Quantitative analysis of shadow x-ray magnetic circular dichroism photoemission electron microscopy

Abstract

Shadow X-ray Magnetic Circular Dichroism Photo-Emission Electron Microscopy (XMCD-PEEM) is a recent technique, in which the photon intensity in the shadow of an object lying on a surface, may be used to gather information about the three-dimensional magnetization texture inside the object. Our purpose here is to lay the basis of a quantitative analysis of this technique. We first discuss the principle and implementation of a method to simulate the contrast expected from an arbitrary micromagnetic state. Text book examples and successful comparison with experiments are then given. Instrumental settings are finally discussed, having an impact on the contrast and spatial resolution : photon energy, microscope extraction voltage and plane of focus, microscope background level, electric-field related distortion of three-dimensional objects, Fresnel diffraction or photon scattering.Progress is continuous in the decreasing size and increasing complexity of nanosized magnetic systems being designed for either fundamental science or devices. Magnetic microscopies are crucial tools to monitor and understand the properties of such systems. Various types of information are desirable to gather, leading to multiple criteria to classify microscopies: spatial and time resolution, compatibility with environmental parameters such as variable temperature and applied magnetic field, requirements on the sample preparation and compatibility for ex-situ processing such as lithography, correlation with structural information, elemental sensitivity, quantity measured (magnetization, induction, stray field etc.), sensitivity. The most common magnetic microscopies offering spatial resolution below 50 nm and direct sensitivity to magnetization are X-ray Magnetic Circular Dichroism PhotoEmission Electron Microscopy (XMCD-PEEM) [ [7][8][9].Yet another criterion is the volume of the sample probed. This criterium is gaining in importance in the context of the emergence of three-dimensional (3D) magnetic objects and textures. The distribution of magnetization may be truly 3D if along the three directions in space the size of a system lies above magnetic characteristic length scales, such as the dipolar exchange length ∆ d for soft magnetic materials, or the anisotropy exchange length ∆ u for a hard magnetic material [10]. While this is obviously fulfilled in macroscopic materials, the complexity of magnetic textures is such that it cannot be measured in detail, and besides it cannot be controlled to achieve specific functions. The progress in nanofabrication techniques now allows to design suitable systems, both with top-down and bottom-up approaches. Let us give some examples. Flat magnetic elements are basic building blocks in spintronics, being patterned with great 2D versatility with thin film and lithography technologies. Large lateral dimensions may give rise to 2D magnetic textures, such as the so-called vortex state in disks [11]. Such textures are being investigated to design RF oscillator components, relying on the gyrotropic motion ...

Fast Domain Wall Motion Governed by Topology and Œrsted Fields in Cylindrical Magnetic Nanowires

“…DWs were monitored with both magnetic force microscopy (MFM) and X-ray Magnetic Circular Dichroism Photo-Emission Electron Microscopy (XMCD-PEEM) in the shadow mode ( Fig. 1c) to reveal the three-dimensional texture of magnetization [16,30,31]. While in MFM, sharp ns-long pulses could be sent, in XMCD-PEEM the shape of current pulses was distorted to a minimum width of 10 − 15 ns, due to long cabling, UHV feedthroughs and the sample holder contacts.…”

Imaging magnetic vortex configurations in ferromagnetic nanotubes

“…Following the procedure described by Jamet et al, which takes into account the progressive absorption of the x-ray beam through the sample crosssection [7], a vortex configuration in our NTs should result in perpendicular XMCD-PEEM contrast of the form shown in Fig. 1(a).…”

“…At equilibrium, the hollow magnetic geometry is expected to stabilize vortex-like flux-closure configurations with magnetization pointing along the NT circumference. Although vortex ends states have been observed in ferromagnetic nanowires (NWs) [7], the exchange energy penalty for the magnetic singularity along the vortex axis tends to favor non-fluxclosure states. In a NT, the lack of this axial Bloch point structure [8] is also expected to allow for fast magnetization reversal that begins with vortices nucleating at its ends and propagating along its length [9][10][11].…”

Observation of end-vortex nucleation in individual ferromagnetic nanotubes

“…In particular, we show the experimental signatures of vortex end-domain nucleation in individual FNTs and reveal their dependence on the slant angle of the ends. Magnetization reversal in FNTs offers some potential advantages over the equivalent and well-understood process in ferromagnetic nanowires (NWs) [19][20][21]: In particular, the core-free geometry of FNTs has been predicted to favor uniform switching fields and high reproducibility [14,22,23]. Understanding and controlling the switching process in real FNTs is a crucial step toward enabling practical applications.…”