X-ray Magnetic Linear Dichroism Research Articles

Insights into the electronic structure of Co2FeSi from x-ray magnetic linear dichroism

Experimental evidence both for and against a half-metallic ground-state of the Heusler compound Co2FeSi has been published. Density functional theory based calculations suggest a non half-metallic ground state. It has been argued, that on-site Coulomb interaction of the d electrons has to be taken into account via the LDA+U method, which predicts a half-metallic ground-state for U = 2.5...4.5 eV. X-ray magnetic linear dichroism (XMLD) can be used as a tool to assess the appropriateness of the LDA+U approach: the calculated spectra within the LDA+U or GGA+U schemes are different from those within the LDA or GGA. Due to its ability to separate different orbital symmetries, XMLD allows us to distinguish between different models of the electronic structure of Co2FeSi. In this article we discuss experimental XMLD spectra and compare them with detailed first principles calculations. Our findings give evidence for the inadequacy of the LDA+U or GGA+U band structures, whereas constrained calculations with the GGA and a fixed spin moment of 6 \mu_B give better overall agreement between experiment and theory.

Exchange bias in epitaxially grown CoO/MgO/Fe/Ag(001)

CoO/MgO/Fe/Ag(001) films were grown epitaxially and were studied using magneto-optic Kerr effect and x-ray magnetic linear dichroism (XMLD). For thick CoO films, we show that both the Fe coercivity and the exchange bias exhibit the expected monotonic decrease with increasing MgO thickness. For thin CoO films, however, we find that while the coercivity decreases monotonically, the exchange bias increases initially and then decreases with increasing MgO thickness. By measuring the CoO XMLD as a function of MgO thickness, we show that the unusual behavior of the exchange bias at thinner CoO is due to a transition of the CoO spins from rotatable-to-frozen spins as the MgO thickness increases.

X-Treme beamline at SLS: X-ray magnetic circular and linear dichroism at high field and low temperature

X-Treme is a soft X-ray beamline recently built in the Swiss Light Source at the Paul Scherrer Institut in collaboration with École Polytechnique Fédérale de Lausanne. The beamline is dedicated to polarization-dependent X-ray absorption spectroscopy at high magnetic fields and low temperature. The source is an elliptically polarizing undulator. The end-station has a superconducting 7 T-2 T vector magnet, with sample temperature down to 2 K and is equipped with an in situ sample preparation system for surface science. The beamline commissioning measurements, which show a resolving power of 8000 and a maximum flux at the sample of 4.7 × 10(12) photons s(-1), are presented. Scientific examples showing X-ray magnetic circular and X-ray magnetic linear dichroism measurements are also presented.

Three-dimensional spin orientation in antiferromagnetic domain walls of NiO studied by x-ray magnetic linear dichroism photoemission electron microscopy

A determination of the three-dimensional spin directions in all types of domain walls (DWs) of antiferromagnetic NiO has been successfully performed by photoemission electron microscopy combined with x-ray magnetic linear dichroism (XMLD), both for $s$- and $p$-polarized light. By comparing the azimuthal angle dependence of the XMLD contrast in the DWs with cluster model calculations which include the crystal symmetry and full-multiplet splitting, we determine the spin structures in the ${001}$ T walls, ${011}$ T walls, 120\ifmmode^\circ\else\textdegree\fi{} S walls, and 180\ifmmode^\circ\else\textdegree\fi{} S walls. In some cases, distinct S walls are not formed between two adjacent S domains, and the spin direction changes gradually over a wide range of the S domain structures. In the S walls, the spin direction is parallel to the magnetic easy ${111}$ plane. These spin configurations arise from the large difference in anisotropy energy between the in-plane and out-of-plane directions. Unexpectedly large widths in the several hundred nanometer range were observed for all the DWs. This also shows that NiO has a small magnetocrystalline anisotropy energy. Together with Monte Carlo simulation results, the qualitative phenomena concerning the wall energies are discussed. We further investigated the difference in wall energy between the ${001}$ T wall and the ${011}$ T wall. From the Monte Carlo simulation and an experimental study of heating effects, it is revealed that the ${001}$ T wall energy is smaller than the ${011}$ T wall energy.

Direct imaging of both ferroelectric and antiferromagnetic domains in multiferroic BiFeO3 single crystal using x-ray photoemission electron microscopy

In this work, we propose to study the magnetic and ferroelectric configurations in ferroelectric multidomain BiFeO3 single crystals. Using x-ray (magnetic) linear dichroism in a photoemission electron microscope (X-PEEM), we are able to directly image both the antiferromagnetic and ferroelectric domains. We find that inside one single ferroelectric domain several antiferromagnetic domains coexist. This is different from what was observed on epitaxial thin films, where the ferroelectric domains perfectly match the antiferromagnetic ones, but also from previous neutron measurements on ferroelectric monodomain single-crystals for which one single antiferromagnetic domain was identified. This underlines the fundamental differences between thin films, bulk samples, and single versus ferroelectric multidomain samples.

Itinerant and localized magnetic moments in ferrimagnetic Mn2CoGa thin films probed by x-ray magnetic linear dichroism: Experiment andab initiotheory

Epitaxial thin films of the half-metallic ${X}_{a}$ compound Mn${}_{2}$CoGa (Hg${}_{2}$CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality films with a bulk magnetization of $1.95(5)\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per unit cell were obtained. The ${L}_{3,2}$ x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory (DFT) and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism, in good agreement with theory as well, allows us to distinguish between itinerant and local Mn moments. Based on noncollinear spin DFT, it is shown that one of the two Mn moments has local character, whereas the other Mn moment and the Co moment are itinerant.

Surface antiferromagnetic domain imaging using low-energy unpolarized electrons

We report on the surface antiferromagnetic (AFM) domain imaging of the prototype antiferromagnet NiO(100) using an electron-microscopy-based laboratory method. Coherently exchange-scattered electrons from the surface AFM lattice are utilized in a low-energy electron microscope to resolve the surface domain structure. Direct comparison with x-ray magnetic linear dichroism photoemission electron microscopy demonstrates a complimentary nature. High surface sensitivity combined with improved spatial resolution of this method enables studying of nanosized magnetic domains in NiO(100) thin films and their thickness-dependent evolution.

Determination of the Fe magnetic anisotropies and the CoO frozen spins in epitaxial CoO/Fe/Ag(001)

CoO/Fe/Ag(001) films were grown epitaxially and studied by X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD). After field cooling along the Fe[100] axis to 80 K, exchange bias, uniaxial anisotropy, and 4-fold anisotropy of the films were determined by hysteresis loop and XMCD measurements by rotating the Fe magnetization within the film plane. The CoO frozen spins were determined by XMLD measurement as a function of CoO thickness. We find that among the exchange bias, uniaxial anisotropy, and 4-fold anisotropy, only the uniaxial magnetic anisotropy follows thickness dependence of the CoO frozen spins.

Strain control spin reorientation transition in DyFeO3/SrTiO3 epitaxial film

We grew single phase DyFeO3 (110) epitaxial films on SrTiO3(001) substrates in the layer-by-layer mode using the pulsed laser deposition. The Fe L2,3-edge x-ray magnetic linear dichroism and magnetic hysteresis confirm that the film shows the spin-canted antiferromagnetism at room temperature and undergoes the spin reorientation transition (SRT) upon cooling as the bulk. The SRT temperature is significantly reduced in the thin films due to the compressive strain and becomes below 5 K for the thickness ≲ 50 Å.

