In this study, the raw materials including Ni(NO₃)₂・6HO₂, Cu(NO₃)₂・3HO₂, Fe(NO₃)₃・9HO₂ and PVP (polyvinylpyrrolidone, Mw ≈ 1,300,000) were of analytical grade and purchased from Tianjin Guangfu. DMF (N, N-Dimethylfor- mamide, 99.7% purity, Tianjin Guangfu, China) and ethanol (100% purity) were used as solvents. In the solution preparation, 0.1 g of PVP was dissolved in mixture of ethanol and DMF with a weight ratio of 1:1, followed by magnetic stirring for 2 h to ensure the dissolution of PVP. Then Ni(NO₃)₂・6HO₂, Cu(NO₃)₂・3HO₂, Fe(NO₃)₃・9HO₂ with a molar ratio of (1 − x):x:2 were added into the mixture solution. After having been stirred for 2 h, the homogeneous viscous solution was transferred into a plastic syringe in which a needle made of stainless steel was connected to a high-voltage equipment. The applied voltage was kept at +15 kV during the electrospinning process. The nanofibers were collected on a piece of aluminum foil about 15 cm below the tip of needle. All electrospinning processes were carried out at room temperature. The collected Ni_1-xCu_xFe₂O₄/PVP precursor nanofibers were dried at 80˚C for 3 h, and calcined at 650˚C for 3 h in ambient atmosphere with a heating rate of 1˚C/min.

The calcined nanofibers were characterized by X-ray diffraction (XRD) pattern using CuKα radiation with λ = 0.15418 nm (PANalytical diffractometer). The scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, Tecnai^TM G² F30, FEI) were employed to analyze morphology and microstructure of samples. Infrared spectra were obtained using Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700) in the 400 - 4000 cm⁻¹ range. The magnetic properties of nanofibers were measured at room temperature using vibrating sample magnetometer (VSM. Lakeshore 7403, USA) with a maximum applied field of 12 kOe.

The effect of Cu substitution on structural and morphology of NiFe₂O₄ nanofibers was been studied by XRD, SEM and TEM. The XRD patterns of synthesized Ni_1-xCu_xFe₂O₄ nanofibers calcined at 650˚C for 3 h are shown in Figure 1. All of main diffraction peaks are indexed as the cubic spinel structure, no second phase can be detected. The position of peaks is slightly shifted to lower angle with increasing Cu²⁺ content. The diffraction peaks of samples for x ≤ 0.5 correspond to

NiFe₂O₄, while the diffraction peaks of samples for x ≥ 0.7 correspond to CuFe₂O₄. The lattice constant a is calculated by using equation:

a = d ( h 2 + k 2 + l 2 ) , (1)

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 Table 1. From Table 1 it can been seen that lattice constant increases with increasing Cu²⁺ content. The increased a may be explained on the bigger ionic radii of Cu²⁺ ions (0.72 Å) than Ni²⁺ ions (0.69 Å), indicating Cu²⁺ ions can be effectively built into NiFe₂O₄ lattice. The average crystallite size increases initially with Cu²⁺ contents, the maximum D occurs at x = 0.5 (D = 24.1 nm), and then decreases. The variation of a and D with Cu²⁺ content mainly attributed to the Cu²⁺ ions insert into the cubic spinel structure, the similar trends were observed in Ni_1-xCu_xFe₂O₄ nanoparticles prepared by sol-gel combustion method [11] .

The morphology of Ni_1-xCu_xFe₂O₄ nanofibers were investigated by SEM and TEM. Figure 2 shows the SEM images of Ni_1-xCu_xFe₂O₄ 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 CuFe₂O₄ nanofibers(x = 1.0) consists of small open porosity. A similar result was also observed in

Ni_0.5-xCu_xZn_0.5Fe₂O₄ nanofibers with x = 0.0 - 0.5 prepared by electrospinning [22] . The Cu²⁺ content has some influences on morphology of Ni_1-xCu_xFe₂O₄ nanofibers. Figure 3 shows the typical TEM (a-b) and HRTEM (c-d) images of Ni_0.5Cu_0.5Fe₂O₄ 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_0.5Cu_0.5Fe₂O₄ nanofibers (Figure 3(c)), the crystalline phase has well-resolved lattice fringes. The value of distance between the adjacent

lattice is 2.95 Å, which is in agreement with the XRD patterns. As shown in Figure 3(d), selected area electron diffraction (SEAD) of Ni_0.5Cu_0.5Fe₂O₄ nanofibers consists of multiple intense rings, indicating that the sample has a polycrystalline nature.

