Structure and magnetic properties of Co x Cu 1x nanowires in self-assembled arrays M. Almasi Kashi ⇑ , A. Ramazani, F. Adelnia Najafabadi Department of Physics, University of Kashan, Kashan, Iran article info Article history: Received 6 May 2012 Received in revised form 11 June 2012 Accepted 13 June 2012 Available online 19 June 2012 Keywords: Alloys Porous anodic aluminum oxide Pulse electrodeposition CoCu nanowire Magnetic properties abstract CoCu alloy nanowire arrays were ac-pulse electrodeposited into porous anodic aluminum oxide. The effect of off-time between pulses and Cu concentration on the magnetic properties, crystalline structure and weight percentage of Co x Cu 1x alloy nanowires have been studied by alternating gradient force mag- netometer (AGFM), X-ray diffraction pattern (XRD) and energy dispersed spectrometry (EDS), respec- tively. Increasing the off-time between pulses decreased the weight percentage of Co in the range of (x = 0.84  0.24). Results of EDS were in accordance with saturation magnetization per unit area of the samples. Coexistence of a moderate off-time and Cu concentration provided excellent conditions for fab- rication of the composite nanowires which were proved by XRD patterns. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Magnetic nanowires have been widely used in fundamental sci- entific issues such as magnetic interactions, magnetic reversal and nano-scale magnetic structures [1,2] in magnetic devices such as magnetic field sensors and perpendicular magnetic recording med- ia [3–6]. There are many different methods to prepare nanowires. Among these, electrodeposition into an aluminum oxide template is a simple and economical technique. Self-assembled nanopores in anodic alumina have been used to grow cylindrical magnetic nanowires with various diameters and high aspect ratios [7,8]. The addition of nonmagnetic impurities is a suitable way to control the magnetic properties of the magnetic nanowire arrays [9,10]. Among the magnetic metals, Co is the best choice due to its high spin diffusion length and large magnetocrystalline anisotropy. Based on deposition conditions, it can be grown in different crystalline structures. Many studies on magnetic-nonmagnetic nanowires have been reported [11–15]. For example Zhang et al. [11], have studied the magnetization modes of the CoCu nano- wires. For 30 nm Co 77 Cu 23 nanowires a coercivity of 244 Oe has been reported. In another research, Lio and his co-workers [12], have investigated the effect of pH values and deposition potentials on the structure and composition of CoCu nanowire arrays. Segregation of magnetic grains through nonmagnetic elements increases the coercivity of alloy nanowire arrays which is primary goal of the magnetic based researches. The effect of Giant Magneto resistance (GMR) in multilayered nanowires such as Co/Cu has been also studied. Controlling crystalline structure of layers can be obtained thorough changing electrodeposition conditions [13–15]. In present work, the alloy nanowire arrays were grown in the alumina template using ac-pulse electrodeposition in an electro- chemical simple cell. The effect of Cu concentration and off-time between pulses on the magnetic properties of nanowires such as coercivity and saturation magnetization per unit area were studied. 2. Experimental Anodic aluminum oxide (AAO) templates were prepared by the two-step anod- ization process. 0.3 mm thick aluminum foils (99.999%) were electropolished in a 1:4 volume mixture of perchloric acid and ethanol. Then, they were anodized in a 0.3 M oxalic acid solution at 17 °C for 10 h, under a constant direct current (dc) with 40 V [16]. To remove the oxide layer, the anodized foils were immersed into a mixed solution of 0.2 M chromic acid and 0.5 M phosphoric acid at 60 °C for 10 h. The foils were re-anodized for 2 h under the same conditions as the first step. Following the second step anodization, the voltage was systematically reduced to promote the thinning of the barrier layer. The voltage was reduced by 4 V min 1 to 20 V and then 2 V min 1 to 10 V. The voltage was dropped down to 8 V in an interval of 1 V min 1 . Anodization was then continued for 2 min at this final voltage to allow the equilibration of the barrier layer [17,18]. Pulse electrodeposition was carried out using a sine waveform, in a solution with different concentrations of CuSO 4 . A solution composed of 0.75 M CoSO 4 7H 2 O, different concentrations of CuSO 4 .5H 2 O (0.025 M, 0.1 M) and 45 g L 1 H 3 BO 3 was used as electrolyte. The pH value of electrolyte solution was adjusted by adding the diluted NaOH. The electrodeposition voltage was 15 V. The off-times between pulses varied in the range of 0–250 ms and reductive and oxidative times to 4 had the same value of 10 ms. The morphology of the AAO templates was evaluated by scanning electron microscopy (SEM) and scanning probe microscopy (SPM). The structure and composition of Co x Cu 1x alloy nanowire arrays were studied by XRD and EDS. The magnetic properties were studied by AGFM. 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.061 ⇑ Corresponding author. Tel./fax: +98 361 5552935. E-mail address: almac@kashanu.ac.ir (M. Almasi Kashi). Journal of Alloys and Compounds 540 (2012) 133–136 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom