High-Aspect-Ratio Fe Nanowire Arrays with Tailored Texture via Pulsed-Potential Electrodeposition: Uniaxial Magnetization Performance

Background

Nanowire arrays offer a high surface‑to‑volume ratio that underpins unique magnetic and electronic properties, making them attractive for sensors, data storage, and energy devices. Template‑based fabrication, especially using AAO membranes, provides precise control over pore diameter and length, enabling reproducible one‑dimensional growth at low cost.

Previous studies have demonstrated electrodeposition of Ni, Co, and Fe nanowires into AAO templates. Hu et al. showed that Fe nanowires grown by direct current electrodeposition in an acidic chloride bath exhibited enhanced coercivity (~2 kOe at 5 K) and squareness when oriented along the (200) plane. Irfan et al. reported that post‑annealing could further refine the magnetic properties of Fe nanowires with aspect ratios of 80–100. Cornejo et al. used AC electrodeposition at 15 V to produce Fe nanowires 3–5 µm long (aspect ratio ~100). Although high aspect ratios are desirable for permanent magnets, earlier work rarely exceeded 1000.

In this study, we fabricated Fe nanowire arrays with an aspect ratio up to 2000 by employing a pulsed‑potential deposition technique that allows high cathodic overpotential for rapid growth while avoiding the detrimental effects of hydroxide formation that limit growth at elevated temperatures. We also investigated how the deposition overpotential influences crystal orientation and magnetic performance.

Experimental

AAO membranes were produced by anodizing 99.99 % pure aluminum rods (10 mm diameter) in a two‑step process: first, mechanical and electrochemical polishing in ethanol with 20 % perchloric acid at 3.0 A cm⁻² for 120 s; second, anodization in 0.3 M oxalic acid at 12 °C for 22 h under a constant 30 V. After anodization, the membrane was separated from the aluminum substrate and a 200 nm gold layer was sputtered onto one side (10 mA for 900 s). The gold‑coated side was affixed to a copper plate with silver paste to serve as the working electrode.

Fe deposition was performed in 0.05 M FeSO₄·7H₂O (pH 2) at 30 °C, using a gold wire counter electrode and an Ag/AgCl reference. For potentiostatic growth, a constant cathodic potential of –1.2 V was applied. In contrast, the rectangular‑pulsed potential protocol used –1.5 V (or –1.8 V) during a 0.1 s on‑time and –1.0 V during a 1.0 s off‑time.

Fabrication process of free‑standing metallic nanowire array: (a) anodized aluminum oxide template, (b) sputter‑deposited metallic film, (c) electrodeposited metallic nanowires, and (d) free‑standing array.

After electrodeposition, the AAO template was removed by immersion in 5 M NaOH, leaving free‑standing Fe nanowires. Their morphology and crystallography were examined by field‑emission SEM (JEOL‑JSM‑7500FA, 5 kV) and TEM (JEOL‑JEM‑ARM200F, 200 kV), and XRD (Rigaku‑SmartLab, Cu Kα). Magnetic properties were measured with a vibrating sample magnetometer (VSM) at room temperature, recording hysteresis loops with the field applied perpendicular (along the wire axis) and parallel to the membrane surface, up to 10 kOe.

Results and Discussions

Electrodeposition of Fe Nanowire Arrays

Figure 2a shows the cathodic polarization curve (–0.2 V to –1.0 V, 30 mV s⁻¹) at 30 °C. The current density remained ~4.5 × 10⁻⁴ A cm⁻² between –0.2 V and –0.5 V, increasing sharply at –0.55 V, indicating the onset of hydrogen evolution. The equilibrium potential for Fe²⁺/Fe in the experimental conditions is ~–0.68 V vs. Ag/AgCl, so the rise in current beyond –0.7 V corresponds to the commencement of Fe deposition. Figure 2b (Tafel plot) shows that the slope decreases with increasing cathodic overpotential and plateaus below –1.4 V, reflecting an electrophoretic‑migration‑limited regime. Accordingly, –1.2 V was chosen as the optimal potentiostatic potential for Fe growth in the AAO pores.

a Cathodic polarization curve; b Tafel plot (scan rate 30 mV s⁻¹).