Element-specific magnetic properties of Co2(Mn1−xFex)Si films probed by x-ray magnetic circular/linear dichroism

Element-specific magnetic properties of epitaxial Co${}_{2}$Fe${}_{x}$Mn${}_{1\ensuremath{-}x}$Si/MgO(100) Heusler films prepared by laser ablation are measured by circular dichroism in x-ray absorption spectroscopy (XMCD). Surface and bulk magnetization of the 100-nm-thick films are equal as tested by comparing the total electron yield and transmission measurements and follow the generalized Slater-Pauling rule. For Co${}_{2}$FeSi, the large magnetization of 6 ${\ensuremath{\mu}}_{B}$ per formula unit indicates half-metallic behavior. Calculations using the local spin density approximation (LDA) result in a half-metallic band structure for this compound when an additional electron-electron-correlation correction ($\text{LDA}+U$) is considered. Simulated XMCD spectra using the LDA and $\text{LDA}+U$ approaches are compared to the experimental spectra. While the electron-electron Coulomb correlation has only a small effect on the observable XMCD spectra, the calculations predict that it will cause a significant change in the x-ray magnetic linear dichroism (XMLD).

In-plane spin reorientation transition in Fe/NiO bilayers on Ag(1,1,10)

Magnetic properties of Fe/NiO bilayers grown on a (1,1,10) vicinal surface of Ag(001) were studied by magneto-optical Kerr effect (MOKE) and x-ray magnetic linear dichroism (XMLD). The orientation of the antiferromagnetically (AFM) aligned spins of NiO films shows an in-plane spin-reorientation transition (SRT) from parallel to perpendicular to the steps with increasing NiO thickness. Two in-plane SRTs of Fe moments are found in dependence of the NiO thickness. The first SRT of the Fe magnetization from parallel to perpendicular to the steps is observed at NiO thickness where antiferromagnetic order is found, the second at the NiO thickness of the SRT in the NiO film. Also, the in-plane SRT of Fe spins driven by a temperature increase is observed. The perpendicular coupling between the Fe and NiO spins is proven. The induced uniaxial anisotropy energy is estimated to be 0.12 erg/cm${}^{2}$.

Effect of small magnetic corrections to susceptibility on the angular dependences of the reflectivity of polarized X-rays from multilayer structures

During the last fifteen years, a new method for investigating the magnetic properties of multilayer structures—Xray resonant magnetic scattering (XRMS)—has been actively developed at all synchro� trons. Having the same magnetic sensitivity and ele� mental selectivity as Xray magnetic circular or linear dichroism (XMCD or XMLD), XRMS allows for the reconstruction of the features of magnetic ordering and the depth profile of the magnetic moments of res� onant atoms in multilayer nanofilms (see, e.g., (1-5)). This information is very important for testing and cre� ating spintronic devices. Another problem that can be solved with XRMS is the experimental determination of the refraction of waves in the medium, i.e., the absolute values of Xray susceptibility, including magnetic corrections near the absorption edges. This information is almost absent in the existing databases for the scattering amplitudes and refractive indices of various materials. The prob� lem of the determination of the optical constants for X rays near the absorption edges is of fundamental interest for the theory of solids. Furthermore, this information is necessary for processing experimental spectra and angular dependences of XRMS in order to obtain depthselective information on the magnetic structure.

Anisotropic x-ray magnetic linear dichroism and spectromicroscopy of interfacial Co/NiO(001)

