Since July 2006, the European Union (EU) had implemented the Reduction of Hazardous Substances (RoHS) directive [1], which states that no electronic products can be made in or shipped to the EU if they contain lead. The main source of lead used in electronics can be found in the most commonly used tin-lead (Sn-Pb) solders. Lead-free alternatives containing tin and other alloying materials, such as copper, bismuth, silver and gold, have been proposed to address this directive.
Electrodeposition is one method of fabricating tin-copper (Sn-Cu) alloys which could become important as chip sizes and device dimensions shrink. Until now commercial electrodeposition of Sn-Cu alloys is carried out using two separate baths to successively plate a tin layer on top of a copper layer followed by annealing to form the alloy [2]. The use of two plating baths increases the cost and time of solder manufacturing.
The aim of this study was to determine if the Sn-Cu eutectic, i.e. 99.3Sn0.7Cu (wt%), was obtainable via electrodeposition from a single plating bath. Alloy plating was carried out using methanesulfonic acid (MSA), an acid electrolyte, which is less corrosive and toxic than the traditional fluoborate or cyanide based baths. Another advantage of using an acidic MSA electrolyte is that it is compatible with positive photoresists.
Cyclic voltammetry (CV) was carried out at a gold tipped rotating disc electrode (RDE) to determine the current-potential relationship of the system. Varying bath conditions such as the individual metal concentrations and the addition of surfactants were examined to observe the effect on the quality of the deposits. To keep the bath chemistry simple, brighteners or other organic complexing agents were not added. Chronoamperometry and anodic stripping voltammetry (ASV) were then carried out to determine the parameters needed to electrodeposit Sn-Cu alloys with high tin composition.
For electrodeposition on a gold surface, ASV experiments confirm that Sn-Cu alloys start to form at potentials more negative than -0.42 V vs. SCE. The deposits, however, were mostly composed of copper and contained very low amounts of tin. High tin composition only started to occur at potentials more negative than -0.45 V vs. SCE. The Sn-Cu alloys with high tin composition have a matte surface microstructure. This is preferable in electronics as it is less susceptible than bright tin to form tin whiskers that may lead to short circuits.
[1] Official Journal of the European Union. OJ L37, 13.2.2003, p. 19-23.
[2] Doyle, C., et al. Metal Finishing, 2002. 100(1), p. 10-17.
Keywords: alloy plating, electrodeposition, lead-free, methanesulfonic acid, tin-copper
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Electrochemical deposition of Au and Au/Ag alloy nanostructures Ina Alber1, M.E. Toimil Molares1, R. Neumann1
1 GSI Helmholtz Center for Heavy Ion Research, Planckst. 1, 64291 Darmstadt, Germany
Due to their promising optical properties, metallic nanowires are attracting increasing interest. Their optical properties are dominated by the excitation of antenna-like surface plasmon resonances (SPR) that lead to strong charge accumulations at the surface of the nanowires and thus to very strong field enhancements in their proximity. These strong field enhancements and the possibility to tune the resonance frequency of the wires by varying material and morphology make them excellent candidates for applications in surface enhanced infrared spectroscopy [1,2] and surface enhanced Raman spectroscopy [3].
This work presents the potentiostatic electrochemical deposition of Au and Au/Ag alloy nanowires using a cyanidic electrolyte containing silver and gold ions. Alloy nanowires with variable silver content are synthesized potentiostatically. Consequent etching of Ag results in nanowires with different porous characteristics. In addition, the same electrolyte is used for the synthesis of Ag/Au multilayer nanowires by pulsed deposition. Optical properties of these nanostructures are investigated as a function of nanowire composition, morphology and geometry.
[1] S. Karim et al., Nanotechnology 17, 5954 (2006).
[2] F. Neubrech, A.M. Pucci et al., Phys. Rev. Lett. 101, 157403 (2008).
[3] K. Kneipp et al., Phys. Rev. Lett. 78, 1667 (1997).
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MODELLING THE MAGNETORESISTANCE VS. FIELD CURVES OF GMR MULTILAYERS WITH VARIOUS COUPLINGS AND ANISOTROPIES
K. Szász and I. Bakonyi*
Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences.
H-1525 Budapest, P.O.B. 49, Hungary
It was concluded from previous experimental results that the often observed strongly non-saturating behaviour of the field-dependence of the giant magnetoresistance (GMR) of electrodeposited (ED) multilayer films originates from the presence of superparamagnetic (SPM) regions in the magnetic layers and a procedure was introduced to separate the SPM contribution from the conventional ferromagnetic GMR contribution (GMRFM) [1].