The ideal spinel structure consists of two sub-lattices, namely tetrahedral sites (A) and octahedral sites (B). Different charge combinations of metal cations are distributed in A and B sites. Therefore, the magnetic properties of spinel ferrite are to a large extent determined by the class of metal ions and cation distribution among the A and B sites. In Ni_1-xCu_xFe₂O₄ nanofibers, the replacement of Ni²⁺ ions with Cu²⁺ ions at B sites will influence the magnetic properties of samples. FT-IR spectra is usually assigned to the vibration of ions in crystal lattice, which can been used to confirmed the positions of Ni²⁺, Cu²⁺, and Fe³⁺ ions in spinel structure. The vibrating sample magnetometer is used to measure the magnetic properties of samples.

Figure 4 shows the typical FT-IR spectra of Ni_1-xCu_xFe₂O₄ nanofibers recorded

between 4000 and 400 cm⁻¹. In the range of 1000 - 400 cm⁻¹, two main absorption bands of ferrite are appear. The absorption band υ₁ = 580 cm⁻¹ is assigned to the stretching vibration of tetrahedral complexes (Fe³⁺-O²⁻), and the absorption band υ₂ = 400 cm⁻¹ is attributed to the octahedral complexes (Fe³⁺-O²⁻). The peak intensity of υ₁ decreases with increasing Cu²⁺ contents, while the position band is shifted to lower frequencies. Synchronously, the intensity and position of υ₂ changed slightly with x. Similar results are observed in Ni_1-xCu_xFe₂O₄ nanoparticles prepared by sol-gel combustion method [11] [23] . The difference in band position of υ₁ and υ₂ can be related to the difference in Fe³⁺-O² bond lengths at A sites and B sites. It was found that the Fe-O distance at A sites (1.89 Å) is smaller than that of the B sites (2.03 Å) [24] [25] . When Ni²⁺ ions is replaced by Cu²⁺ ions, due to charge imbalance some Fe³⁺ ions shift from A sites to B sites, making the Fe³⁺-O² stretching vibration in greater. So the decrease in peak intensity of υ₁ with increasing Cu²⁺ content is mainly attributed to the change in Fe³⁺-O² bands.

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-xCu_xFe₂O₄ nanofibers, the composition and cation distribution among the A and B sites will influence the magnetic properties of samples. Figure 5 shows magnetic hysteresis loops for Ni_1-xCu_xFe₂O₄ nanofibers measured at room temperature, the values of saturation magnetization (M_s), remanent magnetization (M_r) and coercivity (H_c) are calculated from loops and given in Table 1. From the figure it is noticed that the value of M_s decreases linearly with increasing Cu²⁺ content, while M_r gradually decreases up to x = 0.7, then increases small for x = 1.0. The H_c decreases up to x = 0.5 with x, after it sharply

increase to 723.9 Oe for x = 1.0.

According to Neel’s model, the magnetic moment per formula is expressed as:

μ B = M B ( x ) − M A ( x ) , (2)

where M_B and M_A are magnetic moments of B and A sites in μ_B. It is well know NiFe₂O₄ is an inverse spinel structure, all Ni²⁺ ion and a Fe³⁺ ion occupy B sites, other Fe³⁺ ion occupy A sites. CuFe₂O₄ is a partial inverse spinel structure with 85% Cu²⁺ at B sites, other 15% at A sies [25] . The magnetic moment of Cu²⁺ ions (1.0 μ_B) is smaller than Ni²⁺ ions (2.3 μ_B) [23] . The Cu²⁺ ions are substituted instead of Ni²⁺ ions result in a decrease in net magnetic moment of samples. A part of Cu²⁺ ions occupy A sites leading to a migration of Fe³⁺ ions from A sites to B sites. With increasing Cu²⁺ content, the super-exchange interaction between A and B sites decreases. Therefore, the decrease trend in saturation magnetization and remanent magnetization is agreed with that of an expected decrease in Ni_1-xCu_xFe₂O₄ nanofibers.

The variation of H_c with Cu²⁺ 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 CuFe₂O₄ nanofibers in present work is higher than the value of 93.7 Oe and 151.0 Oe of CuFe₂O₄ nanoparticles prepared by double-sintering method and coprecipitation method, respectively [10] [23] . This values is also higher than H_c = 625.0 Oe for CuFe₂O₄ 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 CuFe₂O₄ nanofibers is about 0.6 × 10⁵ erg cm⁻³, while shape anisotropy is calculated to be k_s = 1.7 × 10⁵ 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 CuFe₂O₄ nanofibers mainly come from shape anisotropy if we neglect the dipolar interactions between nanofibers. In Refer. 15, the breakdown of fibers morphology result to lower H_c than this work, while in this paper CuFe₂O₄ sample retain favorable nanofibers morphology.