Photoemission electron microscopy (PEEM) with linearly polarized x rays is used to determine the orientation of antiferromagnetic domains by monitoring the relative peak intensities at the $3d$ transition metal ${L}_{2}$ absorption edge. In such an analysis it is necessary to take into account the orientations of the x-ray polarization $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and magnetization $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ with respect to the crystalline axes. We address this problem by presenting a general expression of the angular dependence for both x-ray absorption (XA) spectroscopy and x-ray magnetic linear dichroism (XMLD) for arbitrary directions of $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ in the (001) cubic plane. In cubic symmetry the angular-dependent XMLD is a linear combination of two spectra with different photon energy dependencies. As a result the intensity maxima of the XA and XMLD are no longer found for $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ parallel to $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ and their directions will vary with photon energy. When $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ or $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ is along a high-symmetry axis there is only one spectrum. The angular-dependent XMLD can be separated into an isotropic part, which is symmetric around $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$, and an anisotropic part, which depends on the orientation of the crystal axes. The anisotropic part has maximal intensity when $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}$ and $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}$ have equal but opposite angles with respect to the [100] direction. The Ni${}^{2+}$ ${L}_{2}$ edge has the peculiarity that the isotropic part vanishes, which has the interesting consequence that the maximum in the XMLD intensity is obtained not only for $\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [100] but also for ($\mathbf{E\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [110], $\mathbf{H\ifmmode \hat{}\else \^{}\fi{}}\ensuremath{\parallel}$ [1$\mathrm{1\ifmmode \bar{}\else \={}\fi{}}$0]). We apply the angular-dependent theory to determine the spin orientation near the Co/NiO(100) interface. The PEEM images show that the ferromagnetic Co spins and antiferromagnetic NiO spins are aligned perpendicular to each other. By rotating the sample with respect to the linear x-ray polarization we furthermore find that the perpendicular coupling with the ferromagnetic Co layer at the interface causes a canting of the antiferromagnetic Ni moments. This shows that taking into account the angular dependence of the XMLD in the detailed analysis of PEEM images can lead to an accurate retrieval of the spin axes of the antiferromagnetic domains.

Direct observation of imprinted antiferromagnetic vortex states in CoO/Fe/Ag(001) discs

In magnetic thin films, a magnetic vortex is a state in which the magnetization vector curls around the centre of a confined structure1. In a thin-film disc, vortex states are characterized by the vortex polarity and the winding number2,3. In ferromagnetic (FM) discs, these two parameters have been shown to govern many fundamental properties of the vortex, such as its gyroscopic rotation4, polarity reversal5,6,7, core motion8 and vortex-pair excitation9. In antiferromagnetic (AFM) discs10, in contrast, there has been only indirect evidence for a vortex state, obtained through the observation of induced FM-ordered spins in the AFM disc11,12,13,14. Here we report the direct observation of an AFM vortex state in the AFM layer of an AFM/FM bilayer system. We have fabricated single-crystalline NiO/Fe/Ag(001) and CoO/Fe/Ag(001) discs, and using X-ray magnetic linear dichroism techniques we observe two types of AFM vortex, one of which has no analogue in FM structures. We also show that a frozen AFM vortex can bias an FM vortex at low temperature.

Anisotropy of theL2,3x-ray magnetic linear dichroism of Fe films on GaAs: Experiment andab initiotheory

We report investigations of the anisotropy of the x-ray magnetic linear dichroism (XMLD) of thin single-crystalline Fe films on GaAs substrates with film thicknesses of 3 nm and 7 nm, respectively. The orientation of the in-plane Fe magnetization with respect to the crystal axes is stepwise rotated from the [1 1 0] to the [0 1 0] direction and the $\text{Fe}\text{ }{L}_{2,3}$ XMLD spectrum is measured at every orientation step. A highly anisotropic XMLD signal at the $\text{Fe}\text{ }{L}_{2}$ and ${L}_{3}$ edges is observed, which origin is analyzed in detail. We show that the measured anisotropic XMLD spectra are in reasonable agreement with ab initio calculated spectra. The XMLD anisotropy is shown to be a consequence of the cubic crystal-field split density of $3d$ states, which are selectively probed by transitions from the spin-orbit and exchange-split $2p$ core levels. We furthermore investigate computationally the influences of the spin-orbit interaction of the $\text{Fe}\text{ }3d$ valence electrons as well as of stress in the Fe films. The $\text{Fe}\text{ }3d$ spin-orbit interaction only leads to small modifications of the XMLD spectra, whereas tetragonal deformation of the films can cause a larger modification of the spectra.

