One-dimensional nanowires with a large aspect ratio have received much attention due to their unique shape anisotropy and extremely large surface area. Metallic nanowires such as Au, Ag and Cu nanowires [1-3] and semiconductor nanowires [4-9] would show unique electron transport properties which are not observed in bulk metals. Ferromagnetic nanowires such as Ni, Co and Fe alloy nanowires as well as Co/Cu multilayered nanowires would be best candidate materials for magnetic field sensors with anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) effect. Nanowires can be fabricated by manipulating metallic atoms one by one using scanning tunneling microscopy (STM) or catalyst-assisted wet chemical etching technique , while they can be also prepared by electrochemically depositing metallic atoms into a nanochannel template with numerous nanochannels . Nanochannel templates such as polycarbonate membrane films or anodized aluminum oxide films with high density of nanochannels (about 108~1010 cm–2) can be used in the template synthesis technique .
Using polymer membrane filters, ferromagnetic metal nanowires have been synthe sized so far. Whitney
On the contrary, using anodized aluminum oxide pores on the surface of metallic aluminum substrates, Ni, Co and Fe homogeneous ferromagnetic nanowires have been also electrodeposited and characterized in terms of their magnetization properties. Kawai
Electrodeposition of metallic nanowires into the as anodized aluminum template can be carried out using an alternating current or a pulsed current in order to reduce the charging up effect of the barrier layer at the interface between the anodized aluminum oxide layer and the metallic aluminum substrate. However, this resistive barrier layer makes it difficult to achieve good electric contacts at the pore bottom and well-controlled layered structure of electrodeposited nanowires. In this chapter, fabrication process of anodized aluminum oxide template without the barrier layer and the magnetoresistance properties of electrodeposited Ni, Co alloy nanowires and Co/Cu multilayered nanowires were investigated to synthesize novel functional ferromagnetic devices with anisotropic magnetoresistance (AMR) effect and giant magnetoresistance (GMR) effect.
2. Fabrication of Anodized Aluminum Oxide Nanochannels Templates
2.1. Barrier Layer Thinning by Chemical Etching Technique
Aluminum sheets with thickness of 500 μm were used as a starting material to prepare anodized aluminum templates. First, the aluminum sheets were electrochemically polished in a ethanol solution containing 25 vol% of perchloric acid to achieve a mirror like surface. During the electrochemical polishing, the cell voltage was kept at 8 V for 10 min at room temperature. Then, the polished aluminum sheets were anodized in an aqueous solution containing 0.3 mol/L oxalic acid to obtain a nanoporous aluminum oxide layer on the surface. During the anodization, the cell voltage was kept at 50 V for 10 min at room temperature. Subsequently, the anodized aluminum sheets were immersed in an aqueous solution containing 5 vol% of phosphoric acid for 50 min to widen the pores, and to thin the oxide layer at the pore bottom. These processing parameters give pores 2 μm long with a diameter of 60 nm.
Figure 1 shows the time dependence of current density on the fabrication of self-organized nanoporous anodized aluminum templates. Anodization was conducted under 50 V in aqueous solution containing 0
Figure 2 shows SEM images of a cross section view of an anodized aluminum template. The oxide layer has a typical porous columnar structure and the pore length is approximately 2μm as shown in Figure 2. The pore diameter is approximately 60 nm and the order of pore density is 1010 pores/cm2 as shown in Figure 2. It is well known that an anodized aluminum template has a resistive oxide layer (barrier layer) at the interface between the anodized aluminum oxide film and the metallic aluminum substrate. Figure 3 shows SEM images of a pore top planar view (a) and a pore bottom back side view (b) of an anodized aluminum template. For observation of pore bottom back side (barrier layer), the massive metallic aluminum substrate was dissolved in hydrochloric acid containing traces of cupric ions. The pore diameter is approximately 60 nm and the order of pore density is 1010 pores/cm2 as shown in Figure 3(a). The existence of a barrier layer is made obvious by looking at the pore bottom of anodized aluminum oxide layer (Figure3(b)).This resistive barrier layer makes well-controlled electrodeposition of layered structures difficult and lowers their AMR and GMR performance. Electrodeposition of metals into the pores of an anodized aluminum surface is usually carried out using alternating current or a pulsed current technique [37–40] in order to reduce the charging up effect of the barrier layer. To remove this barrier layer, the massive metallic aluminum backing is usually dissolved in an aqueous solution containing HgCl2 prior to thinning the barrier layer [41–45]. However, this aluminum substrate dissolving technique can be applied to thick aluminum oxide layers with the thickness of several tens micro meters. On the contrary, when this barrier layer is removed or thinned without removing the aluminum backing, anodized aluminum templates with short pores length can be obtained. Since the thickness of the barrier layer is about several tens of nanometres, this layer can be removed or thinned using a chemical etching technique from the pore side direction . Therefore, after the anodization process, to remove or thin this barrier layer without dissolving the metallic aluminum backing, the anodized aluminum templates were subsequently immersed in an aqueous solution containing phosphoric acid prior to the electrode position process.
2.2. Barrier Layer Thinning by Anodization Voltage Controlling
Figure 4 shows experimental apparatus and time-dependence of applied voltage to synthesize an anodized aluminum oxide membrane filter. Anodized aluminum oxide thick films with numerous nanochannels were exfoliated mechanically by the pressure of hydrogen gas generated at the interface between an oxide layer and a metallic aluminum during the subsequent cathodic reduction process after the growing of anodic aluminum oxide layer. At first, 70 V was applied to growing aluminum oxide long nanochannel. Then, the anodization voltage was decreased gradually down to 0 V for thinning the pore bottom oxide layer (barrier layer). Finally, cathodic voltage was applied to exfoliate an anodized aluminum oxide membrane film from the metallic aluminum rod due to the hydrogen evolution. Figure 5 shows surface appearance of anodized aluminum oxide membrane filters exfoliated from a metallic aluminum rod. These membrane filters were obtained by anodizing at 50 V (Figure 5(a)) and 70 V (Figure 5(b)). The membrane filter anodized at 70 V had round and disc shape with the diameter of 10 mm (Figure 5(b)) while that anodized at 50 V had several cracks (Figure 5(a)).
Figure 6 shows SEM images of barrier layer side (plan view (a), cross section (b)) and surface side (plan view (c), cross section (d)) of an anodized aluminum oxide membrane filter exfoliated from a metallic aluminum rod. The anodization was conducted at 70 V. The average channel diameter at the surface side was around 60 nm while that at the barrier layer side was ca. 20 nm. The oxide layer has a typical porous columnar structure and the channel length was ca. 60 μm while the channel density was around 108 channels / cm2.
Figure 7 shows the effect of anodization voltage on the growth rate of anodized aluminum oxide layer. The oxide layer thickness was determined by observing the cross sectional SEM images. With increasing the anodization voltage
According to the equation (1), if
According to the equation (2), if
Figure 8 shows the relationship between film thickness of anodized aluminum oxide layer and anodization time. The anodization was conducted at 70 V. The film thickness
Therefore, the real time growth rate of the anodized aluminum oxide layer
Therefore, the real time growth rate of the anodized aluminum oxide layer
As mentioned previously, the real time growth rate of the anodized aluminum oxide layer
3. Electrodeposition and Magnetoresistance of Ni and Co Nanowires Array
3.1. Electrodeposition of Ni and Co Nanowires Array
Figure 9 shows experimental apparatus and a nanochannel template for electrodeposition of nanowires array. The exfoliated anodized aluminum oxide nanochannels were used as templates for growing nanowires. On surface of the membrane filter, a gold layer was sputter-deposited to cover the pores and make a cathode. A gold wire and Ag/AgCl electrode were used as an anode and a reference electrode. For example, aqueous solution containing NiSO4 (120 g/L) and H3BO3 (45 g/L) was used as electrolyte for electrodeposition of Ni nanowires.
