X-ray Magnetic Linear Dichroism Research Articles

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.

Effect of NiO spin orientation on the magnetic anisotropy of the Fe film in epitaxially grown Fe/NiO/Ag(001) and Fe/NiO/MgO(001)

Single-crystalline Fe/NiO bilayers were epitaxially grown on Ag(001) and on MgO(001), and investigated by low-energy electron-diffraction, magneto-optic Kerr effect, and x-ray magnetic linear dichroism (XMLD). We find that while the Fe film has an in-plane magnetization in both Fe/NiO/Ag(001) and Fe/NiO/MgO(001) systems, the NiO spin orientation changes from in-plane direction in Fe/NiO/Ag(001) to out-of-plane direction in Fe/NiO/MgO(001). These two different NiO spin orientations generate remarkable different effects that the NiO induced magnetic anisotropy in the Fe film is much greater in Fe/NiO/Ag(001) than in Fe/NiO/MgO(001). XMLD measurement shows that the much greater magnetic anisotropy in Fe/NiO/Ag(001) is due to a $90\ifmmode^\circ\else\textdegree\fi{}$ coupling between the in-plane NiO spins and the in-plane Fe spins.

Quadratic X-Ray Magneto-Optical Effect upon Reflection in a Near-Normal-Incidence Configuration at theMEdges of3d-Transition Metals

We have observed a quadratic x-ray magneto-optical effect in near-normal-incidence reflection at the M edges of iron. The effect appears as the magnetically induced rotation of approximately 0.1 degrees of the polarization plane of linearly polarized x-ray radiation upon reflection. A comparison of the measured rotation spectrum with results from x-ray magnetic linear dichroism data demonstrates that this is the first observation of the Schäfer-Hubert effect in the x-ray regime. Ab initio density-functional theory calculations reveal that hybridization effects of the 3p core states necessarily need to be considered when interpreting experimental data. The discovered magneto-x-ray effect holds promise for future ultrafast and element-selective studies of ferromagnetic as well as antiferromagnetic materials.

X-ray magnetic dichroism in III-V diluted magnetic semiconductors: First-principles calculations

The electronic structure of the (Ga,Mn)As, (Ga,Mn)N, and (Ga,Gd)N, diluted magnetic semiconductors (DMSs) were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as the $\text{LSDA}+U$ method. The x-ray magnetic circular and linear dichroism (XMCD and XMLD) spectra at the Mn, As, Ga, and $\text{N}\text{ }K$ and Gd, Mn, As, and $\text{Ga}\text{ }{L}_{2,3}$ edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The effect of interstitial Mn atoms was found to be crucial for the x-ray magnetic dichroism at the Mn and $\text{As}\text{ }K$ and ${L}_{2,3}$ edges in the (Ga,Mn)As DMS. The influence of the exchange splitting and spin-orbit coupling strength on each of the constituent atoms were furthermore analyzed. The orientation dependence of the XMCD and XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges in the (Ga,Mn)As DMS was investigated by calculating the XMCD and XMLD spectra for the $⟨001⟩$ and $⟨110⟩$ magnetization axis. We found a quite small anisotropy in the XMCD and a giant anisotropy in the XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges. The exchange splitting of the initial $\text{Mn}\text{ }2p$ core level was found to be responsible for huge magnetocrystalline anisotropy of the XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges. The calculated results are compared with available experimental data.

Complete Assignment of Spin Domains in Antiferromagnetic NiO(100) by Photoemission Electron Microscopy and Cluster Model Calculation

Spin domains in bulk NiO have been observed by photoemission electron microscopy combined with X-ray magnetic linear dichroism (XMLD) using s - and p -polarized light. The azimuthal angle dependence of domain contrast at Ni L 2 -edge XMLD has been analyzed with a NiO 6 cluster model taking into account crystal symmetry and multiplet splitting. The complete assignment of spin domains has been achieved. The results evoke the reconsideration of the domain assignment and XMLD studies based on the existing atomic multiplet theory.

