XAS) And Magnetic Circular Dichroism Research Articles

Direct observation of oxygen polarization in Sr2IrO4 by O K -edge x-ray magnetic circular dichroism

X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) measurements at the oxygen (O) $K$ edge were performed to investigate the magnetic polarization of ligand O atoms in the weak ferromagnetic (WFM) phase of the Ir perovskite compound ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$. With the onset of the WFM phase below ${T}_{\mathrm{N}}\ensuremath{\simeq}240$ K, XMCD signals corresponding to XAS peaks respectively identified as originating from the magnetic moments of apical and planar oxygen (${\mathrm{O}}_{\mathrm{A}}$ and ${\mathrm{O}}_{\mathrm{P}}$) in the ${\mathrm{IrO}}_{6}$ octahedra were observed. The observation of magnetic moments at ${\mathrm{O}}_{\mathrm{A}}$ sites is consistent (except for the relative orientation) with that suggested by prior muon spin rotation ($\textmu{}\mathrm{SR}$) experiment in the noncollinear antiferromagnetic (NC-AFM) phase below ${T}_{\mathrm{M}}\ensuremath{\approx}100$ K. Assuming that the ${\mathrm{O}}_{\mathrm{A}}$ magnetic moment observed by $\textmu{}\mathrm{SR}$ is also responsible for the corresponding XMCD signal, the magnetic moment of ${\mathrm{O}}_{\mathrm{P}}$ is estimated to be consistent with the previous $\textmu{}\mathrm{SR}$ result. Since the ${\mathrm{O}}_{\mathrm{A}}$ XMCD signal is mainly contributed by Ir $5d\phantom{\rule{4pt}{0ex}}zx$ and $yz$ orbitals which also hybridize with ${\mathrm{O}}_{\mathrm{P}}$, it is inferred that the relatively large ${\mathrm{O}}_{\mathrm{P}}$ magnetic moment is induced by Ir $5d\phantom{\rule{4pt}{0ex}}xy$ orbitals. Moreover, the inversion of ${\mathrm{O}}_{\mathrm{A}}$ moments relative to Ir moments between the two magnetic phases revealed by XMCD suggests the presence of competing magnetic interactions for ${\mathrm{O}}_{\mathrm{A}}$, with which the ordering of ${\mathrm{O}}_{\mathrm{A}}$ moments in the NC-AFM phase may be suppressed to ${T}_{\mathrm{M}}$.

Element-specific contributions to improved magnetic heating of theranostic CoFe2O4 nanoparticles decorated with Pd

Decoration with Pd clusters increases the magnetic heating ability of cobalt ferrite (CFO) nanoparticles by a factor of two. The origin of this previous finding is unraveled by element-specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) combined with atomic multiplet simulations and density functional theory (DFT) calculations. While the comparison of XAS spectra with atomic multiplet simulations show that the inversion degree is not affected by Pd decoration and, thus, can be excluded as a reason for the improved heating performance, XMCD reveals two interrelated responsible sources: significantly larger Fe and Co magnetic moments verify an increased total magnetization which enhances the magnetic heating ability. This is accompanied by a remarkable change in the field-dependent magnetization particularly for Co ions which exhibit an increased low-field susceptibility and a reduced spin canting behavior in higher magnetic fields. Using DFT calculations, these findings are explained by reduced superexchange between ions on octahedral lattice sites via oxygen in close vicinity of Pd, which reinforces the dominating antiparallel superexchange interaction between ions on octahedral and tetrahedral lattice sites and thus reduces spin canting. The influence of the delocalized nature of Pd 4d electrons on the neighboring ions is discussed and the conclusions are illustrated with spin density isosurfaces of the involved ions. The presented results pave the way to design nanohybrids with tailored electronic structure and magnetic properties.

First principles calculations of intersite disorder influence on the electronic structure and X-ray magnetic circular dichroism in CoFeMnSi Heusler alloy

Electronic structure, X-ray absorption, and magnetic circular dichroism (XMCD) spectra in the CoFeMnSi Heusler alloy were studied from first principles. Fully relativistic Dirac linear muffin-tin orbital band structure method was implemented with various exchange–correlation functionals tested. The supercell approach was used to study the influence of intersite disorder, at the levels of 6.25%, 12.5%, and 25% within transition metal sites, on the XMCD spectra at [Formula: see text] edges and spin polarization (SP) at the Fermi level. It is found that most sensitive to Fe–Mn and Co–Fe disorder are XMCD spectra at [Formula: see text] edges of Fe, while the sensitivity decreases from Mn to Co. It is shown that magnetic moments estimated with the use of magneto-optical (MO) sum rules agree with the ab initio calculated ones to within [Formula: see text], [Formula: see text], and [Formula: see text], for Co, Fe, and Mn, respectively. The calculated SP decreases from 99% for ordered CoFeMnSi alloy, to 96% upon 25% Co–Fe disorder, to 83% for Fe–Mn disorder, and to 42% in the case of Co–Mn disorder. The calculated spectra agree well with the available experimental data. The rich XMCD spectral structures are predicted from first principles at Fe, Co, Mn and Si [Formula: see text] edges.

