37
7th International Workshop on Electrodeposited Nanostructures
Potentiostatic Electrodeposition of Nickel Nanowires inside
38
The impact of a superimposed magnetic field on the free corrosion of iron R. Sueptitz*, K. Tschulik, M. Uhlemann, A. Gebert, L. Schultz
Leibniz Institute for Solid State and Materials Research IFW Dresden, Helmholtzstr. 20, D-01069 Dresden, Germany
*corresponding author: r.sueptitz@ifw-dresden.de Tel.: +49 351 4659 715
The influence of magnetic fields oriented parallel and perpendicular to the electrode surface on the free corrosion of differently shaped iron samples in low concentrated sulphuric acid solutions has been studied. It is demonstrated, that the relative sample-to-magnet configuration, which determines the magnetic flux density distribution in front of the electrode surface, is decisive for the free corrosion activity. In a configuration generating low magnetic flux density gradients the Lorentz force driven micro-convection leads to an anodic shift of the free corrosion potential. In contrast, a configuration yielding high magnetic flux density gradients causes a cathodic potential shift and leads to a suppression of the corrosion reaction. These effects are discussed on the basis of the Lorentz force and the magnetic field gradient force acting on the partial reaction steps during the corrosion process.
39
GIANT MAGNETORESISTANCE STUDY OF ELECTRODEPOSITED Co-Ni/Cu MULTILAYERS
B.G. Tóth*, L. Péter, J. Dégi and I. Bakonyi
Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences.
H-1525 Budapest, P.O.B. 49, Hungary
Among multilayers prepared with physical methods, Ni-Co/Cu multilayers have simultaneously large giant magnetoresistance (GMR) and low GMR saturation fields [1]. Therefore, in this work, several series of electrodeposited Co-Ni/Cu multilayers were prepared in order to study the effect of the Co:Ni ratio of the magnetic layer, the thickness of both the non-magnetic and the magnetic layer as well as the total multilayer thickness on the GMR.
Si wafers were used as substrate with evaporated Cr(5 nm) and Cu(20 nm) layers.
Electrodeposition was carried out from a sulfate+sulfamate bath with a galvanostatic/potentiostatic (G/P) pulse combination [2]. The Cu-deposition potential was optimized [2] to avoid both Co dissolution and the codeposition of Co. The Co-concentration in the magnetic layer was varied from 6.9 to 99.7 at.% by changing the c(CoSO4):{c(CoSO4)+c(NiSO4)} concentration ratio. The deposition conditions ensured that the Cu-content in the magnetic layer was about 0.6 at.% only [3].
The magnetoresistance (MR) was determined by a four-point-in-line probe in magnetic fields up to H = 8 kOe. Both the longitudinal (LMR, magnetic field parallel to current) and transverse (TMR, magnetic field perpendicular to current) components were measured. All multilayers investigated exhibited a GMR effect since the LMR and TMR components had the same sign with a small difference in their magnitude due to the anisotropic magnetoresistance of the magnetic layers. Except for the smallest values of both layer thicknesses, the MR(H) curves saturated for magnetic fields above about 2 kOe and this indicated the absence of a significant superparamagnetic contribution to the GMR.
When varying the Co:Ni ratio in the magnetic layer while keeping both layer thicknesses constant, the GMR showed a maximum at about 50 at.% Co content. The layer thickness dependences were subsequently studied at this magnetic layer composition.
The effect of the variation of the Cu layer thickness and the total sample thickness (Σd) was investigated for a constant magnetic layer thickness of 2.0 nm. For all total thicknesses (50, 100, 300 and 700 nm), the GMR showed a maximum at around 5 nm Cu layer thickness (left figure). By increasing the total thickness of the multilayer, the GMR increased until 300 nm and a slight decrease was observed for 700 nm. However, if the resistivity change ∆ρGMR due to the GMR effect (calculated by using the measured values of the zero-field resistivity) is displayed (right figure), the GMR contribution increases permanently with total multilayer thickness. These data can be analyzed by assuming specific models for the evolution of multilayer microstructure quality with total multilayer thickness (bilayer number).
1 2 3 4 5 6 7 8 9
-5 -4 -3 -2 -1 0
Σd=50nm LMR Σd=50nm TMR Σd=100nm LMR Σd=100nm TMR Σd=300nm LMR Σd=300nm TMR Σd=700nm LMR Σd=700nm TMR dNiCo = 2.0 nm
GMR (%)
dCu (nm) -0.50 1 2 3 4 5 6 7 8 9 10
-0.4 -0.3 -0.2 -0.1 0.0
Σd=50nm ∆ρL Σd=50nm ∆ρT Σd=100nm ∆ρL Σd=100nm ∆ρT Σd=300nm ∆ρL Σd=300nm ∆ρT Σd=700nm ∆ρL Σd=700nm ∆ρT
dNiCo = 2.0 nm
∆ρGMR (µΩcm)
dCu (nm)
[1] Y. Bian et al., J. Appl. Phys. 75, 7064 (1994); H. Kubota et al., J. Magn. Magn. Mater. 129, 383 (1994) [2] L. Péter et al., Electrochim. Acta 49, 3613 (2004)
[3] I. Bakonyi et al., J. Electrochem. Soc. 155, D688 (2008)
*Corresponding author. E-mail: tothb@szfki.hu
40
STRUCTURE AND ELECTRICAL TRANSPORT PROPERTIES OF ELECTRODEPOSITED Ni-Co ALLOYS
B. G. Tóth1,*, L. Péter1, Á. Révész2, J. Pádár1 and I. Bakonyi1
1Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences.
