Structural, Magnetic Properties of the Electrochemically Deposited Arrays of Nickel Nanowires
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magnetoresistive properties as a function of the diameter and length of the nanowires are investigated and compared with an electroplated continuous nickel layer of 5 prm thickness on copper substrate and 25 nm crystallite size. EXPERIMENTAL DETAILS The fabrication of the Ni-nanowires consists of two step process: First anodic alumina of different pore diameters is obtained by electrochemically oxidizing an (Al-l% Mg) substrate in electrolytes at different temperatures and potentials e.g. A: Pores of 18 nm diameter: 7 wt.% H 2 SO 4 electrolyte, 25 V and 01C; B: Pores of 30 nm diameter: 10 wt.% H3P0 4, 30 V and 30'C; C: Pores of 78 nm diameter: 3wt.% Cr0 3, 40 V and 40°C. The pore morphology was checked by scanning electron microscopy. Table I: Anodic oxidation of (A1-1% Mg) in various electrolytes and the pore characteristics Electrolyte
Pore diameter
Inter-pore
(nm)
distance (nm)
18
10 wt.% H 3P0 4; 30 V; 30 130°C 3. 3 wt.% CrO 3; 40 V; 78 400C
1. 7 wt.% 0°C
H 2 S0 4 ; 25
V;
2.
Wall thickness
Pore
42
30
7.5 x 10'u
46
19
6.5 x 10'u
89
15
2 x 10'0
(nm)
density/cm2
The nickel nanowires of different diameters and lengths were deposited into the pores of anodic alumina using a sulphate bath (112 g/l NiSO 4o6H 20 + 40 g/l H3 B0 3) with a pH value of 3.9 at 50'C temperature using an a.c. voltage (20 - 40 V; 50 Hz). A Ni layer of 5 pm was also deposited on a copper substrate using the same electrolyte and a current density of 10 mA/cm2 . The crystallographic structure and texture of the Ni-nanowires and layer were checked by X-ray diffraction technique using CuKa-radiation. The crystallite size of the Ni layer was determined using the Scherrer formula: 0.9*" / B * cosO [18] (13is the full width at half maximum of a (0 20) diffraction line of Bragg angle 0 and ), is the wave length of the CuK&-radiation). The magnetization measurements were made at room temperature using a vibration magnetometer. The magnetoresistive measurements were made by two point technique by applying magnetic field parallel and perpendicular to the substrate plane. RESULTS AND DISCUSSION Fig. I shows the SEM-images of anodized (A]-I% Mg) substrate using different electrolytes. The pore characteristics are listed in table I. The wall thickness decreases with increasing pore diameter. Fig. 2 shows the (0 - 20) X-ray diffractograms of Ni-layer (5gm) on a copper substrate and the Ni-nanowires of diameters 18, 30 and 78 nm in the pores of anodic alumina. A crystallographic preferred orientation of and is observed in the Nilayer of fc.c. structure. The Ni crystallite size estimated from the Scherrer formula is - 25 nm. The Ni-nanowires of 18 nm diameter show only crystallographic orientation, where as the 30 nm and 78 nm diameter nanowires show both and crystallographic preferred orientations similar to the Ni-layer but with different intensities. This shows that the
246
Fig. I SEM-images (x 200,000) of the pores in anodic alumina (AIMgl) after the anodic oxidation by H 2SO 4, CrO3 and H3P0 4 electrolytes
S
c U S S
c
30
100
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