Direct detection of induced magnetic moment and efficient spin-to-charge conversion in graphene/ferromagnetic structures

This article shows that the spin-to-charge current conversion in single-layer graphene (SLG) by means of the inverse Rashba-Edelstein effect (IREE) is made possible with the integration of this remarkable 2D-material with the unique ferrimagnetic insulator yttrium iron garnet (YIG = $Y_{3}Fe_{5}O_{12}$) as well as with the ferromagnetic metal permalloy (Py = $Ni_{81}Sb_{19}$). By means of X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) techniques, we show that the carbon atoms of the SLG acquires an induced magnetic moment due to the proximity effect with the magnetic layer. The spin currents are generated in the magnetic layer by spin pumping from microwave driven ferromagnetic resonance and are detected by a dc voltage along the graphene layer, at room temperature. The spin-to-charge current conversion, occurring at the graphene layer, is explained by the extrinsic spin-orbit interaction (SOI) induced by the proximity effect with the ferromagnetic layer. The results obtained for the SLG/YIG and SLG/Py systems confirm very similar values for the IREE parameter, which are larger than the values reported in previous studies for SLG. We also report systematic investigations of the electronic and magnetic properties of the SLG/YIG by means of scanning tunneling microscopy (STM).

Electronic structure of Fe and magnetism in the 3d/5d double perovskites Ca2FeReO6 and Ba2FeReO6

The Fe electronic structure and magnetism in (i) monoclinic ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ with a metal-insulator transition at ${T}_{MI}\ensuremath{\sim}140\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and (ii) quasicubic half-metallic ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$ ceramic double perovskites are probed by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). These materials show distinct Fe ${L}_{2,3}$ XAS and XMCD spectra, which are primarily associated with their different average Fe oxidation states (close to ${\mathrm{Fe}}^{3+}$ for ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ and intermediate between ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ for ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$) despite being related by an isoelectronic (${\mathrm{Ca}}^{2+}/{\mathrm{Ba}}^{2+}$) substitution. For ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$, the powder-averaged Fe spin moment along the field direction ($B=5\phantom{\rule{0.28em}{0ex}}\mathrm{T}$), as probed by the XMCD experiment, is strongly reduced in comparison with the spontaneous Fe moment previously obtained by neutron diffraction, consistent with a scenario where the magnetic moments are constrained to remain within an easy plane. For $B=1\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, the unsaturated XMCD signal is reduced below ${T}_{MI}$ consistent with a magnetic transition to an easy-axis state that further reduces the powder-averaged magnetization in the field direction. For ${\mathrm{Ba}}_{2}{\mathrm{FeReO}}_{6}$, the field-aligned Fe spins are larger than for ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ ($B=5\phantom{\rule{0.28em}{0ex}}\mathrm{T}$) and the temperature dependence of the Fe magnetic moment is consistent with the magnetic ordering transition at ${T}_{C}^{Ba}=305\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Our results illustrate the dramatic influence of the specific spin-orbital configuration of Re $5d$ electrons on the Fe $3d$ local magnetism of these Fe/Re double perovskites.

Circular dichroism and angular deviation in x-ray absorption spectra of Dy2ScN@C80 single-molecule magnets on h−BN/Rh(111)

Endohedral fullerenes, such as Dy2ScN@C80, are single-molecule magnets with long relaxation times of their magnetization. An open and anisotropic 4f electron shell in the lanthanides (here Dy) imposes a magnetic moment that maintains its orientation at liquid-helium temperatures for macroscopic times. If these molecules shall be used as single-bit information storage elements or for quantum operations, the orientation of the endohedral units and the orientation of the magnetic moments has to be controlled. X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD)—with variation of the angle of x-ray incidence—allows for the detection of these two structural elements. We present XMCD data of Dy2ScN@C80 on an h−BN/Rh(111) nanomesh that display at 2 K a large hysteresis with a coercive field of 0.4 T. The angular dependence of the XAS data at the Dy M5 edge indicates partial ordering of the endohedral units. In order to quantify anisotropic orientation we introduce the “deviation” D as an operational quantity that measures differences between two spectra.

