The I3EM-CEMES group, in collaboration with the group of Nanomagnetism and Magnetization Processes (ICMM – Madrid – Spain), has performed a quantitative magnetic characterization of isolated diameter-modulated FeCoCu nanowires by combining electron holography technique and micromagnetic simulations. Electron holography was carried out in the I2TEM (Hitachi – Toulouse – Equipex MIMETIS) microscope. The detailed description of the magnetic distribution is essential to move to the next step towards the manipulation of the domain walls propagation in cylindrical nanowires for spintronic applications in data storage and logical operation.
The investigations for efficient methods to manipulate the motion of magnetic domain walls (DWs) in 1D nanostructures is nowadays a main requirement to build high-performance DW-based devices such as magnetic racetrack memories  and/or magnetic logic systems . Although great efforts have been focused for studying DWs in planar nanowires (NWs), or nanostrips, cylindrical NWs begin to attract attention due to high DW propagation velocity favoured by the absence of the Walker breakdown phenomenon . A potential way to control such DW motion in cylindrical NWs is to create NWs where the diameter is periodically varied.
In this article, we analysed the local remanent magnetic distribution in diameter-modulated (DM) cylindrical FeCoCu NWs. A full description of the magnetization and demagnetizing field was carried out by combining the advantages of electron holography (EH) (high spatial resolution, high sensitivity, quantitative information of the local magnetization) and the three dimensional capabilities of micromagnetic simulations to describe the local spin configuration. Polycrystalline DM NWs were fabricated following an electrochemical route where an alloy of Fe28Co67Cu5 is grown by into DM nanopores of anodic aluminium oxide (AAO) templates. Although the two dimensional mapping of the magnetic induction measured by EH exhibits a single-domain remanent state, micromagnetic simulations reveal a more complex spin configuration, strongly affected by the DM geometry of the cylindrical NWs (see Figure): (i) in the transition zones where the diameter changes and the NW ends, vortex-like states are formed; (ii) in segments of small diameter, the spins are perfectly aligned along the NW axis; (iii) in the largest diameter segments, the longitudinal alignment of the spins is curled due to proximity of the vortex-like states in (i). In addition, the DM geometry of the wires induces local magnetic charges that control the local stray field.
The local spin changes and the demagnetizing field configuration we evidenced in such diameters modulated nanowires are a key step to be able tuning the DW motion in such nanostructures.
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This work has been supported by ESTEEM2 (Reference No. 312483), the French National Research Agency under the ‘Investissement d'Avenir' (Reference No. ANR-10-EQPX-38-01), the project MAT2013-48054-C2-1-R from the MINECO-Spain and CPER programs.
L. A. Rodríguez, C. Bran, D. Reyes, E. Berganza, M. Vázquez, C. Gatel, E. Snoeck and A. Asenjo. “Quantitative Nanoscale Magnetic Study of Isolated Diameter-Modulated FeCoCu Nanowires” ACS Nano, Article ASAP (2016). DOI: 10.1021/acsnano.6b05496
Luis Alfredo Rodriguez (rodriguez at cemes.fr) tel: +33562257891
Christophe Gatel (gatel at cemes.fr) tel: +33567524349