Determination of Crystal Growth Geometry Factors and Nucleation Site Densities of Electrodeposited Ferromagnetic Cobalt Nanowire Arrays

Ryusei Saeki,Takeshi Ohgai

doi:10.3390/cryst9030142

Abstract

The time-dependence of electrochemical reduction current, which was observed during the one-dimensional (1-D) crystal growth of ferromagnetic cobalt nanowire arrays, was analyzed by Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory. Textured hcp-Co nanowire arrays were synthesized by potentio-static electrochemical reduction of Co2+ ions in anodized aluminum oxide (AAO) nanochannel films. Crystal growth geometry factor n in the JMAK equation was determined to be ca. 1. Hence, the electrochemical crystal growth process of a numerical nanowires array can be explained by 1-D geometry. The crystal nucleation frequency factor, k in JMAK equation was estimated to be the range between 10−4 and 10−3. Our experimental results revealed that the crystal nucleation site density Nd increased up to 2.7 × 10−8 nm−3 when increasing the overpotential for cobalt electrodeposition by shifting the cathode potential down to −0.85 V vs. Ag/AgCl. The (002) crystal orientation of hcp-Co nanowire arrays was, remarkably, observed by decreasing Nd. Spontaneous magnetization behavior was observed in the axial direction of nanowires. By decreasing the overpotential for cobalt electrodeposition, the coercivity of the nanocomposite film increased and reached up to 1.88 kOe, with a squareness of ca. 0.9 at room temperature.

Highlights

It is well known that when the rare-earth elements are added to ferromagnetic metals, the large magneto-crystalline anisotropy effect occurs and the coercivity of the hard-magnetic material can be enhanced
Each (b) and bottom-surface (c) of an anodized aluminum oxide (AAO) nanochannel film that was exfoliated from an aluminum rod
The cobalt nanowire arrays were synthesized by using a potentio-static electrochemical reduction process inside the numerous nanochannels of AAO films

Summary

Introduction

It is well known that when the rare-earth elements (e.g., samarium, neodymium and dysprosium) are added to ferromagnetic metals (e.g., iron, cobalt and nickel), the large magneto-crystalline anisotropy effect occurs and the coercivity of the hard-magnetic material can be enhanced. Ferromagnetic metal nanowires have attracted attention as a substitute for rare-earth compounds because they have large shape magnetic anisotropy along their long axis [2]. Cobalt alloys have a high Curie temperature and excellent corrosion resistance [3]. Cobalt alloy nanowires can be used for the magnetic application utilized under high-temperature conditions and can, have a range of potential applications such as magnetic data storage media, anisotropic magnetoresistance sensors and microwave-absorbing materials with heat-resistance performance.

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Results

Conclusion