Figure 10 shows cathode polarization curves for Ni and Co electrodeposition from aqueous solution containing Ni2+ or Co2+ions. According to Nernst equation, the equilibrium potentials of Ni/Ni2+ and Co/Co2+ are estimated to be around −0.46 and −0.48V (vs. Ag/AgCl). As shown in Figure 10, the cathodic currents occur at around -0.2V which is more nobler potential region than the equilibrium potential of Ni and Co. Therefore, this cathodic current would be mainly caused by the reduction current of hydrogen ions. With increasing the cathodic current density, at around 2A/m2, the potential polarizes significantly to the less noble region. This polarization would be caused by the diffusion limit of hydrogen ions. At around -0.8V which is less nobler potential region than the equilibrium potential of Ni and Co, the cathodic current increases again. It is well-known that the electrodeposition of iron-group metals such as Ni, Co and Fe is accompanied by the over potential due to the rate determination of multi-step reduction process even in the form of their aqua ions. Therefore, in the present work, Ni2+ and Co2+ ions would electrodeposit with accompanying the overpotential even in simple aqueous solutions containing sulfuric and boric acid. Consequently, this cathodic current would be mainly caused by the reduction current of Ni2+ and Co2+. These characteristics are identical to those obtained with the pores in nanochannel polycarbonate templates with metallic gold cathode. This result supports the observation that the barrier layer at the pore bottom of an anodized aluminum template is well removed by the exfoliation process from a metallic aluminum rod and the nanowires growon metallic gold cathode. Using the polarization curves, the optimum electrodeposition potential range for growing Ni and Co nanowires are determined to be from −1.0 to -1.2V (vs. Ag/AgCl) which is the potential region more nobler than the diffusion limit of each metal ions.
Time dependence of cathode current was monitored during the electrodeposition to investigate the growing process of nanowires. Ni nanowires and Co nanowires were potentio-statically electrodeposited. Figure 11 shows the effect of cathode potential on the time-dependence of cathode current during the electrodeposition of Ni nanowires and Co nanowires. During the electrodepostion, cathode potentials were fixed to -0.8, –0.9, –1.0, –1.1 and –1.2 V. If the potential was kept to –1.0 V for Ni deposition, the cathode current reached up to ca. 1 mA at the beginning of electrolysis within several tens of minutes. Then, the current rapidly decreased to be ca. 0.4 mA and kept the constant current until around 1000 sec. During this process, electrodeposition of Ni proceeds in the nanopores. At the initial stage of the electrodepostion, large cathode current was observed in each cathode potential. The concentration of metal ions in the nanopores will decrease with increasing the electrodeposition time due to the reduction of metal ions, while the metal ions will be provided from the bulk solution to the nanopores, where the metal ions are consumed on the cathode due to the electrodeposition process. Finally, as shown in Figure 11, the cathode current rapidly increases at the deposition time more than 1000 sec. At this stage, electrodeposited nanowires reach the surface of the membranes and large hemispheric Ni deposits are formed. Growth rate of Ni nanowires can be estimated from dividing channel length by the filling time. For example, growth rate of Ni nanowires can be estimated as ca. 6 nm/sec at the cathode potential of –1.0 V. Time-dependence of cathode current for Co deposition also showed similar behavior as well as Ni deposition. Growth rate of the nanowires increases up to ca. 30nm/sec with increasing the cathode potential up to -1.2 V.
3.2. Structure of Ni and Co Nanowires Array
After the growing nanowires, anodized aluminum oxide membrane filters were dissolved in aqueous solution containing sodium hydroxide and the remains consisted of nanowires and a gold layer was served as a sample for scanning electron microscope (SEM) and transmission electron microscope (TEM) observation. Figure 12 shows SEM images of Ni nanowires separated from the anodized aluminum oxide templates. Diameter (60 and 300 nm) and length (6 and 30μm) of the nanowires corresponds well to that of nanopores and the cylindrical shape was precisely transferred from the nanopores to the nanowires. Aspect ratio of the nanowires reaches up to around 100.