Magnetoresistance in Hybrid Pt/CoFe2O4 Bilayers Controlled by Competing Spin Accumulation and Interfacial Chemical Reconstruction.

Pure spin currents have potential for usein energy-friendly spintronics. They can be generated by a flow of charge along a nonmagnetic metal with large spin-orbit coupling. This produces a spin accumulation at the surfaces, controllable by the magnetization of an adjacent ferromagnetic layer. Paramagnetic metals typically used are close to ferromagnetic instability and thus magnetic proximity effects can contribute to the observed angular-dependent magnetoresistance (ADMR). As interface phenomena govern the spin conductance across the metal/ferromagnetic-insulator heterostructures, unraveling these distinct contributions is pivotal for a full understanding of spin current conductance. Here, we report X-ray absorption and magnetic circular dichroism (XMCD) at Pt M and (Co, Fe) L absorption edges and atomically resolved energy electron loss spectroscopy (EELS) data of Pt/CoFe2O4 bilayers, where CoFe2O4 layers have been capped by Pt grown at different temperatures. It was found that the ADMR differs dramatically, dominated either by spin Hall magnetoresistance (SMR) associated with the spin Hall effect or by anisotropic magnetoresistance. The XMCD and EELS data indicate that the Pt layer grown at room temperature does not display any magnetic moment, whereas when grown at a higher temperature, it becomes magnetic due to interfacial Pt-(Co, Fe) alloying. These results enable differentiation of spin accumulation from interfacial chemical reconstructions and tailoring of the angular-dependent magnetoresistance.

Soft x-ray absorption spectroscopy study of the electronic structures of the MnFe Prussian blue analogs (RbxBay)Mn[3−(x+2y)]/2[Fe(CN)6]H2O

The electronic structures of Prussian blue analog $({\mathrm{Rb}}_{x}{\mathrm{Ba}}_{y}){\mathrm{Mn}}_{[3\ensuremath{-}(x+2y)]/2}[\mathrm{Fe}{(\mathrm{CN})}_{6}]$ cyanides have been investigated by employing soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the Fe and Mn $L$ $(2p)$ edges. The measured XAS spectra have been analyzed with the configuration-interaction (CI) cluster model calculations. The valence states of the Fe and Mn ions are found to be ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ mixed valent, with an average valency of $v(\mathrm{Fe})\ensuremath{\sim}2.8$ and nearly divalent $({\mathrm{Mn}}^{2+})$, respectively. Our Mn/Fe $2p$ XMCD study supports that ${\mathrm{Mn}}^{2+}$ ions are in the high-spin states while ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}{\mathrm{Fe}}^{3+}$ ions are in the low-spin states. The Fe and Mn $2p$ XAS spectra are found to be essentially the same for $80\ensuremath{\le}T\ensuremath{\le}300$ K, suggesting that a simple charge transfer upon cooling from ${\mathrm{Fe}}^{3+}\text{\ensuremath{-}}\mathrm{CN}\text{\ensuremath{-}}{\mathrm{Mn}}^{2+}$ to ${\mathrm{Fe}}^{2+}\text{\ensuremath{-}}\mathrm{CN}\text{\ensuremath{-}}{\mathrm{Mn}}^{3+}$ does not occur in $({\mathrm{Rb}}_{x}{\mathrm{Ba}}_{y}){\mathrm{Mn}}_{[3\ensuremath{-}(x+2y)]/2}[\mathrm{Fe}{(\mathrm{CN})}_{6}]$. According to the CI cluster model analysis, it is necessary to take into account both the ligand-to-metal charge transfer and the metal-to-ligand charge transfer in describing Fe $2p$ XAS, while the effect of charge transfer is negligible in describing Mn $2p$ XAS. The CI cluster model analysis also shows that the trivalent ${\mathrm{Fe}}^{3+}$ ions have a strong covalent bonding with the $\mathrm{C}\ensuremath{\equiv}\mathrm{N}$ ligands and are under a large crystal-field energy of $10Dq\ensuremath{\sim}3$ eV, in contrast to the weak covalency effect and a small $10Dq\ensuremath{\sim}0.6$ eV for the divalent ${\mathrm{Mn}}^{2+}$ ions.

