Current-Driven Domain Wall Motion in Curved Ferrimagnetic Strips Above and Below the Angular Momentum Compensation

Abstract

Magnetic domain walls (DWs) in ferromagnetic nanostrips with perpendicular magnetic anisotropy have widely been a subject of research, not only due to its intrinsic fundamental interest but also to develop spintronic devices where information is coded in domains between walls [1,2]. To that purpose, ferrimagnetic materials (FiM) are recently attracting much attention due to their fast DW dynamics, almost zero interaction with external fields and a wide availability in nature. Controlling the current-driven DW along curved FiMs strips is essential to develop DW-based circuitry, but it faces problems such as DW distortion [3]. A proper design for the strip geometry may overcome these problems, as shape is intimately connected with DW motion via spin currents and interfacial phenomena in the structure. Studying the influence of temperature (or composition) is also key for developing real applications. Therefore, understanding the influence of shape and temperature to fully control the DW motion is nowadays a fundamental topic in spintronics research.In this work we perform a micromagnetic (μm) study on DW motion in ring-like FiM strips, characterising DW terminal velocities and inertial times, as a function of temperature, width, curvature of the strip (see insets in Fig. 1(a)) and density currents (J). Our full μm model treats spin dynamics in each sublattice separately, which yields results close to ‘realistic’ FiM. Results show that strips with low width and high curvature give high DW velocities (VDW > 1000 m/s), with ultra-short response times (τ <<0.1 ns, see Fig.1 (b)). This is optimal for developing compact and high-speed tracks for domains since opposite DWs travel at lower but similar speeds, leading to negligible domain distortion. Our results will help to understand curvature effects to develop efficient and reliable DW-based devices.  Fig.1. (a) DW velocity as a function of J from sub-lattice i = 1 in a FiM strip of width w = 256 nm and different effective radius re (nm) at the angular momentum compensation temperature TA = 260 K. (b) Instantaneous DW velocities for three values of J and for a curved strip of re = 384 nm and w = 256 nm at T = TA. Relaxation times (τ) are extracted from the equation (solid curve) fitted to the data (symbols).