X-ray Magnetic Linear Dichroism Spectra Research Articles

Antiferromagnetism of CoO-NiO bilayers studied by XMLD spectroscopy

CoO-NiO epitaxial bilayer system grown on MgO(001) substrate is investigated using x-ray magnetic linear dichroism (XMLD) spectroscopy with varying CoO overlayer thickness. An analysis of the Ni L 2 edge XMLD spectra using anisotropic XMLD formulation within a two-domain model reveals that the Ni moments undergo a spin reorientation with increasing CoO thickness. Such a spin reorientation is attributed to the competing magnetic interactions at both the NiO film interfaces, suggesting the existence of a sharp horizontal domain wall separating the in-plane and out-of-plane NiO domains. Our study also demonstrates a possible way to investigate the spin-structure along the thickness within the same chemical structure using a model-based approach, in a noninvasive manner.

Anisotropic Charge Distribution Induced by Spin Polarization in La0.6Sr0.4MnO3 Thin Films Studied by X-ray Magnetic Linear Dichroism

Magnetic anisotropy of epitaxially grown thin films is affected by the strain from the substrates due to a combined effect of distorted electronic structure and spin-orbit interaction (SOI). As an inverse process, one expects an anisotropy of the electronic structure induced by magnetization in the presence of SOI. We have studied the charge-density anisotropy induced by magnetization in thin films of the ferromagnetic metal La$_{1-x}$Sr$_{x}$MnO$_3$ via x-ray magnetic linear dichroism (XMLD). XMLD measurements on thin films with various thicknesses have shown that the XMLD intensity is proportional to the square of the ferromagnetic moment. Using the XMLD sum rule and cluster-model calculation, it has been shown that more Mn 3$d$ electrons are distributed in orbitals elongated along the direction parallel to the spin polarization than in orbitals elongated in the direction perpendicular to it. The cluster-model calculation has shown that the effect of tensile strain from the SrTiO$_3$ substrate on the XMLD spectra is also consistent with the observed XMLD spectral line shapes.

Control of antiferromagnetic spin axis orientation in bilayer Fe/CuMnAs films

Using x-ray magnetic circular and linear dichroism techniques, we demonstrate a collinear exchange coupling between an epitaxial antiferromagnet, tetragonal CuMnAs, and an Fe surface layer. A small uncompensated Mn magnetic moment is observed which is antiparallel to the Fe magnetization. The staggered magnetization of the 5 nm thick CuMnAs layer is rotatable under small magnetic fields, due to the interlayer exchange coupling. This allows us to obtain the x-ray magnetic linear dichroism spectra for different crystalline orientations of CuMnAs in the (001) plane. This is a key parameter for enabling the understanding of domain structures in CuMnAs imaged using x-ray magnetic linear dichroism microscopy techniques.

Evidence for asymmetric rotation of spins in antiferromagnetic exchange-spring

We demonstrate an asymmetric rotation of the antiferromagnetic (AFM) spins in the exchange-spring driven by perpendicularly magnetized Co/Pt. The static and dynamic behaviors of the twisted spin structure are directly revealed by a combination of element specific soft-x-ray absorption spectra and magnetoresistance measurements. X-ray magnetic linear dichroism spectra as a function of AFM thickness clarify the features of the whole exchange-spring, while the interfacial uncompensated spins are identified by the x-ray magnetic circular dichroism spectra. Moreover, the observed tunneling anisotropic magnetoresistance (TAMR) in AFM-based junctions based on this asymmetric rotation provides an electrical approach to monitoring the dynamic twist of the AFM spins. These investigations not only provide a deep insight into the spin structure of the exchange coupling layers but would also advance the development of AFM spintronics.

Detection of the Magnetocrystalline Anisotropy in X-Ray Magnetic Linear Dichroism Reflection Spectra Across the Fe 3p and 2p Edges

The magnetocrystalline anisotropy of X-ray magnetic linear dichroism (XMLD) reflection spectra measured on single-crystalline bcc Fe films across the 3p and 2p edges are presented. The XMLD spectra were obtained from a series of reflection spectra by aligning the electric field vector of linearly polarized undulator radiation with respect to the crystal axes. Our results show the presence of a huge magnetocrystalline anisotropy in the XMLD reflection spectra. The XMLD signal is further investigated as a function of the Fe film thickness in Au/Fe/Ag/GaAs layered systems. Simulations of the reflection spectra reveal the influences of interference effects, which can enhance or diminish the XMLD signals. The measured spectra are in good agreement with ab initio calculated spectra.

