Width-controlled M-type hexagonal strontium ferrite (SrFe12O19) nanoribbons with high saturation magnetization and superior coercivity synthesized by electrospinning

Width-controlled M-type hexagonal SrFe12O19 nanoribbons were synthesized for the first time via polyvinylpyrrolidone (PVP) sol assisted electrospinning followed by heat treatment in air, and their chemical composition, microstructure and magnetic performance were investigated. Results demonstrated that as-obtained SrFe12O19 nanoribbons were well-crystallized with high purity. Each nanoribbon was self-assembled by abundant single-domain SrFe12O19 nanoparticles and was consecutive on structure and uniform on width. PVP in the spinning solution played a significant influence on the microstructure features of SrFe12O19 nanoribbons. With PVP concentration increasing, the ribbon-width was increased but the particle-size was reduced, which distributed on a same ribbon were more intensive, and then the ribbon-surface became flat. The room temperature magnetic performance investigation revealed that considerable large saturation magnetization (Ms) and coercivity (Hc) were obtained for all SrFe12O19 nanoribbons, and they increased with the ribbon-width broadening. The highest Ms of 67.9 emu·g−1 and Hc of 7.31 kOe were concurrently acquired for SrFe12O19 nanoribbons with the maximum ribbon-width. Finally, the Stoner-Wohlfarth curling model was suggested to dominate the magnetization reverse of SrFe12O19 nanoribbons. It is deeply expected that this work is capable of opening up a new insights into the architectural design of 1D magnetic materials and their further utilization.

Recently, considerable progress in synthesizing of one-dimensional (1D) magnetic nanostructures, such as nanofibers, nanotubes and nanoribbons, has been inspiring researchers to either investigate their magnetic-domain configurations and magnetization reversals or develop their electromagnetic nanoscale device applications [11][12][13][14] . As is known to all, 1D magnetic nanostructures possess remarkable geometrical limitations being comparable to the critical magnetic lengths such as exchange length and domain wall width and can effectively overcome the serious aggregation of nanoparticles [15][16][17] . It means that 1D permanent magnetic nanostructures may get more remarkable magneto-crystalline anisotropy and shape anisotropy 18 . Hence they are capable of offering more creativity for magnetic media, especially the perpendicular magnetic recording media. Jaya Sarkar has pointed out that nanowire arrays potentially enable generating bit densities in excess of 100 Gbit/in 2 in 2007 19 . Furthermore, the theoretical recording density of 500 Gbit/in 2 ~ 1 Tbit/in 2 may be achieved in the quantum magnetic disks with perpendicular recording pattern 20 . Thus far, a few techniques containing hydrothermal route 21 , electrospinning 2,22 and template approach 23 have been used to synthesize 1D SrFe 12 O 19 nanostructures. What needs to be stressed is that electrospinning technique is more simple and flexible for producing continuous 1D nanostructures of various materials by using a high voltage dc source 24,25 . The diameter or width of electrospun products can be ranged from tens of nanometers to several micrometers. Hence it has got increased attention for many years in a wide range of biomedical and industrial applications, such as drug delivery 26 , wound dressing 27 , air filtration 28 , water purification 29 , sensors 30 and among others 31,32 . Out of the numerous 1D nanostructures synthesized via electrospinning, interestingly, nanoribbons are slightly different from others (nanotubes and nanofibers) because the former has a rectangular cross section but the later have a round cross section. Moreover, nanoribbons also could be considered as a development by cutting a finite-width slice from the 2D nanosheets. Therefore, it is urgently expected that SrFe 12 O 19 nanoribbons could perform some more interesting magnetic properties.
For the first time in this content, width-controlled SrFe 12 O 19 nanoribbons were synthesized via a polymer-sol assisted single-spinneret electrospinning route followed by heat treatment in air. Their chemical component, crystalline structure, morphologies and room temperature magnetic properties were investigated in detail. All of the as-synthesized SrFe 12 O 19 nanoribbons have considerable saturation magnetization and superior coercivity. Moreover, it demonstrated that the ribbon-width and particle-size can be availably modulated by the PVP concentration in the spinning solution.