TEM bright field images and electron diffraction patterns of electrodeposited Ni and Co nanowires were also investigated as shown in Figure 13. According to TEM bright field images, shape of the nanowires was almost cylindrical and the electron diffraction patterns are composed of spots, which suggests a nanowire consists of a crystalline phase with preferential orientation.
3.3. Magnetic Properties of Ni and Co Nanowires Array
Magnetization and anisotropic magnetoresistance (AMR) of Ni alloy and Co alloy nanowires were measured using a vibrating sample magnetometer (VSM) and LCR meter with increasing the magnetic field up to 10 kOe. Figure 14 shows magnetic hysteresis loops of pure Ni, Ni-1.5%Co, Ni-0.8%Fe, pure Co, Co-0.9%Ni and Co-0.1%Fe alloy nanowires electrodeposited into anodized aluminum oxide templates with channel-diameter of 60 nm. Magnetic field was applied to in-plan direction and perpendicular direction to the membrane film plan. The perpendicular direction to the membrane film plan corresponds to the parallel direction to the long axis of nanowires. These nanowires were hardly magnetized in in-plan direction and the magnetization reached to saturation at more than 5 kOe as shown in Figure 14. On the contrary, these nanowires were easily magnetized in perpendicular direction and the coercive force reached up to around 1 kOe. These magnetization curves revealed that the electrodeposited nanowires have a typical perpendicular magnetization behaviour due to the uni-axial shape anisotropy.
Figure 15 shows magnetoresistance curves of pure Ni, Ni-1.5%Co, Ni-0.8%Fe, pure Co, Co-0.9%Ni and Co-0.1%Fe alloy nanowires electrodeposited into anodized aluminum oxide templates with channel-diameter of 60 nm. Magnetic field was applied to in-plan direction and perpendicular direction to the membrane film plan. The perpendicular direction to the membrane film plan corresponds to the parallel direction to the long axis of nanowires. Here, the anisotropic magnetoresistance (AMR) ratio is defined by the following equation:
The magnetoresistive hysteresis of the Ni alloy and Co alloy nanowires depended strongly on the direction of the magnetic field as shown in Figure 15. In the magnetic field direction of 0° (the long axis of nanowires is parallel to the magnetic field), the AMR ratio was almost zero, while a maximum AMR ratio was observed in the magnetic field direction of 90° (the long axis of nanowiresis perpendicular to the magnetic field). Resistance of the nanowires decreased with increase in the magnetic field and the AMR ratio reached 1.0–3.2% with the Ni alloy nanowires. The saturation field of the Ni nanowires was estimated to be about 7 kOe. Ferre
4. Electrodeposition and Magnetoresistance of Co/Cu Multilayered Nanowires
4.1. Electrodeposition of Co/Cu Multilayered Nanowires
For growing Co/Cu multilayered nanowires, the exfoliated anodized aluminum oxide nanochannels were used as templates. Aqueous solution containing CoSO4 (120 g/L), CuSO4 (1.6 g/L) and H3BO3 (45 g/L) was used as electrolyte for electrodeposition of Co/Cu multilayered nanowires. A cathode polarization curve was measured over a wide range of cathode potential to determine the optimum potential for Cu and Co deposition. Figure 16 shows cathode polarization curves for electrodeposition of Cu and Co from the mixed solution (containing Cu2+ and Co2+ ions) and the solution containing only Co2+ ions. The equilibrium potentials of Cu/Cu2+ and Co/Co2+ are estimated to be around +0.05 V and –0.48 V (vs.Ag/AgCl) on the basis of Nernst equation.The cathode current occurs at the potential region close to the equilibrium potential of Cu as shown in Figure 16. It is well-known that Cu2+ ions begin to electrodeposit without accompanying overpotential from the aqueous solution. Therefore, this cathode current corresponds to the deposition current of Cu. With increasing cathode current, at around 10-2A, the potential significantly polarizes to the less-noble region. This polarization would be caused by the diffusion limit of Cu2+ ions. In the potential region which is less-nobler than the equilibrium potential of Co, the cathode current increases again at around –0.7 V. It is also well-known that the electrodeposition of iron-group metals such as Ni, Co, and Fe is accompanied by the overpotential due to the rate determination of multi-step reduction process. Therefore, this increase in cathode current would be mainly caused by the reduction current of Co2+ ions.
