Domain wall spin structures in mesoscopic Fe rings probed by high resolution SEMPA

Abstract

We present a combined theoretical and experimental study of the energetic stability and accessibility of different domain wall spin configurations in mesoscopic magnetic iron rings. The evolution is investigated as a function of the width and thickness in a regime of relevance to devices, while Fe is chosen as a material due to its simple growth in combination with attractive magnetic properties including high saturation magnetization and low intrinsic anisotropy. Micromagnetic simulations are performed to predict the lowest energy states of the domain walls, which can be either the transverse or vortex wall spin structure, in good agreement with analytical models, with further simulations revealing the expected low temperature configurations observable on relaxation of the magnetic structure from saturation in an external field. In the latter case, following the domain wall nucleation process, transverse domain walls are found at larger widths and thicknesses than would be expected by just comparing the competing energy terms demonstrating the importance of metastability of the states. The simulations are compared to high spatial resolution experimental images of the magnetization using scanning electron microscopy with polarization analysis to provide a phase diagram of the various spin configurations. In addition to the vortex and simple symmetric transverse domain wall, a significant range of geometries are found to exhibit highly asymmetric transverse domain walls with properties distinct from the symmetric transverse wall.Simulations of the asymmetric walls reveal an evolution of the domain wall tilting angle with ring thickness which can be understood from the thickness dependencies of the contributing energy terms. Analysis of all the data reveals that in addition to the geometry, the influence of materials properties, defects and thermal activation all need to be taken into account in order to understand and reliably control the experimentally accessible states, as needed for devices.2

Direct Imaging of Chiral Domain Walls and Néel‐Type Skyrmionium in Ferrimagnetic Alloys

“…[25] In this study, we investigate the chirality of spin textures in a ferrimagnet, namely Ta/Ir/Fe/GdFeCo/Pt, by imaging the spin structure of the DW using scanning electron microscopy with polarization analysis (SEMPA). [26][27][28] This surfacesensitive imaging technique has already been successfully used to determine the chiral character of out-of-plane (OOP) magnetized spin textures in ferromagnetic materials [29,30] and here we demonstrate that we can determine the chiral character of spin textures also for ferrimagnetic materials. From the SEMPA images, we are also able to extract the DW width across a wide range of temperatures, which allows us to determine the exchange stiffness evolution with temperature as a crucial parameter that governs the stability and operation temperature range.…”

“…In this study, we investigate the chirality of spin textures in a ferrimagnet namely Ta/Ir/Fe/GdFeCo/Pt by imaging the spin structure of the domain walls using SEMPA [25][26][27]. This surface-sensitive imaging technique has already been successfully used to determine the chiral character of out-of-plane (OOP) magnetized spin textures in ferromagnetic materials [28][29] and here we demonstrate that we can determine the chiral character of spin textures also for ferrimagnetic materials.…”

Magnetic Imaging and Microscopy

“…Via careful data analysis, this also opens up opportunities for improvements in the signal-to-noise ratio, as well as the detection of competing magnetic switching pathways that are not accessible in conventional pump-probe imaging [49,50]. The high spatial resolution is a key advantage of the technique, enabling imaging not only of domain configurations but also of domain wall spin structures [51]. The impetus to increase the spin signal means that it is usual to operate the SEM at large beam currents and low voltages of typically 1-3 kV, for which the emission of the spin-polarized low-energy secondaries is increased [52].…”

Magnetic Imaging and Microscopy

“…The long acquisition times are a barrier to investigation of magnetization dynamics, however very recently the feasibility of imaging on nanosecond timescales with advanced signal processing based on the time of detection of the individual electron counts has been demonstrated [42,43]. The high spatial resolution is a key advantage of the technique, enabling imaging not only of domain configurations but also of domain wall spin structures [44]. The impetus to increase the spin signal means that it is usual to operate the SEM at large beam-currents and low voltages of typically 1-3 kV for which the emission of the spin-polarized low energy secondaries is increased [45].…”