Magnetocrystalline anisotropy in x-ray magnetic linear dichroism at the 3pedges of crystalline Fe thin films

X-ray magnetic linear dichroism spectra measured in reflection (XMLD-R) on crystalline bcc Fe thin films across the 3$p$ absorption edges are reported. A series of measurements with varying orientation of the electric field vector of the linear polarized synchrotron radiation with respect to the crystal axes reveals a strong magnetocrystalline anisotropy in the XMLD-R spectra. The spectra agree well with theoretical spectra calculated within the framework of the density-functional theory accounting for the spin-orbital and exchange splitting of the $3p$ semicore states on an equal footing.

Influence of the crystal structure of thin Co films on X-ray magnetic linear dichroism—Comparison of ab initio theory and reflectometry experiments

We report an investigation of the influence of the crystal structure of Co thin films on the X-ray magnetic linear dichroism (XMLD) spectrum. We compare XMLD spectra measured in reflection at the 3p-edges for two distinct orientations of the magnetization in the crystalline Co film with ab initio calculated spectra. The latter was computed for the face-centered cubic as well as the hexagonal-close packed crystal structures of Co. We find that the XMLD signal is strongly dependent on the magnetization direction with respect to the crystal axes as well as strongly influenced by the crystal structure.

X-Ray Magnetic Linear Dichroism of Co35Fe65 Alloy Films Annealed with and without Oxygen Gas

X-ray magnetic linear dichroism (XMLD) experiments were carried out at beam line 3W1B of the Beijing Synchrotron Radiation Facility. The results demonstrate the experimental XMLD spectra of Co35Fe65 alloy film annealed with and without oxygen gas, in agreement with the ligand field multiplet calculation. It can be concluded that cobalt atoms are more difficult to oxidize than iron atoms. The oxygen atoms inserting into the sample surface interval lead to the lattice distortion and the fine tuning of the occupation number of absorption atom d-orbits.

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.

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.

Effects of thermal annealing in oxygen on the antiferromagnetic order and domain structure of epitaxial LaFeO 3 thin films

The antiferromagnetic order and domain structure of LaFeO 3 thin films, epitaxially deposited on LaAlO 3(001) substrates, were investigated by X-ray magnetic linear dichroism (XMLD) spectroscopy and spectromicroscopy, using the soft X-ray photoelectron emission microscopy (PEEM) beamline at the Advanced Light Source. In particular, we have examined the effects of thermal annealing in oxygen on the antiferromagnetic domains and spin orientation in the near surface region of such films. The recorded PEEM micrographs show AFM domains for all samples, though the domains are on average larger for the thermally annealed films. XMLD spectra recorded for different angles between the electromagnetic field vector and the film surface normal suggest a characteristic difference in the spin orientation between the as-grown (epitaxially strained) and the thermally annealed (strain relaxed) samples. While temperature-dependent measurements reveal little difference in Néel temperature between an as-grown and a fully relaxed film ( T N=645 K and 610 K, respectively), both are reduced compared to that of bulk LaFeO 3 ( T N=740 K).

Anisotropic x-ray magnetic linear dichroism at theL2,3edges of cubic Fe, Co, and Ni:Abinitiocalculations and model theory

We present an ab initio investigation of the x-ray magnetic linear dichroism (XMLD) in cubic Fe, Co, and Ni. We demonstrate that the XMLD spectra can be understood in terms of a simple model based on the energy derivatives of the $3d$ partial densities of states and the exchange splitting of the core levels, while the valence-band spin-orbit interaction plays only a marginal role for the overall shape of the spectra. Furthermore, our calculations predict the existence of a huge magnetocrystalline anisotropy of the XMLD spectra, which is of the same size as the XMLD spectrum itself. We provide a qualitative explanation of the anisotropy in terms of the above-mentioned model and suggest its experimental use for obtaining more information about the unoccupied d states.

X-ray magnetic linear dichroism in absorption at theLedge of metallic Co, Fe, Cr, and V

It is demonstrated that x-ray magnetic linear dichroism (XMLD) in absorption spectroscopy is a viable technique for element-specific magnetic characterization of metallic thin films and multilayers. XMLD is measured at the Fe, Co, Cr, and V ${L}_{2,3}$ edges, and varies from 1--11% of the edge jump in the absorption coefficient, with a magnitude that scales with the square of the magnetic moment. The XMLD spectra show satisfactory agreement with first principles calculations of this effect.