X-ray Magnetic Circular Dichroism Study Research Articles

Ferromagnetism in Cr doped In2O3

In this work, we deposited 5%Ta–5%Cr–In2O3 film on LaAlO3(001) substrate as an example to investigate the effect of electron doping, oxygen partial pressure and thickness on the ferromagnetism. The ferromagnetism is strongly dependent on oxygen partial pressure. Films deposited under low oxygen partial pressure leads to the enhancement of ferromagnetism. However, the electron doping through Ta dopant has a limit influence on the magnetism, suggesting that the film may not be carrier-mediated. Through the X-ray magnetic circular dichroism study, no magnetic moment can be found in Cr, In and O edge, suggesting that the ferromagnetism may be originated from defects. Magnetoresistance and anomalous Hall effect can be observed at low temperature in the film deposited under low oxygen partial pressure. Hence, these two properties may not be as an indication of intrinsic ferromagnetism.

Magnetic Properties of bcc-Fe(001)/C60 Interfaces for Organic Spintronics

The magnetic structure of the interfaces between organic semiconductors and ferromagnetic contacts plays a key role in the spin injection and extraction processes in organic spintronic devices. We present a combined computational (density functional theory) and experimental (X-ray magnetic circular dichroism) study on the magnetic properties of interfaces between bcc-Fe(001) and C(60) molecules. C(60) is an interesting candidate for application in organic spintronics due to the absence of hydrogen atoms and the associated hyperfine fields. Adsorption of C(60) on Fe(001) reduces the magnetic moments on the top Fe layers by ∼6%, while inducing an antiparrallel magnetic moment of ∼-0.2 μ(B) on C(60). Adsorption of C(60) on a model ferromagnetic substrate consisting of three Fe monolayers on W(001) leads to a different structure but to very similar interface magnetic properties.

Impact of the CoFe microstructure etched surface oxidation on Co spin and orbital moment

Patterned ferromagnetic thin film shows promising applications in ultra-high density magnetic storage, magnetoresistive transducer, magnetic random access memory and many other devices. Since the performance of these devices is closely associated with the magnetic properties of the etched patterns, it is necessary to study the effects of freshly etched surface oxidation on the magnetic properties of the patterned microstructures. In the current work, were carried out an X-ray Magnetic Circular Dichroism (XMCD) study on a 50 nm Co0.9Fe0.1 continuous thin film and a related patterned Co0.9Fe0.1 grating structure etched with a 2 μm period. Based on the sum rules, the spin and orbital moments were calculated for these two samples, respectively. The results indicated that the spin and orbital moments of grating structure (1.34µB and 0.24µB, respectively) decreased 17.3% compared with the corresponding continuous film (1.62µB and 0.29µB, respectively). We proposed that the moment decreasing of the patterned grating structure was mainly caused by the etched surface oxidation during the pattern manufacture process. The oxidation ratio of Co element in the patterned grating structure is 14.4% calculated from X-ray absorption spectroscopy (XAS) measurement. Considering the oxidation ratio, we amend the spin and orbital moment of Co and the amended result is basically in accordance with that of continuous film, demonstrating that the difference of the spin and orbital moments between the sub-micron grating unit and the continuous film is really caused by the oxidation.

Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator

The breaking of time-reversal symmetry by ferromagnetism is predicted to yield profound changes to the electronic surface states of a topological insulator. Here, we report on a concerted set of structural, magnetic, electrical and spectroscopic measurements of \MBS thin films wherein photoemission and x-ray magnetic circular dichroism studies have recently shown surface ferromagnetism in the temperature range 15 K $\leq T \leq 100$ K, accompanied by a suppressed density of surface states at the Dirac point. Secondary ion mass spectroscopy and scanning tunneling microscopy reveal an inhomogeneous distribution of Mn atoms, with a tendency to segregate towards the sample surface. Magnetometry and anisotropic magnetoresistance measurements are insensitive to the high temperature ferromagnetism seen in surface studies, revealing instead a low temperature ferromagnetic phase at $T \lesssim 5$ K. The absence of both a magneto-optical Kerr effect and anomalous Hall effect suggests that this low temperature ferromagnetism is unlikely to be a homogeneous bulk phase but likely originates in nanoscale near-surface regions of the bulk where magnetic atoms segregate during sample growth. Although the samples are not ideal, with both bulk and surface contributions to electron transport, we measure a magnetoconductance whose behavior is qualitatively consistent with predictions that the opening of a gap in the Dirac spectrum drives quantum corrections to the conductance in topological insulators from the symplectic to the orthogonal class.

