pH-controlled magnetic characteristics of electrodeposited FeCo nanowire arrays

Abstract

Controlling magnetic nanowire (NW) systems raises interesting challenges both from fundamental and applied points of view, concerning their characteristics such as magnetic phases, coercive fields and interactions, as well as their applications in high-density storage and spintronic devices. Here, FeCo NW arrays with a diameter of ∼35 nm and an average length of ∼7 μm are fabricated in porous alumina templates using a pulse electrodeposition technique with solution pH in the range of 3.00–5.50. Morphological investigations show no changes in the length of NWs fabricated at the different pH values, whereas the elemental composition of the FeCo alloy slightly changes with increasing pH. Also, X-ray diffraction and high-resolution transmission electron microscopic analyses indicate the formation of magnetic oxides at pH > 5.00, together with polycrystalline structure along the length. The formation of the magnetic oxides significantly changes magnetic properties of the NWs, according to hysteresis loop and first-order reversal curve (FORC) investigations. The presence of interacting soft and hard magnetic phases is confirmed by FORC diagrams, arising from the magnetic oxide and FeCo alloy contents, respectively. Meanwhile, the FORC coercivity of the hard phase increases from 1463 to 2453 Oe with increasing pH from 3.00 to 5.25, indicating the hardening of FeCo NWs. Therefore, the solution pH is proposed as one of the main influencing and determining factors to control structural and magnetic characteristics of the alloy NW arrays electrochemically deposited in porous templates.