The magnetoresistance effect in magnetic nanowires

The magnetoresistance (MR) effect is the variation of the electrical resistance of a magnetic material under the effect of an external magnetic field. The MR effect has been used widely as MR sensor that found applications for measuring or detecting a magnetic field. One-dimensional nanowires with large aspect ratio have received much attention due to unique shape anisotropy and extremely large surface area [1,2]. These nanowires could be used for magnetic field sensors with MR effect. The characteristic magnetic properties of nanowires can be controlled by choice of material and dimensions [3]. In this thesis, MR properties of electrodeposited NiFe, CoNiFe and Co/Cu nanowires are investigated. Firstly, CoNiFe and NiFe nanowires electrochemically produced by dc electrodeposition on highly ordered porous anodic alumina oxide (AAO) templates. Scanning electron microscope (SEM) images of nanowires after removing the alumina membrane showed that these nanowires are uniform with the diameters of about 220-300 nm and about 25-30 µm length. The energy dispersive X-ray (EDX) analysis showed that the composition of the nanowires is Ni93Fe6, Co12Ni64Fe24. NiFe alloy nanowires exhibit nanogranular structure, but CoNiFe alloy nanowires show much smoother surface along the wire. Magnetic field was applied by an electromagnet and resistance was measured by four point technique. The maximum MR change of 0.35 % was observed from NiFe nanowires at 50 mA bias current. Secondly, we have employed the pulsed electrodeposition technique to obtain Co/Cu multilayer nanowires whose layer thicknesses are tuned by applying electrodeposition potential having various pulses time. SEM showed that wires have diameters of about 250-300 nm and length 40-50 µm. The structure and magnetic characteristic of multilayer nanowires are strongly depend on various parameters like electrolytic bath content and concentration, pH, temperature as well as time and potential of the electrodeposition puls. The maximum value of MR ratio of about 0.65 % was obtained for the nanowires array deposited at pulse time 2 s for Co and 10 s for Cu. Keywords: magnetoresistance, nanowire, electrodeposition, nanocontact.