- Nano Express
- Open Access
Uniaxial Magnetization Performance of Textured Fe Nanowire Arrays Electrodeposited by a Pulsed Potential Deposition Technique
© The Author(s). 2017
- Received: 19 June 2017
- Accepted: 9 November 2017
- Published: 21 November 2017
Textured ferromagnetic Fe nanowire arrays were electrodeposited using a rectangular-pulsed potential deposition technique into anodized aluminum oxide nanochannels. During the electrodeposition of Fe nanowire arrays at a cathodic potential of − 1.2 V, the growth rate of the nanowires was ca. 200 nm s−1. The aspect ratio of Fe nanowires with a diameter of 30 ± 5 nm reached ca. 2000. The long axis of Fe nanowires corresponded with the <200> direction when a large overpotential during the on-time pulse was applied, whereas it orientated to the <110> direction under the potentiostatic condition with a small overpotential. By shifting the on-time cathode potential up to − 1.8 V, the texture coefficient for the (200) plane, TC200, reached up to 1.94. Perpendicular magnetization performance was observed in Fe nanowire arrays. With increasing TC200, the squareness of Fe nanowire arrays increased up to 0.95 with the coercivity maintained at 1.4 kOe at room temperature. This research result has opened a novel possibility of Fe nanowire arrays that can be applied for a new permanent magnetic material without rare-earth metals.
Nanowire arrays with a high surface area show novel physical properties and are considered for applications in numerous industrial fields such as the fabrication of electronic and magnetic devices. The preparation processes include template-free methods [1–3] and template-based methods [4–7]. The template-based method using nanochannel structures such as ion track-etched foils or aluminum oxide membranes  is a promising technique to achieve precise length and diameter scales. In this method, the one-dimensional shape is directly adapted to the pore dimension of the membrane using electrodeposition techniques. Because of the possibilities of achieving high porosities and pore aspect ratios at low costs, anodic aluminum oxide (AAO) exhibits many advantages compared with other membrane materials.
Some researchers have reported that Ni, Co, and Fe nanowires can be electrodeposited in the nanochannel of membranes on metals [9, 10]. Hu et al. reported that Fe nanowire arrays can be electrodeposited by applying the direct current electrodeposition technique using acidic chloride bath . In their report, the effects of diameter and crystal orientation of Fe nanowires on the low temperature magnetic properties were examined. They revealed that the coercive force increased up to ca. 2 kOe at 5 K when decreasing the diameter of Fe nanowires down to ca. 30 ± 5 nm. They also found that the magnetic squareness of Fe nanowires with (200)-orientation was larger than that with (110)-orientation. Irfan et al. reported the effects of post-annealing on the magnetic properties of Fe nanowires with the aspect ratio of ca. 80–100, which are potentiostatically electrodeposited at − 1.1 V vs. SCE . Cornejo et al. also reported that Fe nanowires can be prepared using AC electrodeposition at a cell voltage of 15 V. They revealed that the length of Fe nanowires is approximately 3–5 μm and the aspect ratio is ca. 100 . The magnetic force of a permanent magnetic film increases with increase in the surface magnetic flux density. The magnitude of the surface magnetic flux density depends on the thickness of a magnetic film, while the magnetic coercive force of a permanent magnetic film increases with decrease in the diameter of magnetic crystal grains. Hence, for a permanent magnet application, a high aspect ratio of Fe nanowires is required in the industrial production line. However, in the previous works, the aspect ratio of Fe nanowires did not reach 1000. Recently, we have reported that Co nanowires with an aspect ratio of more than 2000 can be electrodeposited by a potentiostatic electrodeposition technique using AAO nanochannels with a large aspect ratio . In our previous study, to obtain the Co nanowires with a large aspect ratio, the electrolytic solution temperature was kept at higher than 80 °C and the cathodic overpotential was kept smaller than 0.2 V to enhance the growth of Co nanowires and to avoid hydroxide formation in the small AAO nanochannels. However, in the case of Fe electrodeposition, a high temperature solution will accelerate the hydroxide formation in the AAO nanochannels and inhibit the growth of Fe nanowires. Potentiostatic electrodeposition in a small cathodic overpotential range at room temperature will cause a small growth of Fe, while the pulsed potential deposition technique, which enables the achievement of a large cathodic overpotential, will prompt a large growth of Fe nanowires with a large aspect ratio. Hence, in this study, we fabricated Fe nanowire arrays with the aspect ratio up to 2000 and examined the effect of the deposition overpotential, which can be controlled by potentiostatic and pulsed potential deposition techniques, on the crystal orientation and magnetic performance of the nanocomposite films with Fe nanowires.
