Superlattice Nanowires
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K. ATTENBOROUGH*, R. HART*, W. SCHWARZACHER*, J-PH. ANSERMET**, A. BLONDEL**, B. DOUDIN** AND J.P. MEIER** *H. H. Wills Physics Laboratory, Tyndall Avenue, Bristol, BS8 1TL. UK **Institut de Physique Experimentale, EPFL, PHB-Ecublens, CH-1015, Lausanne, Switzerland
ABSTRACT
CoNiCu/Cu superlattice nanowires have been grown by electrodeposition in nuclear tracketched nanoporous membranes. Transmission electron microscopy (TEM) images show a good layer structure and allow an estimate of the current efficiency. Current perpendicular to plane (CPP) giant magnetoresistance of up to 22%, at ambient temperature, has been measured but appears to be limited by defects, giving rise to ferromagnetic interlayer coupling, at low nonmagnetic layer thicknesses. Magnetic properties of the superlattice nanowires are influenced by in-plane anisotropy and magnetostatic coupling.
INTRODUCTION
The discovery of giant magnetoresistance (GMR) in the Fe/Cr system and others has resulted in much research into the characterisation and growth of short period metal/metal multilayers. Interest has now moved to studying GMR in the CPP (current perpendicular to plane) direction as it allows a clear separation of interface and bulk contributions to the magnetoresistance"12,3* We have shown that it is possible to grow high-quality epitaxial metal/metal multilayers with individual layer thicknesses as small as - 6A by the relatively simple and inexpensive technique of electrodeposition, using a single electrolyte and switching between two deposition potentials. A GMR of up to 20% was found in the CoNiCu/Cu system using this technique 4 . When an appropriate template is used, electrodeposition also provides a means of growing ultrafine wires having diameters of a few hundred A and a length of several gtm 5' 6' 7 . As a result of combining these techniques and using nanoporous membranes metal/metal multilayer nanowires have now been electrodeposited by various groups8 '9 "0 "'ll. This paper will discuss the preliminary results achieved from CoNiCu/Cu nanowires
EXPERIMENTAL DETAILS
The deposition of the nanowires takes place within the pores of nuclear track-etched polycarbonate membranes (figure 1). The diameter of the pores, and thus the nanowires, was -800A and their length -61tm. A thin layer of gold was evaporated onto the back of the Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
Polycarbonate membrane Pores 6 Jm
Gold coating
800 A Figure 1 Schematic diagram of the polycarbonate membrane and a representation of a nanowire membrane which acted as a substrate and electrode. X-ray diffraction showed that the gold was polycrystalline with mainly (111) and (200) textured regions. The nanowires were grown from a sulfamate electrolyte containing Co 2', Ni2 ÷ and Cu2÷ ions, similar to that used for our conventional large area multilayers4. Experience in growing these large area multilayers has shown that the pH of the electrolyte is an important factor in the growth and influences the magnitude of the GMR in the samples12.
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