It has been shown in a recent work [2] that for ED Co/Cu multilayers, the GMRFM term when properly extracted from the experimental GMR data does not show an oscillatory behaviour as a function of the spacer layer (Cu) thickness which feature is well documented for multilayers prepared by physical deposition methods. For Cu layer thicknesses above 1 to 2 nm, the GMRFM term was found to increase smoothly with Cu layer thickness up to a maximum at about 4 nm. This fact, together with the large relative remanence (∼0.75) of these multilayers [2] suggests the absence of a significant antiferromagnetic (AF) coupling in ED Co/Cu multilayers. In an uncoupled state when the magnetization orientation distribution is random, there is some degree of antiparallel alignment of the neighbouring layer magnetizations which is a sufficient condition for the occurrence of a GMR effect.
In order to better understand the observed evolution of GMR with spacer thickness and the role of possible couplings in ED multilayers, a knowledge of the dependence of the GMR on magnetic field H would be useful but a few specific cases have only been treated theoretically in the literature [3]. Therefore, we started a modelling of the GMR(H) curves of ferromagnetic/non-magnetic (FM/NM) multilayers with various interlayer couplings and anisotropies. For simplicity, we focused on a trilayer structure (FM1/NM/FM2) corresponding fairly well to the case of a large number of FM/NM bilayers [4].
To carry out the calculations, some fundamental assumptions were made:
1. Each FM layer consists of a single domain and the magnetizations are in the layer planes.
2. The magnetization of each layer is the same.
3. The magnetization vectors rotate in the plane of the layers in an external magnetic field.
In order to calculate the GMR(H) function, we need to know the magnetization process in the trilayer, i.e., the M(H) function. Therefore, first we calculate the equilibrium angle θ(H) between the two magnetization vectors as a function of the field by minimizing the total energy of the multilayer. Since the dependence of the GMR on θ was theoretically calculated by Blaas et al. [5], thus we can get the GMR(H) function.
Along this line, the M(H) and GMR(H) curves were calculated for several specific cases:
(i) AF coupling; (ii) orthogonal coupling; (iii) AF coupling and orthogonal coupling; (iv) AF coupling and uniaxial anisotropy with H along either the easy or the hard direction; (v) orthogonal coupling and uniaxial anisotropy with H along the easy axis. Most of these cases have not yet been treated in the literature; where literature results for M(H) [3,4] and GMR(H) [3] exists, our results agreed with those reported previously.
1. I. Bakonyi et al., Phys. Rev. B 70, 054427 (2004) 2. I. Bakonyi et al., Phys. Rev. B 79, 174421 (2009) 3. W. Folkerts, J. Magn. Magn. Mater. 94, 302 (1991) 4. B. Dieny et al., J. Phys.: Cond. Matter 2, 159 (1990) 5. C. Blaas et al. Eur. Phys. J. B 9, 245 (1999)
*Corresponding author. E-mail: bakonyi@szfki.hu
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ELECTRODEPOSITION OF MAGNETITE (Fe3O4) THIN FILMS FOR SINGLE-MOLECULE SPINTRONICS
Meritxell Cortes1*, Elisa Vallés1, Elvira Gómez1, Natasa Vasiljevic2, Walther Schwarzacher2
1Electrodep, Departament de Química Física and Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain.
2 H.H. Wills Physics Laboratory, Royal Fort, Tyndall Avenue, Bristol, BS8 1TL. United Kingdom.
*m.cortes@ub.edu
Spintronics is an emerging technology that exploits the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. Spintronic devices would be smaller, more versatile and more robust than conventional electronic devices and would use less power.
Our interest is focused on fabricating a molecular spin transistor based on thin films of magnetite where magnetite will act as a source of spin-polarized electrons. Moreover, as the basis of molecular electronics is the use of interconnected molecules to perform the basic functions of digital electronics, the formation of different monolayers over the magnetite films has been also studied.
Magnetite contains Fe3+ in its tetrahedral sites while octahedral sites are shared by an equal mixture of Fe2+ ions and Fe3+. For this reason it exhibits a net ferrimagnetic moment associated with the Fe3+ ions and it is reported to have a spin polarization of 100% at the Fermi level which makes it a good candidate to be used in hard disks and spintronic devices.
In order to prepare magnetite thin films, the control of purity, roughness and uniformity of the layers by electrochemical technology has been tested. The magnetite was electrodeposited using the method proposed by Jay A. Switzer et al. [1]. A glass/Ti(4nm)/Au(250nm) substrate previously annealed to obtain a (111) gold orientation, was selected. Galvanostatic control was mainly used and the morphology, crystalline structure, rate of growth and magnetic properties of the films analysed.
