Element-selective mapping of magnetic moments in ultrathin magnetic films using a photoemission microscope

We combine X-ray magnetic circular dichroism (XMCD) and photoelectron emission microscopy to obtain locally resolved magnetic information on a microscopic scale. Scanning the photon energy across elemental absorption edges and recording microscopic images of the local secondary electron intensity for both photon helicities at each photon energy step allows to analyze local XMCD spectra at any position of the imaged area of the sample. With the help of magnetic sum-rules local quantitative information about magnetic moments can be extracted from such microspectroscopic measurements. The full power of XMCD as a spectroscopic tool is so maintained, while microscopic spatial resolution is added.     

Spin state transition in LaCoO3 studied using soft x-ray absorption spectroscopy and magnetic circular dichroism

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L(2,3) edge, we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.     

Magnetic circular dichroism from the impurity band in III-V diluted magnetic semiconductors

The magnetic circular dichroism of III-V diluted magnetic semiconductors, calculated within a theoretical framework suitable for highly disordered materials, is shown to be dominated by optical transitions between the bulk bands and an impurity band formed from magnetic dopant states. The real-space Green's functions incorporate spatial correlations in the disordered conduction band and valence-band electronic structure, and include extended and localized states on an equal basis. Our findings reconcile unusual trends in the experimental magnetic circular dichroism in III-V diluted magnetic semiconductors with the antiferromagnetic p-d exchange interaction between a magnetic dopant spin and its host.     

Vanishing magnetic interactions in ferromagnetic thin films

We have used element-specific hysteresis measurements, based on the x-ray magnetic circular dichroism technique, to investigate magnetic trilayer structures composed of Fe and Ni layers. Within a critical regime we have discovered a class of structures in which the exchange interaction, the mechanism responsible for the macroscopic magnetism, can become vanishingly small. The experimental observations are supported by first principles theory and are explained as arising from a cancellation of several competing magnetic interactions. Hence, we have discovered a system with a novel exchange interaction between magnetic layers in direct contact that replaces the conventional exchange interaction in ferromagnets.     

Quantitative analysis of magnetic excitations in Landau flux-closure structures using synchrotron-radiation microscopy

We have measured the excitation spectrum of six micron magnetic squares using x-ray magnetic circular dichroism. We observe all three excitations expected in a Landau flux-closure pattern. High temporal and spatial resolution allows quantitative analysis of the excitations. A short magnetic in plane pulse excites the magnetic element and we observe precessional motion of the magnetization within the domains as well as a domain wall mode and vortex motion. The vortex moves perpendicular to the excitation field and relaxes without showing a circulating orbit.     

Imaging excitations in magnetic thin film microstructures

We show how a photoemission electron microscope (PEEM) installed at a synchrotron can be used to image magnetic objects with very high spatial and temporal resolution. Sub-nanosecond magnetic field pulses are used to excite the fundamental magnetic modes of micron sized permalloy particles. The time evolution of the magnetization is imaged using a pump-probe technique where the magnetic contrast is given by X-ray magnetic circular dichroism (XMCD). Depending on the shape and size of the magnetic object we can observe modes related to either the homogenously magnetized domains, to the domain walls or to the vortex. All of these can be analyzed quantitatively yielding their frequencies, amplitudes and damping time constants. For objects with controlled defects we show that the magnetic vortex can be switched between defects using magnetic field pulses.     

EMCD real space maps of Magnetospirillum magnetotacticum

In this study we combine energy loss magnetic circular dichroism (EMCD) and energy filtered transmission electron microscopy (EFTEM) to map magnetic properties of nanoparticles. We show that it is a functional tool for investigating the magnetic behaviour of bio-mineralized magnetite crystals of Magnetospirillum magnetotacticum. We find that the spatial resolution of our experimental set-up is in the range of less than 2 nm. The results are compared with EMCD studies of abiogenic magnetite.     

