X-ray Magnetic Circular Dichroism Photoemission Electron Microscopy Research Articles

Determination of optimal experimental conditions for accurate 3D reconstruction of the magnetization vector via XMCD-PEEM.

This work presents a detailed analysis of the performance of X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) as a tool for vector reconstruction of magnetization. For this, 360° domain wall ring structures which form in a synthetic antiferromagnet are chosen as the model to conduct the quantitative analysis. An assessment is made of how the quality of the results is affected depending on the number of projections that are involved in the reconstruction process, as well as their angular distribution. For this a self-consistent error metric is developed which allows an estimation of the optimum azimuthal rotation angular range and number of projections. This work thus proposes XMCD-PEEM as a powerful tool for vector imaging of complex 3D magnetic structures.

Imprint from ferromagnetic skyrmions in an antiferromagnet via exchange bias

Magnetic skyrmions are topological spin textures holding great potential as nanoscale information carriers. Recently, skyrmions have been predicted in antiferromagnets, with key advantages in terms of stability, size, and dynamical properties over their ferromagnetic analogs. However, their experimental demonstration is still lacking. Here, we show the imprint from ferromagnetic skyrmions into a thin film of an IrMn antiferromagnet, at room temperature and zero external magnetic field, using exchange-bias. Using high-spatial-resolution x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM), we observed the imprinted spin textures within the IrMn from the XMCD signal of the uncompensated Mn spins at the interface with the ferromagnet. This result opens up a path for logic and memory devices based on skyrmion manipulation in antiferromagnets.

Research progress of room temperature magnetic skyrmion and its application

<sec>It has been found that many magnetic materials possess the properties arising from skyrmions at room temperature. In addition to the common interaction energy, chiral interaction is also needed to form the skyrmion in magnetic material. There are four chiral magnetic interactions, namely: 1) Dzyaloshinskii-Moriya (DM) interaction; 2) long-ranged magnetic dipolar interaction; 3) four-spin exchange interaction; 4) frustrated exchanged interaction. Through the competition between exchange interaction and chiral interaction, magnetic skyrmion can be realized in magnetic material subject to a certain magnetic field and temperature. The skyrmion generated by the DM interaction features small size (5–100 nm), which is easy to adjust. The skyrmion can be driven by magnetic field or ultralow current density. The magnetic materials with skyrmion can exhibit the properties related to the skyrmion Hall effect, the topological Hall effect and the emergent electrodynamics, which are closely related to the skyrmion number. The existence of skyrmion in the magnetic material can be indirectly measured by topological Hall effect. The movement of skyrmion can be driven by spin polarized current in the direction either parallel or perpendicular to the current direction. The movement of the skyrmion driven by spin polarized currents will continue when the current is present, and will disappear when the current disappears. </sec><sec>In previous studies, magnetic skyrmions were realized in a variety of materials. However magnetic skyrmions were found only in very limited types of single crystal materials at room temperature or near room temperature. In recent years, scientists have discovered a variety of magnetic skyrmion materials at room temperature, including film materials (such as multilayer materials, artificial skyrmion materials) and crystal materialssuch as <i>β</i>-Mn-type Co<sub>10–<i>x</i>/2</sub>Zn<sub>10–<i>x</i>/2</sub>Mn<i><sub>x</sub></i>, Fe<sub>3</sub>Sn<sub>2</sub>. Among all kinds of room temperature magnetic skyrmion materials, the most valuable one is the multilayer film material. The Skyrmion multilayer film has the advantages of small size, adjustable material type, simple preparation, good temperature stability, good device integration,etc. At the same time, skyrmion multilayer film is very easy to optimize by adjusting and constructing a special structure that has the wanted types of materials each with a certain thickness. Artificial skyrmion material obtains artificial skyrmion by constructing a micro-nano structure, therefore the artificial skyrmion with high-temperature stability can be realized by choosing high Curie temperature materials. There are a variety of materials which can realize the skyrmion above room temperature, such as Co<sub>9</sub>Zn<sub>9</sub>Mn<sub>2</sub> (300–390 K) and Fe<sub>3</sub>Sn<sub>2</sub> (100–400 K). These room temperature materials further widen the temperature application range of skyrmion. The room temperature materials can be prepared or characterized by a variety of techniquesincluding sputtering for fabrication and X-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM) for characterization. </sec><sec>The discovery of the magnetic skyrmion materials at room temperature not only enriches the research content of materials science, but also makes the skyrmion widely applicable in novel electronic devices (such as racetrack memory, microwave detector, oscillators). Because the skyrmion has the advantages of small size, ultra-low driving current density, and topological stability, it is expected to produce racetrack memory based on the skyrmion with low energy consumption, non-volatile and high density. The MTJ microwave detector based on skyrmion can be achieved with no external magnetic field nor bias current but with low power input (< 1.0 μW); the sensitivity of the microwave detector can reach 2000 V·W<sup>–1</sup>. The frequency of the oscillator based on skyrmion can be tuned by magnetic field or current, and moreover, the oscillato is very easy to integrate with IC. In this paper, first, the basic characteristic of magnetic skyrmion is introduced; and then room temperature magnetic skyrmion is reviewed; finally the advances of the racetrack memory, microwave detectors and oscillators are introduced, highlighting the development trend of room temperature magnetic skyrmion. </sec>

