Feasibility of Using NiCu/FeCo Coupled Bi-segmented Nanowires as In-line Writing Heads for 3D Racetrack Storage Devices

Abstract

Multi-segmented nanowire (NW) arrays are strong candidates to improve the memory storage capacity due to their lower magnetic bit area [1]. But writing/reading information on such devices is still a challenge. Recently, micromagnetic simulations revealed that the integration of a soft magnetic segment (with lower coercivity) along the racetrack could ease the writing process by using external magnetic fields [2].Here, we demonstrate the feasibility of using NiCu/FeCo magnetic NWs as in-line writing components in 3D racetrack memory devices. Using template-assisted DC electrodeposition [3-5], 4 samples of NW hexagonal arrays (50 nm-diameter, 100 nm of interwire distance) were produced: i) Ni90Cu10; ii) Fe20Co80; iii) bi-segmented NiCu/FeCo; and iv) multi-segmented NiCu/[FeCo/Au]3. Morphological, chemical and structural characterization revealed the formation of single, bi- and multi-segmented NW arrays with tuned dimensions and uniform compositions (Fig. 1).Magnetic hysteresis loops, ferromagnetic resonance (FMR) measurements and first-order reversal curve (FORC) diagram analysis confirmed the hard and soft magnetic properties of the FeCo and NiCu single arrays, respectively. In the interacting NiCu/FeCo bi- and multi-segmented systems, a significant reduction of the parallel coercivity occurred, while keeping the magnetization easy axis lying parallel to the NWs' axis. FORC diagrams also demonstrated a two-step reversal process in systems (Fig. 2), illustrating a FORC fingerprint typically present in soft/hard magnetic interacting bi-layered NWs [6]. Micromagnetic simulations confirmed such a two-step reversal process, revealing the presence of strong interactions at the soft/hard interface [7].This work was supported by projects POCI-01-0145-FEDER-028676, POCI-01-0145-FEDER-031302, UIDB/04968/2020, EXPL/IF/00541/2015 and contract IF/01159/2015 from Portuguese FCT and COMPETE 2020 (FEDER); grant 02.2020/0261 from the National Research Foundation of Ukraine; and project PID2020-117024GB-C42 from the Spanish Ministerio de Ciencia e Innovacíon.  Fig. 1. Scanning electron microscopy images of the electrodeposited multi-segmented NWs.  Fig. 2. FORC diagram of the multi-segmented sample along the parallel direction.