Experimental Section
Materials. Polyvinylpyrrolidone O 19 nanoribbons were synthesized via a polymer-sol assisted single-spinneret electrospinning route followed by heat treatment. A typical preparation is as below. Firstly, 0.018 g of anhydrous strontium nitrate (Sr(NO 3 ) 2 ) and 0.368 g of iron nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O) were quickly dissolved in 1.5 g of deionized water. Subsequently, a certain amount of PVP powders (0.4 g, 0.5 g and 0.6 g as needed) and 2.4 g C 2 H 5 OH were added together into the above red nitrate solution under vigorous stirring and equilibrated for overnight to acquire a homogenous viscous solution. Of which, the PVP concentrations of these obtained solutions were about 8.5%, 10.4% and 12.3%, respectively. Secondly, right amount of the obtained spinning solution was transformed to a glass syringe equipped with an ordinary stainless needle (the tip was flat and the inner diameter was about 0.4 mm) for electrospinning. The needle was connected with a positive voltage of 15 kV and the aluminum collector was grounded. The vertical distance between the tip of needle and the collector plane was controlled at about 20 cm. Moreover, the spinning solution was withdrawn at a rate of 0.3 mL/h by a micro-injection pump. The whole electrospinning process was conducted at room temperature (about 25 °C) in air. Thirdly, the collected precursor PVP/SrFe 12 O 19 nanoribbons were kept in a drying oven for several hours, and then were subjected to annealing at 800 °C for 2 h in a muffle furnace in air, respectively. Then SrFe 12 O 19 nanoribbons were obtained. The heating and cooling rates were both 1 °C/min. For simplicity, we use the S1, S2 and S3 to label the as-obtained SrFe 12 O 19 nanoribbons resulted from the spinning solutions with PVP concentrations of 8.5%, 10.4% and 12.3%, respectively.
Characterization. The morphological and microstructural characterizations of the as-prepared SrFe 12 O 19 nanoribbons were performed by applying field emission scanning electron microscopy (FESEM, Hitachi S-4800) and transmission electron microscopy (TEM, Tecnai TM G 2 F30, FEI) equipped with an energy dispersive X-ray spectroscopy (EDX). The element and phase component and crystalline structure were determined using powdered X-ray diffraction (XRD, Analytical X'Pert Pro) with Cu-Kα radiation (λ = 0.15406 nm) and high-resolution transmission electron microscopy (HRTEM). Room temperature

Results and Discussion
The composition and microstructure of the prepared SrFe 12 O 19 nanoribbons. The element and phase component, crystalline structure of as-annealed samples (S1-S3) correspondingly originated from their spinning solutions have been demonstrated by carrying out EDX and XRD analysis. Figure 1a displays their EDX patterns. Targeted elements of Sr, Fe and O are simultaneously detected for all samples. The calculated mole ratios of Sr: Fe are basically equivalent to the stoichiometric ratio of SrFe 12 O 19 . The detected C and Cu elements should be ascribed to the carbon-coated copper grids used for the TEM measurement. Figure 1b shows their XRD patterns. All labeled diffraction peaks in the range of 20 o ~ 70 o could be well-indexed as (110), (008), (107), (114) crystallographic planes and etc., respectively, and definitely confirm the formation of hexagonal SrFe 12 O 19 (PDF#33-1340) crystallites with a space group of P63/mmc 2 . It means that all samples are well crystallized into SrFe 12 O 19 but polycrystalline. No peaks of other additional phases are detected, indicating that these SrFe 12 O 19 samples are highly purified. Based on these diffraction peaks, the lattice parameters (a, c and V) of S1-S3 are calculated (Table 1) by using the follow equations for the hexagonal crystal system 33 :

