The influence of the synthesis conditions on the magnetic behaviour of the densely packed arrays of Ni nanowires in porous anodic alumina membranes

Abstract

The densely packed arrays of Ni nanowires of 70 nm diameter and 6-12 μm length were obtained via electrodeposition into porous alumina membranes (PAAMs) of 55-75 μm thickness. The morphology, microstructure and magnetic properties between the room and liquid-helium temperature of Ni nanowires in PAAMs have been investigated using scanning electron microscopy, X-ray diffraction and vibrating sample magnetometry. The crystal structure of the Ni nanowires is fcc with (220) preferred orientation. The magnetic characteristics of the Ni nanowires in PAAMs were compared with the same characteristics of bulk Ni and with other researchers' data. The effect of the porous alumina membrane and the Ni nanowires synthesis conditions on the magnetic characteristics of Ni nanowire arrays has been studied. The coercivity reached more than 750 kOe and the squareness ratio up to 0.65 under the proposed optimal synthesis conditions for Ni nanowires. Magnetic parameters of the densely packed arrays of Ni nanowires allow using them in magnetic recording media, hard disk drives, storage systems and sensors. In addition, such structures are of considerable interest for basic research on nanomagnetism which is significantly different from the magnetic properties of bulk and thin films materials.

Fig. 1. (A) – the scheme of two-step process of PAAM formation (1,2,3) and Ni nanowires (4). (B) – the scheme of setup for Ni electrodeposition.

Fig. 2. (A) – PAAM Surface SEM images of a top and bottom (inset) after barrier layer chemical etching and the underlying Al layer removing. (B) – Cross-section SEM images of Ni nanowires in PAAM.

Fig. 3. Dependence of Ni specific deposition rate on deposition current density and PAAM thickness.

Fig. 4. XRD spectra for Ni nanowires in PAAM: (A) – H_PAAM = 70 μm, j = 1.5 mA cm⁻², t = 120 min (sample 1); (B) – H_PAAM = 75 μm, j = 2 mA cm⁻², t = 240 min (sample 2); (C) – H_PAAM = 55 μm, j = 3 mA cm⁻², t = 240 min (sample 3); (D) – H_PAAM = 55 μm, j = 4 mA cm⁻², t = 240 min (sample 4).

Fig. 5. Axial (A, C, E and G) and in-plane (B, D, F and H) hysteresis loops for Ni nanowires without (A and B) and with NiO phase (C–H) in PAAM for samples 1 (A and B), 2 (C and D), 3 (E and F) and 4 (G and H) at 300 K (red lines) and 4.2 K (black lines). In the insets – enlarged fragments of magnetization from a magnetic field near the origin.

Fig. 6. Coercivity versus a.r. and temperature for applied magnetic field axial (parallel) and in-plane (perpendicular) to the Ni/NiO nanowires (samples 2, 3, 4); data points marked with short dots – for Ni nanowires without NiO phase (sample 1).

Fig. 7. Squareness M_r/M_sversus a.r. and temperature for applied magnetic field axial (parallel) and in-plane (perpendicular) to the to the Ni/NiO nanowires (samples 2, 3, 4); data points marked with short dots – for Ni nanowires without NiO phase (sample 1).