Element selective X-ray magnetic circular and linear dichroisms in ferrimagnetic yttrium iron garnet films

X-ray magnetic circular dichroism (XMCD) was used to probe the existence of induced magnetic moments in yttrium iron garnet (YIG) films in which yttrium is partly substituted with lanthanum, lutetium or bismuth. Spin polarization of the 4d states of yttrium and of the 5d states of lanthanum or lutetium was clearly demonstrated. Angular momentum resolved d-DOS of yttrium and lanthanun was shown to be split by the crystal field, the two resolved substructures having opposite magnetic polarization. The existence of a weak orbital moment involving the 6p states of bismuth was definitely established with the detection of a small XMCD signal at the Bi M 1 -edge. Difference spectra also enhanced the visibility of subtle changes in the Fe K-edge XMCD spectra of YIG and {Y, Bi}IG films. Weak natural X-ray linear dichroism signatures were systematically observed with all iron garnet films and with a bulk YIG single crystal cut parallel to the (1 1 1) plane: this proved that, at room temperature, the crystal cannot satisfy all requirements of perfect cubic symmetry (space group: Ia 3 ¯ d ), crystal distortions preserving at best trigonal symmetry ( R 3 ¯ or R 3 m ). For the first time, a very weak X-ray magnetic linear dichroism (XMLD) was also measured in the iron K-edge pre-peak of YIG and revealed the presence of a tiny electric quadrupole moment in the ground-state charge distribution of iron atoms. Band-structure calculations carried out with fully relativistic LMTO-LSDA methods support our interpretation that ferrimagnetically coupled spins at the iron sites induce a spin polarization of the yttrium d-DOS and reproduce the observed crystal field splitting of the XMCD signal.

Element-specific study of the anomalous magnetic interlayer coupling across NiO spacer layer in Co/NiO/Fe/Ag(001) using XMCD and XMLD

Co/NiO/Fe trilayers are grown on Ag(001) substrate using molecular-beam epitaxy and investigated by element-specific magnetic domain images using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism techniques. By comparing the Co, Fe, and NiO magnetic domain images, we identify that the anomalous Co-Fe interlayer coupling from a $90\ifmmode^\circ\else\textdegree\fi{}$ coupling to a collinear coupling with increasing the NiO film thickness is due to a transition from a collinear to $90\ifmmode^\circ\else\textdegree\fi{}$ coupling at the NiO/Fe interface while retaining a $90\ifmmode^\circ\else\textdegree\fi{}$ coupling at the Co/NiO interface. Uncompensated Ni spins are found at the Co/NiO interface but are absent at the NiO/Fe interface. No evidence of spiral NiO spin structure is found in this Co/NiO/Fe sandwich.

Magnetic domain coupling study in single-crystalline Fe/CoO bilayers

We report on a study of the magnetic domain coupling in epitaxial wedge-shaped Fe layersdeposited onto CoO/Ag(001). By using photoelectron emission microscopy (PEEM) incombination with x-ray magnetic circular and linear dichroism (XMCD, XMLD), weimaged the ferromagnetic and antiferromagnetic domains present in the Fe and CoO layers,respectively, below the CoO magnetic ordering temperature. The uncompensated Co spinsat the Fe/CoO interface were revealed by XMCD–PEEM and were found to be coupledparallel to the magnetization of the Fe layer. An increase of the CoO XMLD contrast isvisible for Fe thicknesses below 2 ML, where the Fe layer lacks magnetic long-rangeorder.