Lattice parameters
Grain size where θ is the diffraction angle, λ ≈ 1.5406 Å is X-ray wavelength, h, k and l are Miller indexes. The calculated values are basically similar to the cell parameters (a = 5. 869 Å, c = 23.007 Å and V = 686.307 Å 3 ) of SrFe 12 O 19 nanoparticles reported by R. K. Sahu 33 . It is also observed that there is a slight line broadening of some characteristic peaks from S1 to S3, which indicates a reduction in grain size. Using Debye-Scherrer formula 34 : where K = 0.9 is a constant, λ ≈ 1.5406 Å is X-ray wavelength and β is full-width at half-maximum of diffraction peak at 2θ, the average crystallite sizes of S1-S3 were also given in Table 1. It suggests that PVP has an influence on crystallization and growth of SrFe 12 O 19 crystallites in the resultant nanoribbons. Figure 2 displays the representative SEM, TEM and HRTEM images of the as-prepared SrFe 12 O 19 nanoribbons (S1-S3). Clearly, as revealed by the low-magnification SEM images (Fig. 2a,e,i), all samples present a novel ribbon-like structure constructed by a large number of interconnecting SrFe 12 O 19 nanoparticles, which will be further confirmed later. Each nanoribbon is uniform on width and continuous on structure with several micrometers along the long-axis direction. By counting all nanoribbons in the whole SEM image area, the average widths (W) are estimated to be about 484 ± 15, 812 ± 10 and 1099 ± 18 nm for S1, S2 and S3 in sequence, indicating that the ribbon-width broadens with PVP concentration increasing in their spinning solutions. Figure 2b,f,j show the large-magnification SEM images of several typical nanoribbons selected from S1-S3. Some pores can be observed obviously between SrFe 12 O 19 nanoparticles on nanoribbons. It is easy to deduce that these pores are caused by PVP decomposition and SrFe 12 O 19 nanoparticles crystallization during the heat treatment. For a nanoribbon from S1 to S3, however, these pores shrink gradually and the reduced SrFe 12 O 19 nanoparticles distribute more and more densely, which make the ribbon-surface smooth and neat. Observation from the typical TEM images for S1-S3 (Fig. 2c,g,k), although these SrFe 12 O 19 nanoparticles located in the same nanoribbon are not of uniform on size, they have remarkable polyhedron sections. This indicates that all samples have a good crystallinity being in accordance with the XRD results. The average sizes (d) of SrFe 12 O 19 nanoparticles are also estimated to be about 131 ± 3 nm for S1, 94 ± 2 nm for S2 and 76 nm ± 3 for S3, respectively.   (Fig. 2m). Therefore, it is concluded that these novel SrFe 12 O 19 nanoribbons are self-assembled by abundant SrFe 12 O 19 nanoparticles, and that PVP concentration in the spinning solutions plays a significant role in controlling their ribbon-width and particle-size.