Intricacies of the Co3+ spin state in Sr2Co0.5Ir0.5O4 : An x-ray absorption and magnetic circular dichroism study

We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-$L_{3,2}$ on the hybrid 3$d$/5$d$ solid state oxide Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. Our data indicate unambiguously a pure high spin state $(S=2)$ for the Co$^{3+}$ (3$d^6$) ions with a significant unquenched orbital moment $L_z/2S_z=0.25$ despite the sizeable elongation of the CoO$_6$ octahedra. Using quantitative model calculations based on parameters consistent with our spectra, we have investigated the stability of this high spin state with respect to the competing low spin and intermediate spin states.

Electronic and magnetic properties of phosphorus across the first-order ferromagnetic transition of(Mn,Fe)2(P,Si,B)giant magnetocaloric materials

To study the evolution of the electronic states and magnetism of the metallic and metalloid atoms across the first-order ferromagnetic transition of (Mn,${\mathrm{Fe})}_{2}$(P,Si,B) compounds, x-ray absorption (XAS) and magnetic circular dichroism (XMCD) spectra were recorded at the $K$ edges of P, Mn, and Fe. Surprisingly, while the Fe and Mn XAS spectra do not show pronounced changes, the $K$-edge XAS of P is strongly modified across the ferromagnetic transition and is a fingerprint of electronic structure changes at the phase transition. The evolution of the $3p$ electronic states of phosphorus is linked to changes in metal-metalloid bonding. The amplitude of this effect is correlated to the discontinuity in interatomic distances at the transition and progressively disappears with boron substitution. Moreover, it is found that the P atoms carry a finite magnetic moment in the ferromagnetic state in accordance with the results of band-structure calculations.

Electronic tuneability of a structurally rigid surface intermetallic and Kondo lattice:CePt5/Pt(111)

We present an extensive study of the structure, composition, electronic, and magnetic properties of Ce-Pt surface intermetallic phases on Pt(111) as a function of their thickness. The sequence of structural phases appearing in low-energy electron diffraction may invariably be attributed to a single underlying intermetallic atomic lattice. Findings from both microscopic and spectroscopic methods prove compatible with ${\mathrm{CePt}}_{5}$ formation when their characteristic probing depth is adequately taken into account. The intermetallic film thickness serves as an effective tuning parameter which brings about characteristic variations of the cerium valence and related properties. Soft x-ray absorption (XAS) and magnetic circular dichroism (XMCD) prove well suited to trace the changing Ce valence and to assess relevant aspects of Kondo physics in the ${\mathrm{CePt}}_{5}$ surface intermetallic. We find characteristic Kondo scales of the order of ${10}^{2}$ K and evidence for considerable magnetic Kondo screening of the local Ce $4f$ moments. ${\mathrm{CePt}}_{5}/\mathrm{Pt}(111)$ and related systems therefore appear to be promising candidates for further studies of low-dimensional Kondo lattices at surfaces.

X-ray absorption spectroscopy and magnetic circular dichroism studies of L1-Mn-Ga thin films

Tetragonally distorted Mn3−xGax thin films with 0.1<x<2 show a strong perpendicular magnetic anisotropy and low magnetization and thus have the potential to serve as electrodes in spin transfer torque magnetic random access memory. Because a direct capping of these films with MgO is problematic due to oxide formation, we examined the influence of a CoFeB interlayer and of two different deposition methods for the MgO barrier on the formation of interfacial Mn-O for Mn62Ga38 by element specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). A highly textured L10 crystal structure of the Mn-Ga films was verified by X-ray diffraction measurements. For samples with e-beam evaporated MgO barrier no evidence for Mn-O was found whereas in samples with magnetron sputtered MgO, Mn-O was detected, even for the thickest interlayer thickness. Both XAS and XMCD measurements showed an increasing interfacial Mn-O amount with decreasing CoFeB interlayer thickness. Additional element specific full hysteresis loops determined an out-of-plane magnetization axis for the Mn and Co, respectively.

High-Magnetic-Field X-ray Absorption and Magnetic Circular Dichroism Spectroscopy in the Mixed-Valent Compound YbAgCu4

The X-ray absorption and magnetic circular dichroism (XMCD) spectra at the L 3 edge of Yb have been studied experimentally as well as theoretically in the mixed-valent heavy fermion compound YbAgCu...