Anisotropic X-ray magnetic linear dichroism

In this Chapter, the reader will be able to gain a deeper understanding in the angular dependence of x-ray absorption (XA) and x-ray magnetic linear dichroism (XMLD). The scattering tensor provides a general framework for the XA intensity, which is a function of the relative orientation of the magnetization M, the x-ray polarization E and the crystalline axes. In cubic crystal field, the XMLD is a linear combination of two independent spectra, which means that the maximum of the XMLD will be photon energy dependent and no longer at E ∥ M. The angular dependence of the XMLD can be separated into an isotropic part, which rotates with M, and an anisotropic part, which depends on the orientation of the crystal axes. At the Ni2+ L2 edge, where the isotropic part vanishes, the angular dependence is fully determined by the anisotropic part, which obtains maximal intensity when E and M have equal but opposite angles with respect to the [100] direction. In the past this complication has not been appreciated, leading in confusion and misinterpretation in the literature.

Contribution of Eu 4f states to the magnetic anisotropy of EuO

Anisotropic x-ray magnetic linear dichroism (AXMLD) provides a novel element-, site-, shell-, and symmetry-selective techniques to study the magnetic anisotropy induced by a crystalline electric field. The weak Eu2+ M4,5 AXMLD observed in EuO(001) indicates that the Eu 4f states are not rotationally invariant and hence contribute weakly to the magnetic anisotropy of EuO. The results are contrasted with those obtained for 3d transition metal oxides.

Evidence of local moment formation in Co-based Heusler alloys

In this work we use a combination of x-ray magnetic circular and linear dichroism (XMCD and XMLD) techniques to examine the formation of local moments in Heusler alloys of the composition Co{sub 2}MnX (where X=Si or Al). The existence of local moments in a half-metallic system is reliant upon the band gap in the minority-spin states. By utilizing the element-specific nature of x-ray techniques we are able to explore the origin of the minority-spin band gap in the partial density of states (PDOS), via the degree of localization of moments on Co and Mn atoms. We observe a crucial difference in the localization of the Co moment when comparing Co{sub 2}MnSi (CMS) and Co{sub 2}MnAl (CMA) films that is consistent with the predicted larger minority-spin gap in the Co PDOS for CMS. These results provide important evidence for the dominant role of the Co minority-spin states in realizing half-metallic ferromagnetism (HMF) in these Heusler alloys. They also demonstrate a direct method for measuring the degree of interfacial HMF in the raw materials without the need for fabricating spin-transport devices.

Magnetic structure near the Co∕NiO(001) interface

We investigate the magnetic coupling at the Co∕NiO interface using soft x-ray magnetic linear dichroism (XMLD) and circular dichroism taking explicitly into account the recently observed angular dependence of the XMLD with respect to the crystallographic axes. We find that the Co moments are aligned perpendicular to the NiO moments. We discuss the impact of the anisotropic XMLD on the intensity ratio of the two peaks at the NiL2 edge, which is commonly employed to determine the spin orientation in antiferromagnets using XMLD.

Impact of interface orientation on magnetic coupling in highly ordered systems: A case study of the low-indexedFe3O4/NiOinterfaces

We investigated the influence of the interface orientation on the magnetic coupling for the (110), (001), and (111) interfaces of epitaxial NiO thin films on single-crystal ${\text{Fe}}_{3}{\text{O}}_{4}$. In our analysis, we combine magnetic x-ray spectroscopies with circularly and linearly polarized light, i.e., x-ray magnetic circular and linear dichroism (XMCD and XMLD). A full treatment of the anisotropic XMLD in single-crystalline materials backed by theoretical support allows for a reliable vectorial magnetometry. Both the (110) and the (111) interfaces produce collinear coupling, which at first glance contradicts Koon's theory, while at the (001) interface, a perpendicular (spin-flop) coupling between a ferrimagnet and an antiferromagnet is observed. Furthermore, we find a pronounced enhancement of the interfacial uncompensated magnetization for the (110) interface only, while for the two other interfaces this value is considerably lower. These apparent discrepancies are resolved in terms of a detailed consideration of exchange and magnetoelastic effects for the individual orientations. The results highlight the fundamental role of the crystalline interface orientation in magnetic coupling phenomena involving oxidic systems.

