Micromagnetics of Chemical Barriers Inserted within Permalloy Cylindrical Nanowires: Towards the Control of Domain Wall Motion

Abstract

Cylindrical magnetic nanowires are a three-dimensional system in which magnetization may be manipulated by spin-polarized currents in order to controllably nucleate, move and pin magnetic domain walls (DWs). Such control makes them excellent candidates for three-dimensional storage devices such as in the well-known concept of Racetrack memories [1].Due to their geometry, cylindrical nanowires can host a DW with curling magnetization not subject to the Walker breakdown limitation [2], and thus that can reach velocities over 1 km/s. This kind of DW is known as the Bloch-Point wall (BPW) [3] [4] and it was experimentally reported under static conditions for the first time in 2014 [5]. Recently, velocities above 600 m/s driven by spin-transfer torques have been also reported [6]. In order to achieve the desired control of DW motion, effective pinning sites need to be set along the wire’s axis. Here, we propose a system based on cylindrical Permalloy (Fe20Ni80) nanowires with evenly spaced chemical barriers: Fe-rich segments (Fe80Ni20) of length ranging from 20 to 100 nm and 1 μm separation.We present an overview of their static micromagnetic configuration as well as their response to applied field and nano-second electric current pulses. This is a prerequisite to investigating and understanding the interplay of DWs with chemical modulations. Quantitative understanding is achieved combining magnetic imaging by means of X-ray Magnetic Circular Dichroism (XMCD) coupled to PhotoEmission Electron Microscopy (PEEM), Scanning Transmission X-ray Microscopy (STXM) or X-ray ptychography with <15 nm spatial resolution (Figure 1), and micromagnetic simulations with mumax3 and feeLLGood codes, as well analytical modelling. In addition, Transmission Electron Microscopy (TEM) combined with electron holography was used to extract valuable qualitative and quantitative information on the local magnetic behavior.The Fe-rich barriers have a spontaneous magnetization of 1.4 T whereas the Permalloy segments of 0.8 T. Such magnetization change at the interface generates volume charges and thus a dipolar energy increase. This energy cost can be overcome with a vortex state at the chemical barrier where the magnetization remains axial at the core and curling at the shell.The XMCD images indeed show a curling magnetic state at the chemical barriers (Figure 2). In addition, magnetic images are supported by a quantitative post processing code that simulates the magnetic contrast coming from XMCD-PEEM technique for a specific three dimensional magnetization texture, allowing us to extract the curling angle of the magnetization with respect to the wire’s axis. The strength of curling increases with both the modulation length and wire diameter, with a clear analogy with the vortex state in flat magnetic disks. The strength of curling was assessed by imaging under large axial magnetic field. The chirality of curling is random at rest but can be switched deterministically by means of the Oersted field of nanosecond current pulses, showing again a clear correlation between the strength of curling and the geometry of the modulations. **