Domain wall pinning in FeCoCu bamboo-like nanowires.

The development of devices based on spintronics attracts much interest due to its advantages in electronics regarding non-volatility, reduced power consumption and increased data processing speed1. Particularly, the control of the magnetic domain wall (DW) motion is a key aspect for the functionalization of ferromagnetic-based devices, logic systems or sensing devices2,3,4,5. A large number of works are currently devoted to the study of DW dynamics along ferromagnetic elements driven by electric current6,7,8 or magnetic fields9,10. In order to develop DW-based applications, one has to address some fundamental questions about the DW configuration and the reversal mechanism. Both features play a major role in the propagation speed of the DWs, amongst other parameters. On the other hand, the understanding of pinning/depinning mechanism becomes essential for the use of well-localized artificial pinning sites that enable the trapping of DWs at selected positions. The most widespread method of creating pinning centers in nanostripes is patterning notches with different shapes in planar nanostructures11,12,13. However, individual cylindrical nanowires have attracted less attention14,15,16, despite presenting some features that make them more convenient for applications, such as not showing a Walker breakdown14,17 that limits the DW propagation speed at high external fields. It is also worth noticing that their cylindrical geometry favors the development of vortex domain walls that move uniformly10, contrary to the transverse domain walls extensively studied in two dimensional structures18,19. Thus, it can be of great interest the control over the nucleation and positioning of domain walls in this nanowires for a number of alternative logic and 3D magnetic storage devices, such a race-track memory, where bits are coded as magnetic domain walls along each wire3,11,20. An additional advantage of these nanostructures is the low-cost technique used for the fabrication. In this study, ferromagnetic cylindrical nanowires with negligible crystalline anisotropy and very high aspect ratio have been investigated. Their longitudinal uniaxial magnetic anisotropy makes them ideal systems to study the magnetization reversal process. A significant work has been devoted to the preparation of ferromagnetic cylindrical nanowires grown into templates by electrochemical route21,22. For this work, cylindrical nanowires with periodically distributed small segments of different diameters –labeled as bamboo like NWs- were prepared, as well as NWs of continuous diameter- straight NWs. CoFe based alloy nanowires have been selected due to their large saturation magnetization and high Curie temperature, which makes them good candidates to replace rare-earth free based permanent magnets in certain applications. The final aim of this study is to determine the local magnetic configuration along individual nanowires and to show the pinning effect of the bamboo-like geometry in FeCoCu nanowires making use of an advanced Magnetic Force Microscopy (MFM) technique23,24. MFM is a recognized powerful technique to image the local magnetization configuration25 as well as the magnetization reversal process at the nanoscale26. This technique provides high resolution images of the magnetic configuration (around 20nm) together with the corresponding topographic information. MFM images have been firstly obtained at remanence and under an external applied magnetic field parallel to the cylinder axis. To gain deeper understanding of the magnetization reversal process, a more subtle imaging procedure has been used. Complementary micromagnetic simulations were carried out using object orientated micromagnetic framework27 (OOMMF) package to confirm the experimental results.