Chiral asymmetry detected in a 2D array of permalloy square nanomagnets using circularly polarized x-ray resonant magnetic scattering

The sensitivity of circularly polarized x-ray resonant magnetic scattering (CXRMS) to chiral asymmetry has been demonstrated. The study was performed on a 2D array of Permalloy (Py) square nanomagnets of 700 nm lateral size arranged in a chess pattern, in a square lattice of 1000 nm lattice parameter. Previous x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) images on this sample showed the formation of vortices at remanence and a preference in their chiral state. The magnetic hysteresis loops of the array along the diagonal axis of the squares indicate a non-negligible and anisotropic interaction between vortices. The intensity of the magnetic scattering using circularly polarized light along one of the diagonal axes of the square magnets becomes asymmetric in intensity in the direction transversal to the incident plane at fields where the vortex states are formed. The asymmetry sign is inverted when the direction of the applied magnetic field is inverted. The result is the expected in the presence of an unbalanced chiral distribution. The effect is observed by CXRMS due to the interference between the charge scattering and the magnetic scattering.

Heterojunction-induced magnetic anisotropy and magnetization reversal of Ni wires on LiNbO3 substrate

We report magnetic domain formation control within micro-scale polycrystalline Ni wires on a single-crystal Y-cut 128° LiNbO3 substrate. X-ray magnetic circular dichroism photoemission electron microscopy (XCDM-PEEM), micromagnetic simulations, and magnetoresistance (MR) measurements allowed us to estimate the uniaxial magnetic anisotropy induced by the magnetoelastic effect that originated at the interface between each Ni layer and the LiNbO3 substrate. Comparison of the XMCD-PEEM and MR measurement results shows that the competition between the shape magnetic anisotropy and the uniaxial magnetic anisotropy parallel to the orientation flat (OF) direction of the substrate leads to variations in both the magnetization order and the magnetization reversal process. The uniaxial magnetic anisotropy is estimated to be approximately 3.3 kJ/m3. This heterojunction structure composed of ferromagnetic and ferroelectric layers thus offers alternative ways to produce artificial functional multiferroic materials and devices.

Magnetic patterning of FeRh thin films by energetic light ion microbeam irradiation

Magnetic patterning of FeRh thin films of various shapes and dimensions by 2 MeV H and He ion microbeam irradiation was examined by X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) at the photon energy of the Fe L3 absorption edge. The XMCD-PEEM images clearly show that the various magnetic patterns whose sizes are about 1–2 µm, can be successfully produced by controlling the scan condition and the size of the ion beam probe. Furthermore, the quantitative analysis of the XMCD-PEEM images reveals that the localized magnetization in the micrometer-sized magnetic patterns significantly depends on the irradiation ion fluence. In addition, the annealing condition is also found to be one of the predominant factors that determine the magnetic state even for the micrometer-sized magnetic patterns.

Room temperature ferroelectric and magnetic investigations and detailed phase analysis of Aurivillius phase Bi5Ti3Fe0.7Co0.3O15 thin films

Aurivillius phase Bi5Ti3Fe0.7Co0.3O15 (BTF7C3O) thin films on α-quartz substrates were fabricated by a chemical solution deposition method and the room temperature ferroelectric and magnetic properties of this candidate multiferroic were compared with those of thin films of Mn3+ substituted, Bi5Ti3Fe0.7Mn0.3O15 (BTF7M3O). Vertical and lateral piezoresponse force microscopy (PFM) measurements of the films conclusively demonstrate that BTF7C3O and BTF7M3O thin films are piezoelectric and ferroelectric at room temperature, with the major polarization vector in the lateral plane of the films. No net magnetization was observed for the in-plane superconducting quantum interference device (SQUID) magnetometry measurements of BTF7M3O thin films. In contrast, SQUID measurements of the BTF7C3O films clearly demonstrated ferromagnetic behavior, with a remanent magnetization, Br, of 6.37 emu/cm3 (or 804 memu/g), remanent moment = 4.99 × 10−5 emu. The BTF7C3O films were scrutinized by x-ray diffraction, high resolution transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray analysis mapping to assess the prospect of the observed multiferroic properties being intrinsic to the main phase. The results of extensive micro-structural phase analysis demonstrated that the BTF7C3O films comprised of a 3.95% Fe/Co-rich spinel phase, likely CoFe2 − xTixO4, which would account for the observed magnetic moment in the films. Additionally, x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) imaging confirmed that the majority of magnetic response arises from the Fe sites of Fe/Co-rich spinel phase inclusions. While the magnetic contribution from the main phase could not be determined by the XMCD-PEEM images, these data however imply that the Bi5Ti3Fe0.7Co0.3O15 thin films are likely not single phase multiferroics at room temperature. The PFM results presented demonstrate that the naturally 2D nanostructured Bi5Ti3Fe0.7Co0.3O15 phase is a novel ferroelectric and has potential commercial applications in high temperature piezoelectric and ferroelectric memory technologies. The implications for the conclusive demonstration of ferroelectric and ferromagnetic properties in single-phase materials of this type are discussed.

Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope

This work enlightens the threshold photoemission magnetic circular dichroism (MCD) and its adaption on photoemission electron microscopy (PEEM) using lasers. MCD is a simple and efficient way to investigate magnetic properties since it does not need any spin analyzers with low efficiency, and thus the MCD related techniques have developed to observe magnetic domains. Usually, MCD in a total yield measurement in the valence band with weak spin–orbit coupling (SOC) excited by low photon energy (hν≤ 6eV) does not compete with the X-ray magnetic circular dichroism (XMCD) with strong SOC. XMCD PEEM observation of magnetic domains has been successfully established while MCD PEEM derived from valence bands has not been. However, using angle and energy resolved photoelectron, valence band MCD provides large asymmetry similar to that by XMCD. Threshold measurement of photoelectron in a total electron yield procedure can take advantage of the measurement of photoelectrons with a limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic information and gives more than 10% asymmetry for Ni/Cu(001), which is comparable to that obtained by angle resolved photoemission. Thus the threshold MCD technique is a suitable method to observe magnetic domains by PEEM. For threshold MCD, incident angle dependence and high sensitivity to out-of-plane magnetized films compared with in-plane ones are discussed. Ultrashort pulse lasers make it feasible to measure two photon photoemission MCD combined with PEEM, where resonant excitation has a possibility to enhance dichroic asymmetry. Recent results for valence band magnetic dichroism PEEM are presented.

Magnetization dynamics of Landau structures: tuning the response of mesoscopic magneticobjects using defects

Magnetic vortex cores are interacting with and can even be annihilated by artificial defects,such as holes. These defects have been fabricated by focused ion beam milling (FIB) intothe magnetic domains, domain walls and the center of square-shaped vortices, known asLandau structures. We report the imaging of the magnetization dynamics of Landaustructures containing holes by means of x-ray magnetic circular dichroism photo-emissionelectron microscopy (XMCD-PEEM). Due to the high lateral and temporal resolution ofthis method, the magnetic excitation spectrum, which is characteristic for the vortex–holeinteraction, is investigated in detail. We find that the vortex core as well as domainwalls can be trapped by small holes. With the help of micromagnetic simulationswe show that the vortex gyrotropic motion frequency is enhanced, whereas theamplitude is significantly reduced in the case of non-centric holes in domain walls.

Domain wall velocity measurement in permalloy nanowires with X-ray magnetic circular dichroism imaging and single shot Kerr microscopy

Domain walls (DWs) propagated along nanoscale magnetic wires by current or field pulses could potentially be used for data storage or logic applications, but the understanding of the DW dynamics, particularly under the influence of spin-polarized current, is incomplete. Measuring the velocity can give insights into the physics of the DW motion. Here we demonstrate DW velocity measurements in permalloy ( Ni 80 Fe 20 ) nanowires (1500 nm width and 20 nm thickness) using the techniques of X-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) to image the magnetic contrast in the nanowires, and single shot Kerr microscopy, which allows for dynamic measurements. The magnetic imaging yields the average velocity as well as information on the DW spin structure, whereas the single shot method highlights the stochastic nature of the DW motion.

Nanomagnetism studies with spin‐polarized low‐energy electron microscopy and x‐ray magnetic circular dichroism photoemission electron microscopy

AbstractThe spin reorientation transition (SRT) of ultrathin FeCo alloy films on an Au(111) surface and the magnetic/structural phase transition of epitaxial MnAs films on a GaAs(100) surface are studied by two electron microscopic techniques with very high surface sensitivity, i.e. spin‐polarized low‐energy electron microscopy (SPLEEM) and x‐ray magnetic circular dichroism photoemission electron microscopy (XMCDPEEM), combined with low‐energy electron microscopy (LEEM) and low‐energy electron diffraction (LEED), respectively. A new SRT mechanism is found and the interrelation between ferromagnetism and structure is elucidated. Copyright © 2005 John Wiley & Sons, Ltd.

Imaging low-dimensional magnetism with slow electrons

Two aspects of low-dimensional magnetism are discussed: the spin-dependent quantization effects in perfect two-dimensional single crystal layers and the magnetic domain structure of submicron magnetic patterns. Both require high lateral resolution and surface sensitivity. This is provided by spin-polarized low energy electron microscopy (SPLEEM) and X-ray magnetic circular dichroism photoemission electron microscopy (XMCDPEEM).