Weak Anisotropic X-Ray Magnetic Linear Dichroism at the EuM4,5Edges of Ferromagnetic EuO(001): Evidence for4f-State Contributions

We have observed a weak anisotropic x-ray magnetic linear dichroism (AXMLD) at the Eu M4,5 edges of ferromagnetic EuO(001), which indicates that the 4f states are not rotationally invariant. A quantitative agreement of the AXMLD is obtained with multiplet calculations where the 4f state is split by an effective cubic crystalline electrostatic field. The results indicate that the standard model of rare earths, where 4f electrons are treated as core states, is not correct and that the 4f orbitals contribute weakly to the magnetic anisotropy.

Influence of crystal field on anisotropic x-ray magnetic linear dichroism at theCo2+L2,3edges

We expand the previous theoretical treatment for the strong anisotropy of the x-ray magnetic linear dichroism (XMLD) in a crystal field of cubic point-group symmetry to the more general case of tetragonal point-group symmetry. For the cubic symmetry, there are only two fundamental spectra, which have the same shape for rotation of either linear light polarization $\mathbf{E}$ or magnetization direction $\mathbf{H}$. For the tetragonal symmetry, the XMLD is a linear combination of four fundamental spectra, with a different shape for linear dichroism (rotation of $\mathbf{E}$) and magnetic dichroism (rotation of $\mathbf{H}$). However, only one extra spectrum is required to relate the linear and magnetic dichroism. The validity of the theory is demonstrated using a $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}(011)$ thin film on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$, which has both tetrahedrally distorted symmetry and large magnetic anisotropy. The XMLD at the Co ${L}_{2,3}$ edges was found to exhibit a strong dependence on the relative orientation of external magnetic field, x-ray polarization, and crystalline axes. The large variations in the peak structure as a function of angle are not caused by the spin-orbit-induced magnetocrystalline anisotropy but arise from the symmetry of the measurement geometry. The results are compared with calculated spectra using atomic multiplet theory for ${\mathrm{Co}}^{2+}$ ${d}^{7}\ensuremath{\rightarrow}2{p}^{5}{d}^{8}$ in octahedral and tetragonal crystal field symmetry. Although the magnitude of the dichroism is strongly influenced by the temperature, its spectral shape remains largely unaffected. The measured fundamental spectra are also robust against incomplete magnetization. The influence of the tetragonal distortion is revealed by small differences between the linear and magnetic dichroism. It is shown that the magnetic dichroism spectra can be transferred from $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ to CoO. Therefore, the rich structure in the ${\mathrm{Co}}^{2+}$ ${L}_{3}$ XMLD provides a sensitive probe to determine the orientation of the spin axis with respect to the crystalline axes, hence offering a valuable tool for experimentalists for the study of exchange bias in Co oxides. In contrast, the ${\mathrm{Co}}^{2+}$ ${L}_{2}$ edge, where the fundamental spectra have similar spectral shape but with opposite sign, does not allow an unambiguous determination.