Effects of off-stoichiometry on the spin polarization at the Co2MnβGe0.38/MgO interfaces: X-ray magnetic circular dichroism study

We have studied the electronic and magnetic states of Mn and Co atoms at the Co${}_{2}$Mn${}_{\ensuremath{\beta}}$Ge${}_{0}.38$ (CMG)/MgO ($\ensuremath{\beta}=0.67$--1.8) interfaces, which constitute CMG/MgO/CMG magnetic tunnel junctions, by means of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroisms (XMCDs) at the Mn and Co ${L}_{2},3$ edges and compared the deduced Co and Mn magnetic moments by the density-functional calculations with the coherent potential approximation (CPA). As the Mn content $\ensuremath{\beta}$ increased, the intensity of Mn ${L}_{2},3$ XMCD decreased, in agreement with the result of the CPA calculation, which showed the resultant reduction of charges and spin moment on Mn atoms because of the delocalization of Mn 3$d$ electrons with increasing $\ensuremath{\beta}$. The Co spin magnetic moment for all the samples increased with decreasing $\ensuremath{\beta}$, again consistent with the present calculation and the previous density-functional calculation by Picozzi et al. that the Co${}_{\mathrm{Mn}}$ antisite has a larger magnetic moment than that of Co atoms at the regular sites and reduces the spin polarization at the Fermi level and hence the tunnel magnetoresistance ratio.

Time-resolved x-ray magnetic circular dichroism study of ultrafast demagnetization in a CoPd ferromagnetic film excited by circularly polarized laser pulse

The magnetization dynamics of CoPd films excited by circularly polarized ultrashort laser pulses is studied by time-resolved x-ray magnetic circular dichroism. In those films the ultrafast dynamics measured at the Co-${L}_{3}$ edge is strongly sensitive to the orbital magnetic moment $L$${}_{z}$. The amount of angular momentum transferred by the circularly polarized ultrashort laser pulses to the ferromagnetic films is evaluated to $\ifmmode\pm\else\textpm\fi{}$0.1 $\ensuremath{\hbar}$/atom, which is above the detection limit of the experiment. Despite this, no polarization-dependent difference on the magnetization dynamics could be evidenced. These results are explained by ultrafast electronic relaxation mechanisms of the transferred angular momentum, faster than $\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}100$ fs. This experiment sets the methodology as well as an upper time limit for determination of angular momentum relaxation processes.

X-ray Magnetic Circular Dichroism (XMCD) Study of a Methoxide-Bridged DyIII–CrIII Cluster Obtained by Fluoride Abstraction from cis-[CrIIIF2(phen)2]+

An isostructural series of dinuclear chromium(III)-lanthanide(III) clusters is formed by fluoride abstraction of cis-[CrF2(phen)2](+) by Ln(3+) resulting in LnF3 and methoxide-bridged Cr-Ln clusters (Ln = Nd (1), Tb (2), Dy (3)) of formula [Cr(III)(phen)2(μ-MeO)2Ln(NO3)4]·xMeOH (x = 2-2.73). In contrast to fluoride, methoxide bridges in a nonlinear fashion, which facilitates chelation. For 3, X-ray magnetic circular dichroism (XMCD) provides element-specific magnetization curves that are compared to cluster magnetization and susceptibility data acquired by SQUID magnetometry. The combination of XMCD and SQUID is able to resolve very small magnetic coupling values and reveals a weak Cr(III)-Dy(III) coupling of j = -0.04(3) cm(-1). The Dy(III) ion has a ground-state Kramers doublet of mJ = ±13/2, and the first excited doublet is found to be mJ = ±11/2 at an energy of δ = 57(21) cm(-1). The Cr(III) ion exhibits a uniaxial anisotropy of DCr = -1.7(1.0) cm(-1). Further, we observe that a weak anisotropic coupling of dipolar origin is sufficient to model the data, suggesting that methoxide bridges do not play a significant role in the magnetic coupling for the present systems.

Interplay between magnetism and chemical structure at spinel-spinel interfaces

By utilizing a graded wedge sample geometry in combination with surface sensitive soft x-ray techniques, we explain the enhanced magnetic properties observed at the interface between two dissimilar magnetic spinel oxide thin films in terms of the chemical and magnetic character of the constituent cations. Through x-ray absorption spectroscopy and magnetic circular dichroism studies, we find that the interfacial cations exhibit chemical valences and site-occupancies which differ remarkably from the bulk of either film. This results in enhanced magnetic properties localized to the interface region. While this phenomena likely arises from cation interdiffusion of 1–2 nm near the interface due to the open spinel crystal structure, this dramatic change in the magnetic properties localized to a thin interface region may provide a route to obtaining isolated interfacial properties in other spinel-structured heterostructures.