Orbital and spin magnetic moments of ferrocene encapsulated in metallicity sorted single‐walled carbon nanotubes

AbstractThe nature of the electronic and local magnetic properties of ferrocene (FeCp2) filled single‐walled carbon nanotubes (SWCNT) has been investigated by X‐ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). Metallic, semiconducting, and unsorted ferrocene‐filled tubes have been studied in different conditions of temperature and magnetic field. XMCD signal becomes evident with the application of a magnetic field at low temperature. We find that the molecular states of ferrocene interact with SWCNT of different metallicities. A paramagnetic behavior of encapsulated ferrocene is observed from the magnetic field dependent XMCD measurements which is consistent with theoretical predictions.

Field dependence of the interfacial Cu in Cu-coated γ-Fe2O3 nanoparticles

Evidence of elemental copper magnetism in Cu-coated γ-Fe2O3 nanoparticles was revealed by element specific x-ray absorption spectroscopy and magnetic circular dichroism (XMCD). An interfacial layer of magnetic CuO was discovered that interacted with the Fe3+ surface magnetic moments. This unexpected exchange interaction canted the moments of the CuO, so that a non-zero net magnetic moment was measured. XMCD measurements as a function of field on the copper coated γ-Fe2O3 nanoparticles indicated that the CuO magnetic moment was aligned with the octahedral Fe3+ and its magnetization was modified by the changing applied field. With increasing copper thickness, a stronger field dependence of the Fe3+ and Cu2+ magnetization was observed.

X-ray absorption and magnetic circular dichroism of graphene/Ni(111)

We present systematic investigations of the electronic and magnetic properties of the graphene/Ni(111) system by means of x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) at the Ni L2,3 and C K absorption edges. The XAS C 1s→π∗,σ∗ spectra show dramatic changes as the angle, α, between the electrical vector of light and normal of the sample is varied reflecting the symmetry of the final state (σ or π). XMCD spectra reveal an induced magnetic moment of the carbon atoms in the graphene layer. Our experimental results are discussed in the light of previous results on the observation of induced magnetism in nonmagnetic materials.

X-ray Magnetic Circular Dichroism Investigations of the Origin of Room Temperature Ferromagnetism in Fe-Doped ZnO Nanoparticles

We have studied the electronic structure of Zn0.9Fe0.1O nanoparticles, which have been reported to be ferromagnetic at room temperature, by using soft X-ray absorption (XAS) and magnetic circular dichroism (XMCD). The XAS results indicate that Fe ions are predominantly in the Fe3+ state with mixture of a small amount of Fe2+. It is shown that the room temperature ferromagnetism in the Zn0.9Fe0.1O nanoparticles is primarily originated from the antiferromagnetic coupling between unequal amounts of Fe3+ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of ∼2–3 nm.

Martensite Transition and Microscopic Magnetism of Epitaxial Ni2MnGa Films

AbstractA magnetically induced shape memory effect in Ni2MnGa results in huge magnetostrictive effects of several percent. Using x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) we investigated element specific magnetic moments and electronic structure of single crystalline, (110) oriented Ni2MnGa films on a-plane Al2O3 substrates in the austenite and martensite state. The structural phase transition of the samples is evident from temperature dependent x-ray diffraction and magnetization measurements. The Ni XAS differ significantly for temperatures above and below the martensite transition in agreement with published ab-initio calculations. Using XAS in transmission geometry on our thin film samples we observe the corresponding reduction of the absorption feature as predicted by theoretical calculations. The XMCD analysis shows the orbital contribution of the Ni electrons to be responsible for the magnetic anisotropy.

Mn Distribution and Spin Polarization inGa1−xMnxAs

Using the density functional full-potential linearized augmented plane wave approach, the x-ray absorption and magnetic circular dichroism (XMCD) spectra of Ga(1-x)Mn x As are calculated. Significantly, XMCD of Mn is highly sensitive to the change in environment, and thus can be utilized to characterize impurity distribution. The nature of Mn-induced spin polarization on Ga and As sites, vital for the carrier mediated magnetic ordering, is discussed in light of computational and experimental results.

Mn 2p–3d soft X-ray magnetic circular dichroism study of Mn 5Ge 3

We have investigated the Mn 3d electronic structure of Mn 5Ge 3 by means of the Mn 2p–3d soft X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) experiments. The spectra are similar to those of the itinerant ferromagnetic compound MnSb. The integrated intensity of the XMCD spectrum is almost zero, indicating that the orbital magnetic moment is negligible for the magnetism of the Mn 5Ge 3 compounds. The atomic multiplet calculations suggest the 3d 6 configuration for the Mn atoms at two inequivalent sites.