During potentiostatic deposition at –1.2 V (Figure 3), the current density initially decreases as Fe²⁺ and H⁺ concentrations near the pore walls drop, then stabilizes due to bulk diffusion. A sharp current rise indicates the completion of the nanowire length (~60 µm), confirming a growth rate of ~200 nm s⁻¹.

Time dependence of current density during Fe nanowire growth at –1.2 V.

Figure 4 illustrates the applied potential patterns and the corresponding current responses for potentiostatic and pulsed deposition. In the pulsed regime, the high‑amplitude on‑time pulses (–1.5 V or –1.8 V, 0.1 s) produce a higher instantaneous current, while the off‑time at –1.0 V maintains a lower cathodic current, preventing dissolution. The on‑time pulses dominate nanowire growth, leading to distinct crystallization behaviors compared to the steady potentiostatic case.

a Potentiostatic deposition at –1.2 V; b Pulsed deposition with –1.5 V on‑time; c Pulsed deposition with –1.8 V on‑time.

Structure and Crystallographic Orientation of Fe Nanowire Arrays

Figure 5 shows a SEM cross‑section of the arrays after AAO removal: the wires are densely packed and uniformly oriented. TEM bright‑field images (Figure 6) confirm a consistent diameter of 30 ± 5 nm and an aspect ratio of ~2000 for all samples.

SEM cross‑section of arrayed Fe nanowires.

TEM bright‑field images: (a) potentiostatic deposition at –1.2 V; (b) pulsed deposition –1.5 V on‑time; (c) pulsed deposition –1.8 V on‑time.

XRD patterns (Figure 7a) reveal a strong dependence of crystal orientation on deposition parameters. With small overpotential (potentiostatic), the (110) peak dominates, reflecting the lowest surface energy of this plane. In contrast, pulsed deposition at –1.8 V on‑time suppresses (110) and enhances the (200) peak, indicating a transition to <200> alignment. The peak shift and shoulder observed in pulsed samples suggest tensile stress and crystal defects induced by the high overpotential.

a XRD patterns; b Texture coefficient (TC) vs. on‑time potential.

The texture coefficient, calculated via the Harris formula, quantifies the degree of preferred orientation. Potentiostatic deposition yields TC₍110₎ = 1.52 (<110> alignment). Pulsed deposition at –1.5 V gives TC₍110₎ ≈ 1.0 and TC₍200₎ ≈ 1.0 (random orientation). At –1.8 V, TC₍200₎ rises to 1.9, confirming a strong (200) texture.

Perpendicular Magnetization of Fe Nanowire Arrays

Figure 8 presents hysteresis loops measured with the magnetic field perpendicular and parallel to the membrane surface. All samples exhibit pronounced anisotropy: the perpendicular direction (wire axis) shows higher coercivity and squareness. The coercivity remains ~1.3 kOe for potentiostatic and –1.5 V pulsed samples, increasing to 1.4 kOe for the –1.8 V pulsed array. More notably, the squareness (M_R/M_S) climbs from 0.65 to 0.95 as TC₍200₎ increases, reflecting the alignment of the easy axis along <200>.

a Hysteresis loops (perpendicular vs. in‑plane); b Squareness vs. TC₍200₎ and TC₍110₎.

These observations confirm that both crystal orientation and shape anisotropy govern the magnetic behavior. By controlling the deposition overpotential, the pulsed‑potential method successfully tunes the texture and thereby optimizes the magnetization performance of Fe nanowire arrays.

Conclusion

Adjusting the cathodic overpotential during potentiostatic and pulsed‑potential electrodeposition significantly influences the crystal orientation and magnetic properties of high‑aspect‑ratio Fe nanowire arrays. Potentiostatic deposition at –1.2 V promotes a <110> orientation (TC₍110₎ = 1.52), while pulsed deposition at –1.8 V on‑time yields a pronounced <200> texture (TC₍200₎ = 1.94). All arrays exhibit strong uniaxial anisotropy due to their ~2000 aspect ratio. The <110> samples achieve coercivity of 1.3 kOe; the <200> samples reach 1.4 kOe with a remarkably high squareness of 0.95. This study demonstrates that pulsed‑potential electrodeposition can produce rare‑earth‑free Fe nanowire magnets with superior magnetic performance.