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Nanoporous particle track etched membranes -ptm- and their use as templates for electrodeposited multilayers for giant magnetoresistance applications - gmr

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An efficient process for the realization of patterned nanoPTM from polycarbonate (PC) film has been developed. These patterned nPTMs consists of membranes in which a well-defined number of pores are confined to arranged areas of well-controlled shape and size. First results show that samples with porous areas of 10 µm in diameter can be obtained. Patterned nanoPTMs samples have been realised at the laboratory scale and have been successfully used as templates for the electro-deposition of metals and conducting polymers. These filled patterned nPTMs allow electric or magnetic measurements on a restricted and known number of nanowires. This is very helpful for the precise definition of the nanowires properties at the nanoscale. The patterned membranes are likely to be of particular value in applications requiring differentiated zones of pores and properties across the membrane for example arrays of sensors. A patent has been filed concerning this technology which will be further developed within a new project funded under the Framework 5 programme of the EC. End users are likely to include sensor, display and analytical measurement systems companies.
This result concerns the use of magnetic nanowires (multilayered or not) in the field of sensors and more particularly in the field of magnetoresistive sensors. The result consists of know-how of the magnetoresistive properties of these materials and their dependence with the various material parameters. The result extends to know-how in device processing of these materials. In the field of magnetic sensors based on giant magnetoresistance, this result brings a way to use the particular measurement geometry with the current flowing perpendicular to the plane of the layers, which allow to increase the amplitude of the magnetoresistive effects by about an order of magnitude. The magnetic geometry leads to high electrical resistance (of the order of 1k? for a single wire) which is comparable to the resistance of tunnel junctions and two orders of magnitude higher than the typical values for CIP(Current in Plane) GMR systems. The market sectors which are potentially concerned by this result are the magnetic recording industry, the automotive industry and more generally the magnetic sensor industry including navigation systems, electrical current measurements,
The existing production technology has been significantly enhanced to allow the reliable and reproducible manufacture of nanoporous Polycarbonate (PC) membranes with controlled pore size and shape. The new technology allows the realisation of nanoPTM from PC film characterised by cylindrical smooth pores with diameters in the range 15 - 100nm, and with pore densities in the range 1E6 to 5E9 cm-2. These types of membranes fit within the Ultra-filtration market (pore size <100nm). Contrary to the previous commercially available nanoPTM materials, which exhibit barrelled pore shape with a high degree of pore wall roughness, the new PC nanoPTM are perfectly cylindrical, smooth and accurately controlled. This makes them ideal for their use as templates: e.g. for the synthesis, by electro-deposition, of single and multi-layered metallic nanowires. Due to their controlled and homogeneous diameter all along their length, these nanowires have been found to be especially suitable for the study of the GMR properties, and consecutively for the evaluation of the use of this GMR effect for a range of applications. The new technology is now used for the production of PC nanoPTM lab-scale samples and demonstrated at a production level although scaling - up issues remain to be solved. A patent has been applied for. Applications of the new membranes to other fields (such as filtration /separation) will take time to develop and will probably be in niche areas where there is a specific need for pores with controlled geometry.
Methods have been developed for the electro-deposition of single metals and multilayered combinations of metals into the pores of nano-porous polymer track etched membranes (nPTM). NPTMs with pore sizes in the range 30 - 100nm and above can be used and nanowires of different sizes and lengths made depending on the thickness of the membrane and the pore size. These nanowires can either be used in situ or extracted from the membrane by dissolving the polymer. Single metals such as Co, Cu, Ni, Fe, Au, Ag etc and also alternating multilayered metallic nanowires of controlled layer thickness in the nanometer range for combinations of metals such as Ni/Cu, permalloy(NiFe)/Cu, Co/Ag, Co/Au, .Fe/Cu etc can be electro-deposited into the nanopores. The single metal nanowires are useful for studying the magnetic properties such as switching whilst multilayered structures exhibit strong perpendicular GMR effects. Both of these effects will be useful in next generation sensitive magnetic sensor systems and possibly, also in the longer term, in data storage. In addition, multilayered coatings covering larger areas can be deposited. Electro-deposition is potentially a much lower cost method than for example MBE whilst the nanowire geometry also offers advantages in terms of signal size and localization. The end users of this result are likely to be companies making filled membranes, sensors producers and also people interested in multilayer deposition techniques in for example the electronics industry.
Experimental investigations have revealed a drastic change in the magnetic anisotropy properties of hcp Co nanowires upon varying their diameter. As the wire diameter is reduced below 40-50nm the preferential orientation of the crystal easy axis of magnetization (c-axis) changes from perpendicular to parallel to the wire axis. This strongly affects the zero field magnetic domain structure which form in the nano-wires. For small diameters, both shape anisotropy and crystal anisotropy favour an alignment of the magnetisation along the wire axis. As a consequence, the magnetisation is oriented longitudinally and a single domain structure can be realized after applying a saturating magnetic field along the wire. After the application of a transverse saturating field, the magnetic configuration consists of a succession of several longitudinally magnetized anti-parallel domains. Due to the severe lateral confinement of the magnetisation, these domains are forced to meet head-on, which never occurs in bulk material. In nanowires with large diameters, a single domain state cannot be achieved in zero field, although the very elongated shape of the objects favors such a magnetic configuration. Instead, complex multidomain patterns always form in zero field, in which the domain magnetisation has a strong transverse component. This results from a competition between the shape anisotropy and the crystal anisotropy which tends to align the magnetisation normal (or almost normal) to the wire axis.

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