Novel pressure-induced magnetic transition in magnetite (Fe3O4)

Fe K-edge x-ray magnetic circular dichroism of magnetite (Fe3O4) powders was measured with synchrotron radiation under variable pressure and temperature conditions in diamond anvil cell. The magnetic dichroism was observed to decrease discontinuously by approximately 50% between 12 and 16 GPa, independent of temperature. The magnetic transition is attributed to a high-spin to intermediate-spin transition of Fe2+ ions in the octahedral sites and could account for previously observed structural and electrical anomalies in magnetite at this pressure range. The interpretation of x-ray magnetic circular dichroism data is supported by x-ray emission spectroscopy and theoretical cluster calculations.     

利用扫描透射X射线显微镜观测磁涡旋结构

利用上海光源软X射线谱学显微光束线站(STXM)并结合X射线的磁圆二色效应, 我们对方形、圆形和三角形的Ni₈₀Fe₂₀薄膜微结构中的磁涡旋结构进行了定量实验观测, 并利用同步辐射光源的元素分辨特性, 分别在Fe和Ni的L₃吸收边对涡旋磁结构进行了观测. 我们还对磁涡旋中磁矩的分布进行了定量分析, 发现实验结果与微磁学模拟结果完全符合.     

Theoretical studies on magnetic atom imaging by use of photoelectron diffraction

We theoretically study possible methods how to obtain imaging of magnetic atoms by use of photoelectron diffraction (PD). We propose a novel method to apply Daimon effect where PD peaks are rotated around forward focusing peaks. In usual circular dichroism, we simply use the difference of the PD intensities for different X-ray circular polarization. In contrast to this dichroism, we rather use the difference of the PD intensities for different circular polarizations but - is used for the – circular polarization only in the data handling. This technique allows us to obtain clear atomic image only of spin polarized atoms, and to distinguish magnetic atoms with up-spins from those with down spins. Some illustrative calculations demonstrate the potential use and also the limitation of this technique.     

Atomic-layer resolved magnetic and electronic structure analysis of ni thin film on a Cu(001) surface by diffraction spectroscopy

Up until now there has been no direct method for detecting the electronic and magnetic structure of each atomic layer at the surface, which is an essential analysis technique for nanotechnology. For this purpose, we have developed a new method, diffraction spectroscopy, based on the photon energy dependence of the angular distribution of Auger electron emission. We have applied this method to analyze the magnetic structure of a Ni ultrathin film on a Cu(001) surface around the spin reorientation transition. Atomic-layer resolved x-ray absorption and magnetic circular dichroism spectra were obtained. Surface and interior core-level shifts and magnetic moments are determined for each atomic layer individually.     

New applications of the magnetic X-ray circular dichroism method for surface-magnetism investigations in a photoemission electron microscope

It is shown that magnetic X-ray circular dichroism (MXCD) can be exploited in photoemission electron microscopy not only to visualize the domain structure of ferromagnets, but also to perform quantitative measurements of the stray magnetic fields at the domain boundaries. In the general situation, two MXCD images obtained at different extractor voltages are required. In specific cases, however, it suffices to consider a single image, if it is deformed by the stray magnetic fields compared to a known object geometry. The object geometry means its real shape, scratches or other defects. It is also possible to deposit a paramagnetic film structured in the form of stripes or a grid as a reference on the ferromagnetic sample being investigated. Received: 2 August 2001 / Accepted: 6 September 2001 / Published online: 20 December 2001     

样品电流模式下外磁场引起的X射线吸收谱强度变化

在用样品电流模式的测量过程中发现，磁场强度、磁场与样品表面的夹角以及光斑在样品表面的位置都会对吸收谱强度产生影响；在光斑、磁场和样品的不同几何配置下,测量并分析了表面均匀氧化的铝箔中氧的K边吸收谱，指出外磁场下吸收谱强度随各种条件变化的趋势，并对实验结果给出了合理解释；结果表明所用的模型分析与实验数据符合得很好；所得到的信息对于XMCD实验的设计安排、相应数据的分析以及物理信息的提取具有重要意义.     

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≤ 6 eV) 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(0 0 1), 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.     