Growth rates of nanowires were estimated by the channel filling time, which was determined from the time dependence of deposition current at each potential. Figure 17 shows the effect of cathode potential on the time dependence of cathodic current during the electrodeposition of Co nanowires. The cathode potentials were fixed to -0.80,-0.85, -0.90, -0.95 and -1.0 V. To determine the nanowire growth rate, the channel-filling time was estimated by monitoring the deposition current. When the nanowires reach the membrane surface, the current will increase drastically due to the formation of hemispherical caps. The growth rates were calculated dividing the channel length by the channel-filling time. For example, at –1.0 V, the channel-filling time is around 40 s and the deposition rate is estimated to be ca. 150nm/s. On the basis of the results shown in Figure 16 and Figure 17, the optimum deposition potentials of Cu and Co are determined to be about –0.4 and –1.15 V (vs. Ag/AgCl) which is the potential region nobler than the diffusion limit potential of each metal ion. Typical deposition rates of Cu and Co were roughly 10 nm/s (at –0.4 V) and 200 nm s-1 (at –1.15 V).
Co/Cu multilayered nanowires were synthesized alternatingly switching cathode potential from -0.4 V (for Cu layer) to -1.15 V (for Co layer) as shown in Figure 18. According to this figure, when the potential is switched from -1.15 V to -0.4 V, anodic current is observed. This is resulting from the dissolution of electrodeposited Co, because -0.4V is more nobler than the equilibrium potential of Co. At this potential, it is estimated that the Cu deposition and Co dissolution will proceed simultaneously. According to the time-dependence of cathodic current during electrodeposition of Co/Cu multilayered nanowires as shown in Figure 19, filling time was around 3500 s and the deposition rate was estimated to be about 17 nm/s.
4.2. Structure of Co/Cu Multilayered Nanowires
TEM bright field images of electrodeposited Co/Cu multilayered nanowires were investigated as shown in Figure 20. Shape of the nanowires was almost cylindrical and the multilayered structure was also observed as shown in this images. In the structure, several stacking faults were also observed. There would be significant strain energy in the interphase boundary between Co and Cu layer. The strain energy could be stored as the stacking fault energy. It is well known that the stacking fault structure is usually observed in fcc crystals such as Au, Ag, Cu, Ni, Al, etc. Therefore, the stacking fault structure would be introduced to the Cu layer from the interface between Co and Cu layer.
4.3. Magnetic Properties of Co/Cu Multilayered Nanowires
Typical perpendicular magnetization behavior was observed from the magnetic hysteresis loops of electrodeposited Co/Cu multilayered nanowires with diameter of 60 nm and length of 60μm as shown in Figure 21-(a). Coercive force of Co/Cu multilayered nanowires was ca. 1 kOe. This is resulting from the shape anisotropy of nanowires with super large aspect ratio of ca. 1000. The magnetoresistance curves revealed that 10.5 % of GMR effect was obtained in the multilayered nanowires with Co layer 10nm, Cu layer 10 nm and 3000 bi-layers as shown in Figure 21-(b).
Anodized aluminum oxide films with the thickness ranging from 2μm to 200 μm were synthesized using a bipolar continuous electrolysis process with anodic oxidation and cathodic exfoliation as well as barrier layer thinning process using chemical etching technique. 3.2 % of AMR effect was observed in Ni-1.5at.% Co alloy nanowires with diameter of 60 nm and length of 60μm. Co/Cu multilayered nanowires with Co-10 nm, Cu-10 nm and 3000 bi-layers were synthesized using a pulsed current electrodeposition technique. 10.5 % of GMR effect was observed in Co/Cu multilayered nanowires electrodeposited into anodized aluminum oxide template with super large aspect ratio of ca. 1000.