Chapter 4 - Magnetic Anisotropy in Transition Metal Systems

Publisher Summary Magnetic anisotropy is because of the electronic exchange forces, which can be quite significant in magnetic materials. Even in the absence of external magnetic fields, these forces are strong enough to align magnetic moments present in a given material. Usually an external field is necessary to get these moments to align along, even in materials that are likely to be strongly ferromagnetic. The reason for such behavior is the presence of magnetic domains or subvolumes. Each one of these domains could possess a saturated moment, but different domains may not be aligned with each other. This non-alignment is the cause of unsaturation in typically ferromagnetic materials. An applied (magnetic) field of sufficient strength is necessary to bring such domains to align themselves. The Magnetic Anisotropy Energy (MAE) is the energy associated with the orientation of the magnetic moments in a condensed matter system. For cubic systems, the leading term in anisotropy breaks the lattice (local) symmetry. Magneticanisotropy can be linked with switching the orientation of magnetization of a given magnetic material from its easy axis towards its hard axis. The energy required to change this orientation is identified as the magnetic anisotropy energy. The chapter discusses magnetocrystalline anisotropy (MCA), early work in this field, dipole–dipole interaction and related anisotropy, magnetoelastic anisotropy, perpendicular magnetic anisotropy, x-ray magnetic circular dichroism (XMCD), X-ray magnetic linear dichroism (XMLD) for anisotropy measurements, Mermin–Wagner theorem, spin Hamiltonian, band theoretical treatments, and spin reorientation transitions in multilayers. Examining Hartree–Fock type exchange integrals and nonspherical terms therein shows how orbital polarizations arise. Evaluation of the spin–orbit energy term along various directions of spin quantization in a lattice yields useful information about magnetic anisotropy. There is discussion on the basics of exchange anisotropy and its complexity and its relevance to magnetic data storage.

Magnetic properties of coupled ultrathinNiO∕Fe3O4(001) films

A strong reduction in the magnetic spin and orbital moments as well as large magnetocrystalline anisotropies are observed in ultrathin $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) films compared to ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001). Combining x-ray absorption spectroscopy, x-ray magnetic circular and linear dichroism (XMCD-XMLD), and high-resolution transmission electron microscopy (HRTEM) the magnetic anisotropy of ultrathin $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) films was studied and compared to structural anisotropy at the interface. The evolution of the XMCD signal at the Fe octahedral sites allows a quantitative description of the interface. A simple model is proposed where uncompensated spins are located in a $1.5\text{\ensuremath{-}}\mathrm{nm}$-thick interfacial $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ compound. Angular-dependent measurements of the effective spin and orbital magnetic moments at the Fe ${L}_{2,3}$ edges reveal the presence of a large magnetocrystalline anisotropy, appearing for a particular growth temperature of the films. The magnetocrystalline anisotropy measured at the Fe ${L}_{2,3}$ edges is attributed to the specific anisotropic structure developed at the interface and to crystalline ordering of a strained compound. HRTEM confirms the presence of an ordered anisotropic structure at the $\mathrm{Ni}\mathrm{O}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) interface. The in-plane strained NiO(001) films promote antiferromagnetic order where the spins are oriented parallel to the ⟨100⟩ in-plane direction, collinear to the magnetization direction in the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (001) substrate.

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).

Magnetostructure of MnAs on GaAs revisited

The ferromagnetic to nonferromagnetic (α-β) phase transition in epitaxial MnAs layers on GaAs(100) is studied by x-ray magnetic circular dichroism and x-ray magnetic linear dichroism photoemission electron microscopy in order to elucidate the nature of the controversial nonferromagnetic state of β-MnAs. In the coexistence region of the two phases the β phase shows a clear XMLD signal characteristic of antiferromagnetism. The nature and the possible causes of the elusiveness of this magnetic state are discussed.

Huge magnetocrystalline anisotropy of x-ray linear dichroism observed onCo∕FeMnbilayers

We present an x-ray spectromicroscopic investigation of single-crystalline magnetic $\mathrm{Fe}\mathrm{Mn}∕\mathrm{Co}$ bilayers on Cu(001), using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism (XMLD) at the Co and Fe ${L}_{3}$ absorption edges in combination with photoelectron emission microscopy. Using the magnetic coupling between the ferromagnetic Co layer and the antiferromagnetic FeMn layer, we are able to produce magnetic domains with two different crystallographic orientations of the magnetic easy axis within the same sample at the same time. We find a huge difference in the XMLD contrast between the two types of magnetic domains, which we discuss in terms of intrinsic magnetocrystalline anisotropy of XMLD of the Co layer. We also demonstrate that due to the high sensitivity of the method, the small number of induced ferromagnetic Fe moments at the FeMn-Co interface is sufficient to obtain magnetic contrast from XMLD in a metallic system.