The formation mechanism and ribbon-width controlling of the prepared SrFe 12 O 19 nanoribbons. To obtain a visual understanding for the formation mechanism of SrFe 12 O 19 nanoribbons, a possible schematic diagram is displayed in Fig. 3. The whole preparation can be segmented into electrospinning and heat treatment. Based on the earlier researches, one can be summarized that the novel ribbon-like structure of SrFe 12 O 19 nanoribbons is produced during electrospinning process 25,[35][36][37][38] . When the spinning solution arrived up to the spinneret tip, a Taylor-cone shape colloidal droplet (Fig. 3a) formed under the coaction of electrostatic field force and surface tension. With the accumulation of surface charges, the electrostatic field force eventually overcomes the surface tension and a columnar flow-jet (Fig. 3b) with a circular cross section (Fig. 3c) is ejected from the Taylor-cone. Some works have pointed out that the solvents evaporation starts rapidly from the jet surface and causes the jet to become extremely unstable 39,40 . So, the PVP concentration of jet surface sol increases sharply and is much larger than that of jet internal sol. If the PVP concentration of the surface sol is increased to a coagulated critical value at a relatively earlier stage, in which the jet travels steadily and extends along a single straight line 41 , the PVP-sol transforms to PVP-gel. Namely, the viscous PVP sol shell begins to freeze and transforms to an elastic skin 38,41 . But the internal sol is still viscous flow. Once the elastic behavior of skin overcomes viscous behavior of internal sol, the columnar flow-jet is immediately buckled into a flattened flow-jet (Fig. 3d) with approximately rectangular cross section (Fig. 3e) 38,42 . Subsequently, the jet is subjected an unstable stage with a series of bending instabilities and anisotropic shrinkage, and is finally elongated with or without branching/splitting 39,40,43 and solidified to PVP/SrFe 12 O 19 composite precursor nanoribbons (Fig. 3f). Figure 3g shows the SEM image of the collected PVP/SrFe 12 O 19 composite precursor nanoribbons for S1. When electrospinning is finished, the as-spun PVP/SrFe 12 O 19 precursor nanoribbons are annealed at the temperature of 800 °C in air, the PVP is degraded completely and the Sr 2+ and Fe 3+ ions are compounded to SrFe 12 O 19 . Finally, the SrFe 12 O 19 nanoribbons (Fig. 3h,i) are constructed by crystalline SrFe 12 O 19 nanoparticles. Figure 4a,d,g show the digital photographs grabbed at several stable moments in the spinning process of PVP/SrFe 12 O 19 precursor nanoribbons for S1-S3, which can be utilized to explore vividly the reason why the SrFe 12 O 19 nanoribbon width is controlled by PVP concentration in the spinning solutions. Earlier literature has figured out that the electrospun nanofiber diameter mainly depends on the spinneret aperture, polymer (PVP) solution viscosity, feeding rate and voltage supplied 44 . In our work, the unified parameters (a spinneret with an internal diameter of ~0.4 mm, a rate of ~0.3 mL/h and a voltage of ~15 kV) were applied in the electrospinning except the solution viscosity. Hence a higher spinning solution viscosity results in a larger nanoribbon width. When a polymer is dissolved in the mixed solvents of water and ethanol, the solution viscosity is proportional to the polymer concentration 45 . Thus the width of SrFe 12 O 19 nanoribbons ultimately depends upon the PVP concentration. It should be noted that PVP concentration has a significant influence on the stable stage of electrospinning. Based on the above analysis, besides, the PVP/SrFe 12 O 19 precursor nanoribbons (Fig. 4b,e,h) are formed at the stable stage of jet traveling and then elongated and narrowed at the unstable stage. Herein, we named a "transition point" at the junction of the stable and unstable stages. For S1-S3, their own "transition points (I, II and III)" are respectively marked by cyan, green and yellow circles. It is observed that the jet directions for S1-S3 at their stable stages were respectively deviated a certain angle (ϕ ) from the horizontal direction and they are measured to be small but noticeable change on the PVP concentration increased from S1 to S3, as revealed in Fig. 4j. During the electrospinning, these drift angles should be ascribed to disturbance resulted from the breakage of equilibrium between the surface intension and electric field force. Of which, the surface intension is always used to overcome the electric field force to maintain the equilibrium of flow-jet. If the PVP concentration is bigger, the surface intension is increased and the equilibrium resisting ability of jet is enhanced and then the drift angle