Magnetic Nanostructures Fabricated by Electrochemical Synthesis

Electrochemical processing is a cost effective and low-temperature approach suitable for the fabrication of certain unique nanostructures that are difficult to obtain by other methods. Here we report on the synthesis of nanowires and nanoporous structures with the intention to control the magnetic properties of conventional materials. Nanowires with variable sizes (diameter 15 nm microns, and length up to 100 microns) have been fabricated by template assisted electrodeposition. Utilizing a combined alloy electrodeposition and electrochemical dealloying approach, porous nanostructures with controlled pore size and porosity have also been synthesized. Magnetization, Curie temperature, coercivity, saturation field, and remnant magnetization of these magnetic nanostructures exhibit much wider tunibility compared to bulk and thin film samples. Introduction Fabrication and characterization of nanomaterials are at the heart of nanoscience and nanotechnology because these materials often exhibit novel properties (in comparison to bulk samples) that attract both scientific curiosity and technological application interests. For example, magnetic responses of materials always exhibit strong composition, size, shape, and morphology dependency. Lateral confinement and unique structures in artificial nanomaterials have led to the discovery and device application of a serial of novel magnetic and magneto-transport phenomena in magnetic thin films, multilayers and superlattices[1,2]. Here we extend the research to magnetic nanowires and nanoporous materials. These nanostructures are fabricated by electrochemical synthesis. Compared to conventional synthesis methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), electrochemical processing is a cost effective, high yield, easy to scale-up and low temperature approach with the capability of synthesizing unique nanostructured materials. Experimental Section Electrochemistry is a branch of chemistry concerned with the interrelation of electrical and chemical effects [3,4]. It studies the chemical changes caused by the passage of electric currents. Electrochemical behaviors of nickel, copper and their alloys are the subjects of this research. Two aqueous solutions were used in the experiments: a) 1.6M Ni(H2NSO3)2 ⋅4H2O for pure nickel deposition, and b) 1.6M Ni(H2NSO3)2 and 0.1M CuSO4⋅4H2O for the synthesis of multilayered nanowires, Ni-Cu alloys and porous Ni. Both solutions were buffered to pH 2.5 with H3BO3. Electrochemical experiments were performed under ambient conditions in a three-electrode cell with a Pt mesh counter electrode and an Ag/AgCl reference electrode. An EG&G 263A potentiostat was used to control and monitor the electrochemical processes through a computer. Electrochemical reactions involved in the experiments include [5]: , 2 2 Ni e Ni ⇔ + + Ueq 0 = -0.257 V, (1) , 2 2 Cu e Cu ⇔ + + Ueq 0 = 0.340 V, (2) 2 2 2 2 H e H ⇔ + + , Ueq 0 = -0.000 V, (3) Solid State Phenomena Vols. 121-123 (2007) pp. 839-842 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 129.7.203.139-25/10/06,18:58:35) where Ueq 0 is the standard equilibrium potential with respect to standard hydrogen electrode (SHE). For thin film and porous Ni fabrication, Au/Cr(5 nm)/(001)Si substrates were used as the working electrodes. For nanowire deposition, various porous templates with different pore dimensions, density and chemical compatibility were used (Fig. 1). A gold layer was sputter-deposited on one side of the template to cover the pores and serve as working electrode. Results and Discussion Cyclic voltammetry measurements have been performed to reveal the current-voltage behaviors for the two solutions with a scanning rate of 20 mV/s. As shown in Fig. 2a, equilibrium potential for the nickel sulfamate solution is -0.451 V (vs Ag/AgCl), however, the onset of significant deposition current is at about -0.7 V. For the mixed 1.6M Ni(H2NSO3)2 and 0.1M CuSO4⋅4H2O solution, two copper ion reductions and two oxidation peaks corresponding to Cu(I) and Cu(0) are observed in the scan range of 0.8 to -0.7 V (Fig. 2b). Due to the lower ion concentration of Cu(II) in the solution, Cu deposition is diffusion limited below the onset of nickel deposition. Both solutions show is very limited Ni dissolution during the positive scan which is critical for the selective dissolution of Cu in NiCu alloy to form nanoporous Ni. Pure Ni nanowires have been deposited from the Ni(H2NSO3)2 solution at -1.0 V vs. Ag/AgCl reference electrode. Fig. 3 shows the scanning electron microscopy (SEM) images of the Ni nanowires grown in 200 nm alumina and 50 nm polycarbonate membranes. These nanowires were deposited from water suspension after the matrix materials had been removed by NaOH and chloroform, respectively. Using nuclear track etching, we obtained nanoporous mica templates with diamond shape cross-section (Fig. 1e). Nanowires deposited in these templates provide great opportunities to study the shape anisotropy, size dependent magnetic switching and high temperature magnetic properties. For the magnetic properties reported here, the magnetic measurements were performed on nanowire arrays embedded in the matrices. Fig. 1: (a) Optical micrograph of a nanochannel glass sample with pore diameter of 4 μm, sample provided by Dr. D. Justus at Naval Research Lab; (b) SEM image of a homemade polycarbonate membrane with pore size of 100 nm; (c) SEM image of a alumina membrane with pore size of 200 nm, purchased from Whatman; (d) AFM micrograph of a homemade PS-PMMA diblock copolymer membrane with pore size of 32 nm; (e) SEM image of a homemade single crystalline mica membrane with pore size of 1.5 μm. 200nm 1(m 1(m (a)