H-1525 Budapest, P.O.B. 49, Hungary
2Department of Materials Physics, Eötvös University. H-1528 Budapest, P.O.B. 32, Hungary In the present work, the electrical transport properties (zero-field resistivity, temperature dependence of the resistivity, residual resistivity and the anisotropic magnetoresistance, AMR) were studied for electrodeposited (ED) Ni-Co alloys. For this purpose, Ni-Co alloy layers having a thickness of about 2 µm were prepared on Si wafers with evaporated Cr and Cu underlayers. The Ni-Co alloy deposits were investigated first in the as-deposited state on the substrates and then, by mechanically stripping them from the substrates, as self-supporting layers both without and with annealing.
It was established by X-ray diffraction that the ED Ni alloy deposits exhibited an fcc phase. A strongly textured fcc structure was found in the as-deposited state with an average grain size (D) of about 10 nm. Upon annealing, whereas the crystal structure was retained, D increased by a factor of 3 to 5, though the alloys still remained in the nanocrystalline regime.
The room-temperature zero-field resistivity (ρ0) was found to decrease strongly by annealing due to the increased grain size which led to a reduction of the grain-boundary scattering contribution.
-10 0 10 20 30 40 50 60 70 80 90 100 110 10
20 30 40 50 60
70 (111) annealed
(200) annealed (220) annealed (220) unannealed
D (nm)
cCo (at.%)
0 20 40 60 80 100
0 5 10 15 20
25 with substrate, unannealed -31.3 mA/cm2
with substrate, unannealed -18.8 mA/cm2 without substrate, annealed fcc-Ni [38], fcc-Co [39]
ρ 0 (µΩcm)
cCo (at.%)
By measuring the temperature dependence of the resistivity down to 13 K, the residual resistivity (ρres) of the annealed ED Ni-Co alloys were determined and good agreement was found with previously reported data on metallurgically processed Ni-Co alloys. This means that the nanocrystalline state of the annealed Ni-Co alloys with grain sizes around 30 to 50 nm apparently does not give a significant contribution to ρres. The residual resistivity exhibits a maximum at about 20 to 30 at.% Co, i.e. somewhat shifted from the expected equiatomic composition which would be expected according to Nordheim’s rule.
The AMR data measured at room temperature and 13 K on the substrate-free ED Ni-Co alloys either in unannealed or annealed state were in relatively good agreement with reported values on bulk Ni-Co alloys prepared by metallurgical means. The AMR values also exhibit a maximum in the same composition range as the residual resistivity in this alloy system.
*Corresponding author. E-mail: tothb@szfki.hu
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Magnetoelectrodeposition of CoFe nanowires in ordered alumina templates M. Uhlemann, J. Koza, Ch. Mickel, A. Gebert, L. Schultz
IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany
Nanostructured deposition and in particular deposition of magnetic materials in ordered nanoscaled templates has been of interest since several years. Numerous investigations have been reported for Co, Fe, Ni based metals and their alloys as well as multilayered systems deposited in alumina templates.
CoFe alloys electrodeposited as wires or tubes are of interest due to their magnetic properties [1], high saturation magnetisation and Curie temperature and low coercivity. Due to the cubic structure of CoFe the magnetocrystalline anisotropy is negligible. In dependence on diameter the wires show shape anisotropy that means the wires have a perpendicular anisotropy with the easy axis parallel to wire axis. It has been observed for CoFe thin layers that superposition of magnetic fields during the deposition induces in-plane magnetic anisotropy [2]. Applied magnetic field in wire axis during deposition could also induce magnetic anisotropy.
Additionally stress induced anisotropy has to be taken into account and is expected from TEM investigation [3]. Mainly microstructure determines the magnetic behaviour but very high interactions between the CoFe wires due to the highest magnetic moment have been observed by MFM investigation shown in Fig. 1. No effect of external magnetic field on phase
composition and crystallographic structure of the nanowires have been detected; all wires are polycrystalline and exhibit a fibre texture.
In this work the deposition behaviour and the properties of electrochemically deposited CoFe nanowires have been investigated and characterised by SEM, TEM, VSM, and MFM and discussed with respect to the deposition conditions and applied external magnetic field.
Fig. 1: CoFe nanowires deposited in ordered alumina, polished from the top, (a) SEM, (b) AFM, (c) MFM images
[1] T. Osaka: Electrochim. Acta 50 (2005) 4576
[2] A.Koza, F. Karenbach, M. Uhlemann, J.McCord, Ch. Mickel, A. Gebert, St. Baunack, L.
Schultz: Electrochim. Acta 55 (2010) 819 [3] A.Kumar et.al, Phys.Rev.B73 (2006) 064421
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