XMCD Proof of Ferromagnetic Behavior in ZnO Nanoparticles

The combined element-specific X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) study of ZnO nanoparticles (NPs) capped with organic molecules confirms the occurrence of intrinsic ferromagnetic-like (FML) behavior up to room temperature (HTFM). Zn K-edge XMCD measurements reveal the coexistence of two different magnetic contributions: a paramagnetic response from the core of the NP, and a ferromagnetic-like contribution stemming from the interface formed between the ZnO core of the NP and the organic molecule. The extent and conformation of this interface depends on the capping molecule, and as demonstrated in ZnO/ZnS heterostructures, the FML behavior is reinforced when neat interfaces are formed.

<i>In Situ</i> Observation of Magnetic Anisotropy Energy of Alternately Layered FeNi Thin Films

Magnetic anisotropy of alternately layered FeNi thin films grown on Ni(3--21 ML)/Cu(001) is investigated by means of the X-ray magnetic circular dichroism (XMCD). Perpendicular magnetic anisotropy is observed when the substrate Ni is thicker than 15 and 20 ML for 4 and 6 ML FeNi films, respectively, while in-plane magnetic anisotropy appears at the thinner substrate Ni region. By comparing with the magnetic phase diagram calculated from the magnetic anisotropy energies, which are obtained by the XMCD study [M. Sakamaki and K. Amemiya, Appl. Phys. Express 4, 073002 (2011)], we show that the phase boundary of the present samples shifts so as to prefer the in-plane magnetic anisotropy. This might be caused by the structural relaxation in the substrate Ni. [DOI: 10.1380/ejssnt.2012.97]

X-Ray Absorption Spectroscopy and X-Ray Magnetic Circular Dichroism Studies of Transition-Metal-Codoped ZnO Nano-Particles

We report on x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies of the paramagnetic (Mn,Co)-co-doped ZnO and ferromagnetic (Fe,Co)-co-doped ZnO nano-particles. Both the surface-sensitive total-electron-yield mode and the bulk-sensitive total-fluorescence-yield mode have been employed to extract the valence and spin states of the surface and inner core regions of the nano-particles. XAS spectra reveal that significant part of the doped Mn and Co atoms are found in the trivalent and tetravalent state in particular in the surface region while majority of Fe atoms are found in the trivalent state both in the inner core region and surface region. The XMCD spectra show that the Fe$^{3+}$ ions in the surface region give rise to the ferromagnetism while both the Co and Mn ions in the surface region show only paramagnetic behaviors. The transition-metal atoms in the inner core region do not show magnetic signals, meaning that they are antiferromagnetically coupled. The present result combined with the previous results on transition-metal-doped ZnO nano-particles and nano-wires suggest that doped holes, probably due to Zn vacancy formation at the surfaces of the nano-particles and nano-wires, rather than doped electrons are involved in the occurrence of ferromagnetism in these systems.

X-ray magnetic circular dichroism study of the blocking process in nanostructured iron-iron oxide core-shell systems

Experimental evidence for magnetothermal behavior in iron-iron oxide nanostructured systems has been obtained using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe ${L}_{2,3}$ edges. The purpose of this study is the determination of the blocked state in these spin-glass-like core-shell systems. A first overview of the magnetic species participating in the magnetic response was obtained by analyzing the XMCD at saturating fields. Also, the XAS revealed the existence of an antiferromagnetic FeO phase, likely located at the interface regions. Finally, measurements were performed at low temperature and intermediate field, where a frozen state below the blocking energy is observed. The results show that the oxide phase spins are oriented at low temperature, while the magnetic spins of the metallic core do not contribute to the XMCD, suggesting that the blocking process mainly involves the magnetic particle superspins rather than the oxide coverage phase.

Hybridization-induced oscillatory magnetic polarization of C60 orbitals at the C60/Fe(001) interface

We have studied the electronic and magnetic properties of the interface between C60 molecules and a Fe(001) surface. X-ray absorption spectroscopy and x-ray magnetic circular dichroism studies of C60 monolayers on Fe(001) surfaces show that hybridization between the frontier orbitals of C60 and continuum states of Fe leads to a significant magnetic polarization of C60 π∗-derived orbitals. The magnitude and also the sign of this polarization were found to depend markedly on the excitation energy. These observations underline the importance of tailoring the interfacial spin polarization at the Fermi level of ferromagnet/organic semiconductor interfaces for applications in organic spintronics.