Layer-resolved magnetic moments in Ni/Pt multilayers

The magnetic moments in Ni/Pt multilayers are thoroughly studied by combining experimental and ab initio theoretical techniques. SQUID magnetometry probes the samples' magnetizations. X-ray magnetic circular dichroism separates the contribution of Ni and Pt and provides a layer-resolved magnetic moment profile for the whole system. The results are compared to band-structure calculations. Induced Pt magnetic moments localized mostly at the interface are revealed. No magnetically "dead" Ni layers are found. The magnetization per Ni volume is slightly enhanced compared to bulk NiPt alloys.     

Role of magnetic circular dichroism in all-optical magnetic recording

Using magneto-optical microscopy in combination with ellipsometry measurements, we show that all-optical switching with polarized femtosecond laser pulses in ferrimagnetic GdFeCo is subjected to a threshold fluence absorbed in the magnetic layer, independent of either the excitation wavelength or the polarization of the laser pulse. Furthermore, we present a quantitative explanation of the intensity window in which all-optical helicity-dependent switching (AO-HDS) occurs, based on magnetic circular dichroism. This explanation is consistent with all the experimental findings on AO-HDS so far, varying from single- to multiple-shot experiments. The presented results give a solid understanding of the origin of AO-HDS, and give novel insights into the physics of ultrafast, laser controlled magnetism.     

Domain wall dynamics and interlayer interactions in magnetic trilayer systems studied by XMCD-PEEM

We have used time-resolved x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM) to investigate the layer-resolved microscopic magnetization reversal in FeNi/X/Co (with X=Cu, Al₂O₃) trilayer systems. These measurements were performed in pump-probe mode, synchronizing magnetic pulses with synchrotron x-ray pulses. The good magnetic contrast observed for most samples reveals that in many cases the magnetization reversal is reproducible. We have used the measurements to obtain domain wall propagation speeds as a function of applied magnetic field, and to investigate the influence of domain wall interactions on the magnetic switching.     

Soft X-ray resonant magnetic scattering of magnetic nanostructures

Soft X-ray resonant magnetic scattering offers a unique element-, site- and valence-specific probe to study magnetic structures on the nanoscopic length scale. This new technique, which combines X-ray scattering with X-ray magnetic circular and linear dichroism, is ideally suited to investigate magnetic superlattices and magnetic domain structures. The theoretical analysis of the polarization dependence to determine the vector magnetization profile is presented. This is illustrated with examples studying the closure domains in self-organising magnetic domain structures, the magnetic order in patterned samples, and the local configuration of magnetic nano-objects using coherent X-rays. To cite this article: G. van der Laan, C. R. Physique 9 (2008).     

Circularly polarized soft X-ray generation by Co/Pt thin film polarizer with perpendicular magnetic anisotropy

We have experimentally demonstrated circular polarization generation from linear polarized soft X-ray at synchrotron by adopting a thin magnetic film polarizer. Polarizer is composed of Co/Pt multilayer with a perpendicular magnetic anisotropy, which allows us to easily accommodate without needing any tilting angle into the measurement setup since the circular polarization is generated for the X-ray with normal incidence and transmission. Generated circular polarization is examined by observing magnetic domain features based on the X-ray magnetic circular dichroism, where ∼11% of circular component is estimated compared to the case of full circular polarization.     

Opus musicum

The rotation of the plane of linearly polarized light by an optically-active medium arises from a difference between the refractive indices of the medium for left- and right-circularly polarized light. The circular birefringence is related to a circular dichroism, which is exhibited by all molecules which are not superposable on their mirror image and exist as laevo- and dextro-rotatory isomers. The circular dichroism of a dissymmetric molecule in a particular absorption region originates from an electronic transition with collinear electric and magnetic dipole moments, corresponding to the displacement of a molecular valency electron through a helical path by the absorption of radiation. The right- or left-handed form of the helical path depends upon the molecular structure, and the absolute stereochemical configuration of dissymmetric molecules may be determined by comparing their circular dichroism spectra with the calculated rotational strengths. In addition the polarization direction of an electric moment and the magnitude of a magnetic moment of an electronic transition in a dissymmetric molecule may be derived from circular dichroism measurements.