is reduced. In our case, the difference between the largest ϕ for S1 and the smallest ϕ for S3 is about 6 o , which can cause a difference between the total jet traveling distances of S1-S3. But the difference is small and could be ignored briefly because the actual motion of jet is very complete. Thus we could assume that the jets for S1-S3 can travel an approximately equivalent distance (L) from the tip to collector. For simplicity sake, the L can be considered as the sum of lengths of the stable (L s ) and unstable (L us ) stages, i.e., L = L s + L us . For S1-S3 with PVP concentration in the jet of about 8.5%, 10.4% and 12.3 wt%, the "transition point" shift gradually towards to collector, and their lengths L s are measured to be about 2.7, 5.5 and 8.9 cm, respectively. Therefore, L s is increased with PVP concentration increasing from S1 to S3 (Fig. 4k), i.e., L s-S1 < L s-S2 < L s-S3 . In contrast, the length L us of unstable stage is reduced correspondingly, i.e., L us-S1 > L us-S2 > L us-S3 . Namely, the distance of ribbon elongation and narrowing is reduced from S1 to S3. As a result, the average width of as-spun PVP/ SrFe 12 O 19 precursor nanoribbons (Fig. 4b,e,h) for S1-S3 increases gradually as well as that of the final SrFe 12 O 19 nanoribbons (Fig. 4c,f,i). For the polymer-sol assisted electrospinning technique, furthermore, polymer PVP molecular chains construct the main frame of the as-spun precursor nanoribbons. And then the metal ions (Sr 2+ and Fe 3+ ions) locate in the interspaces among the PVP frame. That is, the PVP frame acts as a stabilizer or a capping for metal ions. When the PVP concentration reaches a certain value, its frame can effectively enable restraining the growth of SrFe 12 O 19 nanoparticles during annealing 45 . To understand visually, we consider the interspaces between PVP molecular chains as cells for metal ions. A cell is defined as a unit. If the PVP content or concentration is larger, the units for metal ions anchoring are more but smaller and denser (Fig. 4b,e,h). And the nucleation of SrFe 12 O 19 grains is then more sufficient during annealing. Although these SrFe 12 O 19 grains are tighter, they can't excessively swallow each other following as the Ostwald-ripening theory 46 because of the PVP restraining effect. Therefore, the resultant SrFe 12 O 19 nanoparticle on a nanoribbon originated from the spinning solution with higher PVP concentration are smaller but distribute intensively, which is just like S3.
The room temperature magnetic performance of the prepared SrFe 12 O 19 nanoribbons.   O 19 nanoparticles and between nanoribbons. Additionally, it is clearly discovered that there is a progressive increase of M s , M r and H c from S1 to S3. Just like what has been mentioned in the introduction, although the magnetic properties of assembly nanostructures depend on the particle size and shape of nanoparticles, they are also strongly influenced by the inter-particle interactions 54 . In our case, particularly, the SEM and TEM characterizations have revealed that SrFe 12 O 19 nanoparticles distribute more and more intensively on nanoribbons from S1 to S3. Thus the amount of magnetic moment per unit volume is increased and then M s is increased. Moreover, the exchange-interaction of SrFe 12 O 19 nanoparticles are increased and then M r and H c are increased, too 54,55 . The understanding of magnetization reverse is very important to magnetic materials for their technic applications 56 . According to Stoner-Wohlfarth nucleation modes of the micro-magnetism theory, the magnetization reverse mechanism of a nano-or micro-scale system is depended on the coherent radius R coh . For 1D and 0D magnetic structures, R coh = 3.655L ex and 5.099L ex , respectively. For R < R coh , the magnetization reverse behavior is realized by coherent rotation, whereas for R > R coh , that is realized by curing 1,57 . For a ferromagnetic material such as SrFe 12 O 19 , L ex is the exchange length following the equation 17

Conclusion
Herein, novel SrFe 12 O 19 nanoribbons with high crystallinity and purity were successfully synthesized for the first time by using PVP sol-gel assisted electrospinning followed by heat treatment, and they were characterized by EDX, XRD, FESEM, TEM and VSM in detail. Each nanoribbon has been demonstrated to be structural continuous and width uniform and be assembled-well by abundant single-domain SrFe 12 O 19 nanoparticles along the long-axis direction. Besides, it was revealed that the PVP concentration in the spinning solution has a significant influence on the ribbon-width, surface-flatness and particle-size of SrFe 12 O 19 nanoribbons, and the potential mechanism was explored systematically.