Isotropic magnetic behavior of multi-segmented FeCo nanowire arrays

Abstract

The study of 1D-nanostructures has been intensified in the last few decades due to the development of fabrication methods of metallic, semiconductors and insulator materials aiming applications in several areas [1]. In particular, multi-segmented magnetic nanowire (NW) arrays are prospective candidates for an extensive range of applications such as racetrack memory devices, spintronic, and drug delivery [1, 2, 3]. These possibilities are due to the number of degrees of freedom related to the intrinsic shape anisotropy combined with the potential to introduce different materials along the NW’s length. Concerning practical applications, understanding the role of layers with distinct structural and magnetic properties can be a path to avoid some issues such as system overheating and Walker breakdown in racetrack devices. For instance, micromagnetic simulations revealed that the combination of Co segments with different structures, bcc and hcp, promotes a faster way of writing information in 3D racetrack memory devices [2]. This observation results from the high magnetocrystalline anisotropy along the wire length induced by the Co-hcp structure. Although these evaluations seem promising, thorough experimental research of multi-segmented NWs with low and high anisotropy components still needs to be performed.Here, we present the FeCo layer properties’ evaluation onto multi-segmented NW arrays produced through DC electrodeposition into ordered porous alumina templates with 40 nm of pore diameter and 60 µm of length. For each layer’s deposition, a Pt electrode was used as the counter electrode, Ag/AgCl as the reference electrode, and an Au layer as the working electrode [5]. Two samples were prepared: 1) Au [200 nm] / Fe33Co67 [24 µm] / Au [10 nm] and 2) Au [200 nm] / Ni90Cu10 [1 µm] / (Fe33Co67 [13 µm] / Au [10 nm])x3, where the indicated length was obtained from scanning electron microscopy (SEM) images and the alloy’s stoichiometry from energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) measurements at room temperature revealed the formation of bcc structure of FeCo with the (110) preferential orientation for both samples and an additional hcp phase with the (110) preferential orientation for FeCo tri-segmented NWs (Figure 1). The magnetic hysteresis loops acquired at room temperature for the Au/FeCo/Au arrays present the easy magnetization axis along the parallel direction, evidencing a typical strong shape anisotropy [5]. Distinctly, when three FeCo segments of 13 µm are deposited along the NW’s length, the arrays present an isotropic magnetic behavior with an increase of the coercive field when applying the magnetic field perpendicular to the nanowires’ axis (Figure 2). This change in magnetic behavior for the segmented NWs might be related to the presence of an hcp phase that induces a perpendicular magnetocrystalline anisotropy. However, further research should be performed to better comprehend the interplay between the NiCu and FeCo layers into the NW arrays.In summary, multi-segmented NWs of FeCo alloys were successfully achieved through the electrodeposition process into porous templates. The growth of FeCo segments leads to the formation of hcp structures and the observation of an isotropic magnetic behavior. This can be related not only to the structural properties but also to the interplay between FeCo segments along the NW’s length. A more complete evaluation of the magnetic properties of the produced samples should be performed to fully understand the role of the different layers in the magnetocrystalline anisotropy of the arrays.This work was financially supported by projects POCI-01-0145-FEDER-028676 and UIDB/04968/2020 from Portuguese FCT and COMPETE 2020 (FEDER) and IF/01159/2015, POCI-01-0145-FEDER-031302/PTDC/FIS-MAC/31302/2017.. **