Wet chemical synthesis and magnetic properties of single crystal Co nanochains with surface amorphous passivation Co layers
© Zhou et al; licensee Springer. 2011
Received: 1 February 2011
Accepted: 4 April 2011
Published: 4 April 2011
In this study, for the first time, high-yield chain-like one-dimensional (1D) Co nanostructures without any impurity have been produced by means of a solution dispersion approach under permanent-magnet. Size, morphology, component, and structure of the as-made samples have been confirmed by several techniques, and nanochains (NCs) with diameter of approximately 60 nm consisting of single-crystalline Co and amorphous Co-capped layer (about 3 nm) have been materialized. The as-synthesized Co samples do not include any other adulterants. The high-quality NC growth mechanism is proposed to be driven by magnetostatic interaction because NC can be reorganized under a weak magnetic field. Room-temperature-enhanced coercivity of NCs was observed, which is considered to have potential applications in spin filtering, high density magnetic recording, and nanosensors.
PACS: 61.46.Df; 75.50; 81.07.Vb; 81.07.
In the last decade, diverse technological applications of magnetic nanostructures in magnetofluid, recording tape, flexible disk recording media, permanent magnets, microwave oscillators as well as biomedical materials, and catalysts have provided an impetus for extensive research in nanometer scale magnets [1–17]. Most of these applications rely on the stability of ferromagnetic ordering with temperature. In nanometer scale magnets, the thermal fluctuations randomize the magnetic moment by overcoming the anisotropy energy leading to unstable state of paramagnetism (non-magnetic materials) or superparamagnetism (corresponding coercivity and hysteresis fall to zero). Cobalt (Co) is superior to other ferromagnetic materials because of its highest Curie temperature (T c) of about 1394 K, which is crucial for thermal stability in high-temperature nanodevice applications [1, 5–16]. For realizing high T c, magnetization, and coercivity, the aim must be directed toward increasing the amount of the ferromagnetic Co phase [7–10]. Owing to its basic metallic characteristic, pure cobalt, especially for nanosized Co, is very reactive and must be unstabilized in ambient air , and therefore, its use has been limited to prepare Co nanostructures in the absence of the shell [11–16]. A very simple bottom-up method is to produce stable film-assisted synthesis for a surface with slightly controlled Co to passivate the surface of the host materials, including organic and inorganic templates, and alloyed technique [1, 2, 5–10, 12–16]. For example, recently, control of magnetism in cobalt nanoparticles by oxygen passivation was reported by Srikala's research group [14, 15], and cobalt nanowires with controlled diameters have been synthesized using electrochemical deposition in etched-ion-track polycarbonate membranes . In this latter case, however, corresponding magnetic properties are inevitably decreased by the addition of non-magnetic materials or natural oxide layers [1, 5–16]. As far as we know, amorphous phases lack long-range crystalline order and have unique electronic, magnetic and corrosion-resistant properties. In this article, based on our earlier study , we report for the first time a wet chemical synthesis of high-pure Co nanochains (NCs) without any oxide shells and templates. The amorphous Co covering layer would be able to protect the active Co core from oxygen in atmosphere. In particular, the room-temperature coercivity (up to 355.8 Oe) of the NCs is larger than that (93.6 Oe) of pure single-crystal Co (PSC) metal, which will make them as promising candidates for advanced magnetic media and investigative studies on novel great magnetoresistive properties.
In a typical experiment, 10.0 mL glycerine was heated to the boiling point (approx 560 K) and refluxed for 3 min. Then, 50 mL of hydrazine monohydrate was added dropwise to the boiling solution. After 1 min, 10.0 mL of Co (NO3)2 6H2O solution (0.5 mol/L, in glycerine) and 10.0 mL of hydrazine hydrate solution (0.5 mol/L, in glycerine) were added rapidly to the boiling solution under vigorous magnetic stirring. After refluxing for approx 80 min, as proposed in our previous article , the final products in the form of loose powders (large quantities of light-gray wool-like products) were obtained by centrifugation under permanent-magnet (approx. 0.5 T). The powders were rinsed repeatedly with absolute ethanol for several times, followed by the removal of the residual solvent through evaporation in vacuum at 500 K. The yield of the as-prepared Co specimens is about 60% according to our calculation. We noticed that no sign of oxidation was observed on the as-synthesized metal Co NCs even after aging for over 1 month under ambient conditions. This indicates that the Co NCs are very stable after surface modification with amorphous Co, which is very important for future applications. The samples were characterized extensively for morphology, phase, and chemical composition using scanning/transmission electron microscopy (SEM/TEM), energy dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), selective area electron diffraction (SAED), high resolution TEM (HRTEM), and X-ray photoelectron spectroscopy (XPS). The temperature dependence of magnetization and room temperature (RT) hysteresis curves were carried out by means of vibrating sample magnetometer (VSM, Model 4 HF) and physical properties measurement system (PPMS, Quantum Design PPMS-7).
Results and discussion
The synthesized NCs can be reorganized in a weak magnetic field. Typically, the purified Co NCs were dispersed into de-ionized water by ultrasonic agitation. A drop of the Co NCs solution was dripped on a copper grid with holes and carbon film to characterize the TEM in the absence or the presence of the weak external magnetic field (about 0.5 T) and dried naturally. The result reveals that the NCs in the weak magnetic field have aligned according to the direction of the magnetic field as shown in Figure 3b, whereas without the external magnetic field, nonaligned NCs appear as shown in Figure 3c. Regarding the mechanism for the growth of the highly branched Co nanoparticle chains, we believe that magnetostatic interaction plays an important role. The magnetic dipole-dipole interaction displayed behavior similar to that of soft templates. Initially, very small Co nanoparticles were formed. With the increase of the growth time, presumably, the small Co nanoparticles diffused and aggregated to form larger nanoparticles. The Co nanoparticles were then assembled into neck-like chains with multiple branches because of the stronger anisotropic magnetic forces, and these findings are in agreement with those of our earlier article .
In this study, this proposed method provides a simple and inexpensive method for the preparation of stable, magnetic Co NCs with the complete absence of impurities. The as-synthesized NCs are produced with increasing H c and M r at room temperature. In addition, the large H c, coupled with the controllable coercive field, makes these NC arrays the preferable candidates for probe-based data-storage systems. Introducing long-range, 1D translational order over macroscopic distances among the NCs will undoubtedly be a key driver in this respect, and is an area that clearly warrants future exploration.
energy dispersive X-ray spectroscopy
pure single-crystal Co
selective area electron diffraction
scanning electron microscopy
transmission electron microscopy
X-ray photoelectron spectroscopy
X-ray powder diffraction
vibrating sample magnetometer
physical properties measurement system
This study was partially supported by the Program for Science & Technology Innovation Talents in Universities of Henan Province (No. 2008 HASTIT002), Innovation Scientists and Technicians Troop Construction Projects of Henan Province (No. 094100510015), and by the Natural Science Foundation of China under Grant No. 20971036.
Open Access: This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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