After the electrodeposition, AAO membranes were dissolved by immersing the samples in 5 mol L−1 NaOH aqueous solution to obtain Fe nanowires. In the alkaline solution, alternation of morphology or crystal orientation of Fe nanowires was not observed. Structure and crystallographic orientation of Fe nanowire arrays were characterized by field emission scanning electron microscopy (JEOL-JSM-7500FA, accelerating voltage 5 kV) and transmission electron microscopy (JEOL-JEM-ARM200F, accelerating voltage 200 kV) as well as by X-ray diffraction (XRD: Rigaku-SmartLab, Cu Kα source). Magnetic properties of Fe nanowire arrays were investigated using a vibrating sampling magnetometer (VSM) at room temperature. The hysteresis loops were obtained in the magnetic field which was applied along the perpendicular and in-plane directions with the external magnetic fields up to 10 kOe. The perpendicular direction corresponds to the long axis of Fe nanowires, which is perpendicular to the plane of a membrane film, while the in-plane direction corresponds to the short axis of Fe nanowires, which is in-plane with a membrane film.
Electrodeposition of Fe Nanowire Arrays
Structure and Crystallographic Orientation of Fe Nanowire Arrays
Equation (1) describes the analysis of the relative peak intensities dependent on I(h i k i l i ), i.e., the intensities observed from h i k i l i lattice planes of the sample, and I 0 (h i k i l i ) denotes the intensities of a standard Fe powder. N is the number of diffraction planes considered for the determination of TC. Figure 7b shows the relationship between the TCs calculated for (200) and (110) planes and the electrodeposition potential of Fe nanowire. Potentiostatic deposition led to a preferred (110) orientation with TC110 of 1.52. In this case, the long axis of the nanowire was <110>. In contrast, pulsed deposition with the on-time pulse potential of − 1.5 V resulted in TCs of almost 1 for both (110) and (200) planes denoting randomly oriented crystals in the deposit. Furthermore, Fe nanowires prepared with the on-time pulse potential of − 1.8 V clearly showed (200) orientation with TC200 of 1.9.
Perpendicular Magnetization of Fe Nanowire Arrays
It is well known that the crystalline orientation can be modified by deposition conditions such as the choice of potentiostatic and pulsed potential deposition . In particular, pulsed deposition is a powerful technique to improve uniform growth avoiding the formation of large and randomly oriented crystallites . Furthermore, the low pH value of the electrolyte should be considered. As discussed above, fabrication of Fe nanowires is preceded by the simultaneous reduction of hydronium ions, which results in local pH changes inside the pores of the AAO membrane . Moreover, hydrogen can be easily absorbed in the deposit, significantly influencing its crystallinity . In this case, the metal Fe deposition rate might be considerably reduced. It is well known that the hard axis for the magnetization of bcc Fe is in the <110> direction, which results in reduction of the squareness in magnetization. This uniaxial magnetization behavior of Fe nanowire arrays was confirmed in this study. Yang et al. reported that Fe nanowires, which were fabricated using potentiostatic electrodeposition at a constant cell voltage of 1.5 V, had a random orientation without texture . Irfan et al. also reported that Fe nanowires, which were potentiostatically electrodeposited at − 1.1 V vs. SCE, exhibited a non-textured orientation and the coercive force of ca. 0.5 kOe . Cornejo et al. also reported that Fe nanowires, which were prepared using AC electrodeposition at a cell voltage of 15 V, had a random orientation without texture and the squareness of ca. 0.5 . In the current study, Fe nanowires with an aspect ratio of 2000, which were electrodeposited using a rectangular-pulsed potential deposition technique to control the crystal orientation, had a strong texture with (200) orientation. The textured Fe nanowires exhibited the coercive force of ca. 1.4 kOe and the squareness of ca. 0.95. Hence, we demonstrated that the rectangular-pulsed potential deposition technique can control the crystal orientation and aspect ratio of Fe nanowires, leading to excellent magnetic properties.
The degree of overpotential during the potentiostatic and the pulsed potential deposition significantly affected the crystal orientation and the magnetization performance of high aspect ratio Fe nanowire arrays. According to the determination of the texture coefficients, potentiostatic deposition at a cathode potential of − 1.2 V led to a preferred (110) orientation, whereas pulsed techniques resulted in either randomly oriented crystallites or a (200) orientation by applying on-potentials of − 1.5 and − 1.8 V, respectively. Magnetic hysteresis loops in perpendicular and in-plane directions to the membrane surface showed a strong magnetic anisotropy because of the high aspect ratios (approximately 2000) of all considered Fe nanowire arrays. Therefore, the crystalline orientation and the shape anisotropy are the most important factors controlling the magnetic properties. The coercivity obtained in the magnetic field for the long axis direction of Fe nanowire arrays with a preferred (110) orientation was 1.3 kOe. This value slightly increased to 1.4 kOe for the nanowires with strong (200) orientation. In contrast, the squareness obtained from Fe nanowire arrays with a preferred (200) orientation significantly increased up to 0.95 from 0.65 with increase in TC200. This study illustrates the feasibility of improving the magnetic properties of Fe nanowire arrays by controlling the degree of overpotential during electrodeposition.
The authors thank New Energy and Industrial Technology Development Organization (NEDO: P14015), Japan Science and Technology Agency (JST: AS262Z02450K), the Japan Society for the Promotion of Science (JSPS: PE14005 and 15K06508), and IKETANI Science and Technology Foundation (0271040-A) for the financial support.
TY, MN, and HF designed the study, supervised the project, and analyzed data. CN and TO carried out experiments, analyzed data, and wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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