Simulation of magnetic circular dichroism in the electron microscope

As electron energy-loss spectroscopy (EELS) and x-ray absorption spectroscopy (XAS) probe the same transitions from core–shell states to unoccupied states above the Fermi energy, it should always be possible to apply the two techniques to the same physical phenomena, such as magnetic dichroism, and obtain the same information. Indeed, the similarity in the expression of the electron and x-ray cross-sections had been already exploited to prove the equivalence of x-ray magnetic linear dichroism and anisotropy in EELS, by noting that the polarization vector of a photon plays the same role as the momentum transfer in electron scattering. Recently, the same was proven true for x-ray magnetic circular dichroism (XMCD) by establishing a new TEM technique called EMCD (electron energy-loss magnetic chiral dichroism) (Schattschneider P et al 2006 Nature 441 486–8), which makes use of special electron scattering conditions to force the absorption of a circularly polarized virtual photon.The intrinsic advantage of EMCD over XMCD is the high spatial resolution of electron microscopes, which are readily available. Among the particular obstacles in EMCD that do not exist for synchrotron radiation, is the notoriously low signal and the very particular scattering conditions necessary to observe a chiral dichroic signal. In spite of that, impressive progress has been made in recent years. The signal strength could be considerably increased, and some innovations such as using a convergent beam have been introduced. EMCD has evolved into several techniques, which make full use of the versatility of the TEM and energy filtering, spectroscopy or STEM conditions (Rubino S 2007 Magnetic circular dichroism in the transmission electron microscope PhD Thesis Vienna University of Technology, Vienna, Austria).

Electronic and magnetic structure ofGaxFe1−xthin films

The electronic as well as magnetic properties of ${\text{Ga}}_{x}{\text{Fe}}_{1\ensuremath{-}x}$ films were studied by soft x-ray measurements. Using x-ray magnetic circular dichroism the Fe majority-spin band was found to be completely filled for $x\ensuremath{\approx}0.3$. With further enhanced Ga content, the Fe moment as well as the angular dependence of the x-ray magnetic linear dichroism decrease strongly, which we attribute to the formation of $\text{D}{0}_{3}$ precipitates. Moreover, the magnetocrystalline anisotropy drops significantly.

Antiferromagnetic Domain Reconfiguration in Embedded LaFeO3 Thin Film Nanostructures

Using photoemission electron microscopy in combination with X-ray magnetic linear dichroism, we report reconfiguration upon nanostructuring of the antiferromagnetic domain structure in epitaxial LaFeO3 thin films. Antiferromagnetic (AFM) nanoislands were synthesized using a dedicated process, devised to define nanostructures with magnetic order embedded in a paramagnetic matrix. Significant impact on the AFM domain configuration was observed. Extended domains were found to form along edges parallel to the in-plane <100> crystalline axes of the cubic substrate, with their AFM spin axis parallel to the edge. No such edge-imposed domain configuration was found for nanoislands defined with the edges at 45° with the in-plane crystalline axes. Epitaxial constraints on the film crystalline structure appear to play an important role in the formation of the edge-bound extended AFM domains. The data indicate a magnetostatic origin of this domain reconfiguration.

Rotatable magnetic anisotropy of CoO/Fe/Ag(001) in ultrathin regime of the CoO layer

CoO/Fe thin films were grown epitaxially onto vicinal Ag(001) and investigated using magneto-optic Kerr effect, x-ray magnetic circular dichroism (XMCD), and x-ray magnetic linear dichroism (XMLD) techniques. We show that the CoO film in the ultrathin regime does not induce a uniaxial magnetic anisotropy but a coercivity enhancement. This result provides a mechanism for the microscopic origin of the rotatable magnetic anisotropy. XMLD measurement further reveals that the underlying mechanism is that the CoO spins are totally rotatable in the ultrathin regime to follow the Fe magnetization.

Direct Measurement of Rotatable and Frozen CoO Spins in Exchange Bias System ofCoO/Fe/Ag(001)

The exchange bias of epitaxially grown CoO/Fe/Ag(001) was investigated using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism (XMLD) techniques. A direct XMLD measurement on the CoO layer during the Fe magnetization reversal shows that the CoO compensated spins are rotatable at thinner thickness and frozen at larger thickness. By a quantitative determination of the rotatable and frozen CoO spins as a function of the CoO film thickness, we find the remarkable result that the exchange bias is well established before frozen spins are detectable in the CoO film. We further show that the rotatable and frozen CoO spins are uniformly distributed in the CoO film.