4 nanoparticles prepared by double-sintering method and coprecipitation method, respectively [10] [23]. This values is also higher than H c = 625.0 Oe for CuFe 2 O 4 nanofibers prepared by electrospinning method [27]. The high value of H c in this paper may be attributed to the magnetocrystalline and shape anisotropy.
The magnetocrystalline anisotropy of CuFe 2 O 4 nanofibers is about 0.6 × 10 5 erg cm −3 , while shape anisotropy is calculated to be k s = 1.7 × 10 5 erg cm −3 using the measured M s (31.8 emu g −1 ) [28], which is higher than magnetocrystalline anisotropy. Therefore, the high H c of CuFe 2 O 4 nanofibers mainly come from shape

Conclusion
The class of metal ions and cation distribution among A and B sites will affect the magnetic properties of spinel ferrite. Nanofibers morphology produced a difference characteristic compare with nanoparticles ones. In this paper,