Electrodeposited nanowires. — Ni-Co/Cu multilayered nanowires were fabricated by template-assisted nanowire growth in anodized aluminium oxid (AAO) membrane from an optimized aqueous electrolyte by using two-pulse plating. The segmented magnetic/non-magnetic nanowire structure was achieved during growth along the length of the nanowires. The electrodeposition parameters were set to obtain equal thicknesses of both kinds of layer with a repetition period of 10 nm. Structural characterization of the nanowires was carried out using SEM, TEM, HRTEM, SAED and EELS techniques in collaboration with the Universitat Autònoma de Barcelona, Spain.
These techniques yielded results confirming the multilayered structure of the nanowires. Figure 1 shows the TEM image of a single multilayered Ni-Co/Cu nanowire which was released from the AAO membrane by dissolving the template. An alternating dark and light layering sequence can be observed at the edges of the wire, both layer types having an approximate thickness of 5 nm. The diffraction pattern was measured by using SAED (see inset in Fig. 1) from the red rectangular area of the nanowire. After indexing the pattern, we found that both the magnetic and non-magnetic layers have an fcc structure. Furthermore, from the magnetic hysteresis loops measured on the nanowire array within the template in a VSM, with the magnetic field being parallel or perpendicular to the wire growth direction, we found a coercivity value of 118 Oe or 89 Oe, respectively.
Figure 1. TEM image of a Ni-Co/Cu multilayered single nanowire (Inset: SAED pattern of the red zone marked on the TEM image).
Thermoelectric properties of electrodeposited multilayered nanowires. — Thermoelectric (TE) measurements have been performed on single electrodeposited nanowire samples exhibiting either the giant or the anisotropic magnetoresistance effect (GMR and AMR, respectively). The temperature-dependent (50–300 K) and magnetic field-dependent (up to 1 T) TE power factor (PF) has been determined for several Co-Ni alloy nanowires with varying Co:Ni ratios as well as for Co-Ni/Cu multilayered nanowires with various Cu layer thicknesses, which were all synthesized via a template-assisted electrodeposition process. A systematic investigation of the resistivity as well as the Seebeck coefficient was performed for both types of nanowire samples. The TE PF was found to increase by up to 13.1 % for AMR Co-Ni alloy nanowires and by up to 52 % for GMR Co-Ni/Cu samples in an external applied magnetic field. The magnetic nanowires exhibit TE PFs that are of the same order of magnitude as TE PFs of Bi-Sb-Se-Te-based thermoelectric materials. This effect offers an opportunity to adjust the TE power output to changing loads by external magnetic fields. (These results were achieved in an international cooperation with five European partner institutes.)
Coercivity of nanoscale electrodeposited cobalt layers. — The magnetic properties and the magnetoresistance behavior were investigated for electrodeposited nanoscale Co films, Co/Cu/Co sandwiches and Co/Cu multilayers with individual Co layer thicknesses ranging from 1 nm to 20 nm. The measured saturation magnetization values supported reasonably the validity of the nominal layer thicknesses. All three types of layered structure exhibited anisotropic magnetoresistance for thick magnetic layers whereas the Co/Cu/Co sandwiches and Co/Cu multilayers with thinner magnetic layers exhibited giant magnetoresistance (GMR), the GMR magnitude being the largest for the thinnest Co layers. The decreasing values of the relative remanence and the coercive field when reducing the Co layer thickness down to below about 3 nm indicated the presence of superparamagnetic (SPM) regions in the magnetic layers which could be more firmly evidenced for these samples by a decomposition of the magnetoresistance vs. field curves into a ferromagnetic and an SPM contribution. For thicker magnetic layers, the dependence of the coercivity (Hc) on magnetic layer thickness (d) could be described for each of the layered structure types by the usual equation Hc = Hco + a/dn with an exponent around n = 1. The common value of n suggests a similar mechanism for the magnetization reversal by domain wall motion in all three structure types and hints, at the same time, at the lack of coupling between magnetic layers in the Co/Cu/Co sandwiches and Co/Cu multilayers.
Composition depth profile measurements on electrodeposited Ni-Cu/Cu multilayers. — The composition depth profile of electrodeposited Ni-Cu/Cu multilayers has been studied with Glow Discharge – Time-of-Flight Mass Spectrometry (GD-ToFMS) technique, a relatively new method that was not used before to investigate electrodeposited materials. The GD-ToFMS technique proved to be suitable to detect the layer structure and also to distinguish Ni layers with various Cu contents (8-50 at.%; see Fig. 2).
Figure 2. Quantitative composition depth profile of an electrodeposited multilayer sample with the following nominal layer structure: Cu(20nm)/Ni-Cu-1(80nm)/Cu(80nm)/Ni-Cu-2(80nm)/Cu(80nm)/Ni-Cu-3(80nm)/Cu(20nm).
It was found that some impurities (especially C but also Na and Cl, all originating from the components of the solution) accumulate in the Cu layer. The presence of the impurities in the Cu layer may cause a high resistivity and can yield an explanation why electrodeposited multilayers obtained from citrate baths have inferior magnetoresistance properties as opposed to nominally identical multilayers obtained from bath with no organic components. (This study was performed in co-operation with the University of Oviedo, Spain.)