Silvia Franz

Microelectrodeposition of Co–Pt alloys for micromagnetic applications

Abstract Miniaturisation of advanced devices, such as sensors and micromotors, requires the development of micromagnets of small size but with a high magnetostatic energy. Cobalt alloys, and in particular cobalt–platinum alloys, are proposed for such applications, as well as for magnetic recording, due to their magnetic and corrosion resistance properties. Such alloys can be electrodeposited with good magnetic properties at high thickness by addition of phosphorus and other elements that favour a stable growth of the microstructure during deposition. Results on electrodeposition of Co–Pt–W(P) and Co–Pt–Zn(P) layers from sulphamate citrate solutions are presented. Thick deposits of high coercivity and layers with different preferred orientation are obtained by changing the operation conditions. The deposits have magnetic anisotropy, which favours their implementation in magnetic circuits where the field is perpendicular or parallel to the surface: in the former case, the magnetic easy direction of cobalt alloys must be perpendicular to the surface and in the latter, it must be parallel to the surface. The magnetic properties of Co–Pt-based alloys can be tailored to obtain the desired magnetic properties in relation to the application envisaged.

Pulse plating of Ni–W alloys from model electrolytes

Abstract Pulse plating of nickel–tungsten alloys has been studied from two citrate ammonia based model electrolytes containing either an excess of nickel or an excess of tungsten. The alloy composition and current efficiency were determined for pulse periods ranging from 1 ms to 50 s at a constant duty cycle of 0˙4. Deposits were analysed by X-ray fluorescence and by gravimetry. In the nickel rich electrolyte the rate of co-deposition of tungsten was found to be controlled by mass transport as previously observed with Ni–Mo alloys. In the tungsten rich electrolyte the tungsten concentration in the deposited alloys was the highest at short pulse periods. This suggests that the deposition rate of tungsten was affected by the rate of transport of nickel which is the highest at short times. As the pulse on time exceeded the transition time for nickel the current efficiency for alloy deposition dropped owing to hydrogen formation.

Microelectrodeposition of cobalt and cobalt alloys for magnetic layers

Abstract Structure and growth characteristics of electrodeposited cobalt are discussed with regard to its electrokinetic behaviour. Three main growth modes are pointed out: outgrowth or perpendicular growth, in conditions of low surface inhibition; lateral growth , with complete coverage of the electrode by hydrolytic species giving a high degree of surface inhibition; cluster growth , in the presence of surface complex species stabilised by boric acid. The structure influence upon the magnetic properties is discussed with reference to the electrodeposition of thick layers of Co–Pt–W alloys, with Pt and W content, respectively in the range 8–15 and 1–5 at.%, having high coercivity and high magnetic moment.

Electrodeposition of micromagnets of CoPtW(P) alloys

Well-oriented permanent magnets of CoPtW(P) alloys with the basal plane parallel to the surface and a columnar structure were obtained by electrodeposition. Relatively high saturation moments and coercivities, both in perpendicular and in parallel magnetic field, are obtained at thickness up to 20 µm. Temperature dependence was also investigated, showing a great decrease of the magnetic properties. Chemical post-treatments can strongly influence magnetic properties, decreasing saturation magnetisation and coercivity while increasing squareness ratio.

Engineered fabrication of ordered arrays of Au–NiO–Au nanowires

In the present paper, a novel method to fabricate ordered arrays of Au/NiO/Au nanowires is described, with the aim of filling the gap between the fundamental study of the electrical properties of scattered single nanowires and the engineered fabrication of nanowire arrays. This approach mainly consists of the following steps: (a) electrodeposition of Au/Ni/Au nanowires into an ordered porous anodic aluminum oxide template; (b) mechanical polishing of the sample to expose the gold tips of Au/Ni/Au nanowires to the template surface; (c) in situ annealing of the Au/Ni/Au nanowires without removing the template. The resulting structure consists in an ordered array of Au/NiO/Au nanowires slightly protruding out of a flat aluminum oxide template. Unlike current approaches, with the described method it is not necessary to remove the template in order to oxidize the middle metal, thus allowing the availability of an entire set of metal/oxide/metal nanowires ordered in a two-dimensional matrix and where single heterojunctions can be accessed individually.

Fundamental aspects and applications of electrodeposited nanostructured metals

A rational understanding of what occurs during electrocrystallization, defined at a nanolevel, is developed to control electrodeposition processes. The electrokinetic behavior of the elements in solutions and the electrodeposits structure resulting from the electron exchange reaction at the cathodic surface are taken into consideration and compared. Transient electrokinetic parameters are measured with the secondary current pulse (SCP) technique, where a square galvanostatic pulse of a few ms duration is superimposed on the cathode while electrodeposition is running. Two parameters are obtained, the transient Tafel slope and the adsorption pseudo-capacitance; whilst a third parameter, the diffusive time constant, must be introduced if the overvoltage does not arrive to a steady state during the short pulse period. These parameters are related to the growth of different structures and permit a good control of the process. Control of the growth with nanodefinition is key to the development of innovative processes to keep pace with more and more demanding applications and environmental challenges. Examples are given to stress the relevance of the theoretical framework and to show possible implications for electrodeposition technology and its applications.

Influence of Magnetic Fields on Autocatalytic Deposition of Co–Fe Thin Films

In the present paper, the effect of magnetic fields (17-98 mT) on the autocatalytic deposition of Co 1―x ―Fe x (35-42 atom %) thin films is addressed. The deposition kinetics was strongly related to field intensity. Low magnetic fields (17 mT) increased the deposition rate by a factor of 2, whereas high magnetic fields (above 76 mT) completely inhibited the deposition process after a few minutes from the beginning of the reaction. Alloy composition was also affected by the application of magnetic fields. Depending on the intensity of the applied field, the Fe content in the alloy gradually changed from 36 to 41 atom % below 200 nm and from 42 to 37 atom % above 200 nm of thickness. On average, the application of magnetic fields during the deposition process increased the coercivity of thin Co-Fe films and made them anisotropic, with the extent of such an effect depending on film thickness.

Calculation of Uniform Corrosion Current Density of Iron in Hydrochloric Acid Solutions based on the Principle of Maximum Entropy Production Rate Applied to Literature Data

In this paper a new procedure for the calculation of uniform corrosion current density is presented. The procedure is based on the following criterion: corrosion current and extent of anodic and cathodic areas are such that the entropy production rate is maximum. Experimental data for the reaction Fe(s) + HCl(aq) = FeCl2(aq) + H2(g), taken from literature, are used to test the new model. A comparison between corrosion current density calculated from weight loss measurements and current density calculated from polarization curves by means of the proposed model is carried out. The proposed model provides a better agreement, between Tafel line extrapolation and gravimetric measurement, in comparison to the mixed potential theory usually adopted.