Deposition conditions were subsequently optimized for the lowest surface roughness value and control over the growth rate.
Films ranging from 80 nm-3.75 µm-thick were obtained and characterised. The SEM and AFM images show granular deposits with a low roughness of 5 to 40 nm. Based on measuring the transversal section of the thin films, a growth rate of 7.5 nm/s was estimated at a current density of 10 mA/cm2. Pure magnetite with an fcc structure is observed in the XRD diffractograms. Magnetisation-applied magnetic field curves show behaviour compatible with ferrimagnetism and the hysteresis loop is comparable to pure magnetite obtained from other methods.
The adsorption of both oleic acid and dodecanethiol on the previously prepared magnetite films has been tested by immersion of the samples in different ethanol solutions containing the organic molecules. Monolayer formation in both cases has been detected from both the contact angle of water and voltammetric measurements.
[1] H. M. Kothari, E. A. Kulp, S. J. Limmer, P. Poizot, E. W. Bohannan and J. A. Switzer. J. Mater. Res.
21(1) (2006) 293.
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ENHANCED GIANT MAGNETORESISTANCE IN Co-Ag ELECTRODEPOSITS
Jose Garcia-Torres, Elisa Vallés, Elvira Gómez
Electrodep, Departament de Química Física and Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain.
jm.garcia@ub.edu
Although giant magnetoresistance (GMR) phenomenon was discovered in the Fe/Cr system, some years later a great number of systems were found to exhibit this property.
Among those systems Co-Ag system seems to be a good candidate as magnetoresistive material, because sharp magnetic/non-magnetic interfaces are expected in view of the total immiscibility between the two metals shown by the phase diagram.
The preparation of the Co-Ag system by electrodeposition was tackled in our research group as a simple and low cost way to prepare magnetoresistive materials and as alternative to the usual physical methods. Due to the highly different electrodeposition conditions for both metals, a complex sulphur-based bath was developed, containing thiourea, gluconate salt and boric acid, and which led to rough granular deposits.
Further optimization of the electrodeposition conditions gave rise to improve deposits coherence, to decrease roughness and to a better control of the film’s composition with the applied potential. Moreover, these deposits contained up to 2 wt.% of sulphur.
However, these deposits did not change their resistance with the applied magnetic field at room temperature. Small GMR values were only measured at low temperatures (40K). Taking into account that sulphur presence could exert a deleterious effect on the GMR, thiourea bath concentration was decreased in order to reduce sulphur incorporation into the deposits. Gluconate concentration was simultaneously increased in order to keep the quality of the deposits. The diminution of the sulphur content up to 0.8 wt.% led to obtain higher GMR values but only at low temperatures. The deleterious effect of sulphur was confirmed preparing Co-Ag deposits from a thiosulphate bath and with no GMR at any conditions. It was explained by the blocking of the magnetic/non-magnetic interface by a sulphur “barrier”.
In order to avoid the incorporation of third species into the deposit, the preparation of the Co-Ag system from a chloride-based electrolyte was explored. Deposits with GMR at room temperature were obtained in all the conditions tested. Influence of Co(II)/Ag(I) ratio in the electrolyte, film’s composition, kind of electrochemical signal applied and film’s thickness on the magnetotransport properties were investigated. Co-Ag deposits prepared from a 0.002 M AgNO3 + x M CoCl2 + 3.5 M NaCl solution with 0.1 M ≤ x ≤ 0.2 M showed the best GMR results. The GMR values measured in this work practically double those previously reported for Co-Ag electrodeposits. The no saturation of the MR(H) curves even at the highest applied magnetic field induced to think in a high superparamagnetic contribution to the total GMR.
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Surface alloying during Pb underpotential deposition on Au(111) Jeerapat Nutariya, Walther Schwarzacher and Natasa Vasiljevic
Physics Department, University of Bristol H.H. Wills Physics Laboratory, Tyndall Avenue,
Bristol BS8 1TL
In the present work we examined Pb underpotential deposition (UPD) on Au(111) thin film electrodes. This system is characterized by large misfit and interesting surface stress behaviour. During Pb monolayer formation, the Pb/Au(111) system exhibits surface alloying/dealloying transition. The changes in the surface structures and the kinetics of surface alloying were examined by cyclic voltammetry and extended polarization measurements at different potentials i.e. Pb coverages. The mechanisms of Pb monolayer formation at different stages of the growth have been studied by combination of in-situ techniques: laser beam reflection and scanning tunnelling microscopy (STM).
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