Influence of the porosity on the magnetic properties of Ni nanowires arrays

The hysteresis properties of ordered arrays of Ni nanowires, electrodeposited into cylindrical nanoporous alumina membranes were investigated. The effect of the nanowire geometry (diameter and length), the array order and the nanowires' volume fraction on the array coercivity and remanence were analyzed. Porous alumina membranes were produced by a two-step anodization process in a potentiostatic electrochemical cell. The pores' diameters were varied from 20 nm to 40 nm by immersing the membranes in a 5% phosphoric acid solution for different times. The membranes were characterized using a scanning electron microscope (SEM) and both the pore size distribution as well as their separation were determined. These membranes were used as templates for the AC electrodeposition of Ni nanowires, using a voltage of 20 V and a frequency of 200 Hz, at room temperature. The Ni nanowire arrays were structurally and magnetically characterized by SEM and with a SQUID magnetometer, respectively, at room temperature. It was found that the magnetic properties of the array strongly depend on the nanowires aspect ratio -length/diameter ratio- which regulates the wire shape anisotropy and also on the porosity, which determines the magnitude of the dipolar interaction field acting on the array. Particulary, it was found that coercitivity decreases with porosity and that the parameter epsilon, which is related to the energy barriers of the system, must depend on the porosity of the ensemble in order to accurately describe the experimental data.