Magnetic state of EuN: X-ray magnetic circular dichroism at the EuM4,5andL2,3absorption edges

We report ${M}_{4,5}$- and ${L}_{2,3}$-edge x-ray magnetic circular dichroism (XMCD) studies of the ion-specific magnetic polarization on Eu in europium mononitride. The absorption spectra at both edges show a main signal originating from Eu${}^{3+}$ with a well-separated small ($~2%$) contribution from a Eu${}^{2+}$ valence state that we propose is associated with a small concentration of N vacancies. Magnetic polarization is observed in both $4f$ and $5d$ orbitals and for both valence states. The temperature and field dependence of the ${M}_{4,5}$-edge XMCD signal shows a van Vleck signature from Eu${}^{3+}$ (main EuN contribution) and a Brillouin paramagnetic signal from the small concentration of Eu${}^{2+}$ ions. In addition, the ${L}_{2,3}$ edge shows that the Eu${}^{3+}$ ions have $5d$ orbitals that are polarized both by intra-ion $f\ensuremath{-}d$ exchange and by inter-ionic $d$-$d$ exchange from neighboring Eu${}^{2+}$ ions. There is no evidence of magnetic order among the Eu ions at temperatures as low as 10 K.

ODE: a new beam line for high-pressure XAS and XMCD studies at SOLEIL

X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) under pressure are probes of local order and microscopic magnetic properties. XMCD is a selective probe that has access to a large variety of elements. The dispersive extended X-ray absorption fine structure (EXAFS) station at SOLEIL (ODE beam line) provides the possibility to perform numerous pressure XAS and XMCD experiments with an excellent statistic. The main advantages of dispersive XAFS are the focusing optics, the short acquisition time (few μs) and great stability during the measurements due to the absence of any mechanical movement. These advantages allow the study of small samples, 70 μm at SOLEIL, which is mandatory in the case of high-pressure studies. We present the new ODE beam line at SOLEIL and its first high-pressure XMCD results.

Investigation on the Magnetic Anomaly and the Role of Orbital Moment on the Magnetic Properties of LaMn0.5Co0.5O3

Two distinct ferromagnetic phases are present in LaMn0.5Co0.5O3 for which the spin-only magnetic moment calculated from the high temperature dc susceptibility is found to be unusually high. Such a high moment can only be accounted for by assigning the valence state of the cations to Mn2+-Co4+. This is unrealistic as the earlier report based on X-ray absorption spectroscopy (XAS) has suggested the valence state to be mainly Mn4+-Co2+ with traces of Co3+. Also from our studies using XAS, it is found that the valence state is mainly Mn4+-Co2+. In addition, no notable difference is observed in the minor Co3+ present in both phases. Our results based on X-ray magnetic circular dichroism studies (XMCD) reveal the presence of ``distinct'' high orbital moment associated with Co2+ for both phases. Thus it is found that the distinctness of the orbital moment also plays a vital role in determining the magnetic moment and T-c of both phases of LaMn0.5Co0.5O3. By considering the orbital moment obtained from XMCD, the anomaly in the paramagnetic susceptibility is resolved and thus we are able to assign the valence state to Mn4+-Co2+ configuration. The difference in the magnitude of orbital moment in both phases is believed to be due to the crystal field effects.

Spin Structure of Surface-Supported Single-Molecule Magnets from Isomorphous Replacement and X-ray Magnetic Circular Dichroism

Surface-supported arrays of Fe(4)-type Single-Molecule Magnets retain a memory effect and are of current interest in the frame of molecule-based information storage and spintronics. To reveal the spin structure of [Fe(4)(L)(2)(dpm)(6)] (1) on Au, an isomorphous compound [Fe(3)Cr(L)(2)(dpm)(6)] was synthesized and structurally and magnetically characterized (H(3)L is tripodal ligand 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol and Hdpm is dipivaloylmethane). The new complex contains a central Cr(3+) ion and has a S = 6 ground state as opposed to S = 5 in 1. Low-temperature X-ray Magnetic Circular Dichroism studies at Fe- and Cr-L(2,3) edges revealed that the antiparallel alignment between Fe and Cr spins is preserved on surfaces. Moreover, the different Fe-L(2,3) spectral features found in the homo- and heterometallic species disclose the opposing contribution of the central Fe(3+) ion in the former compound, proving that its ferrimagnetic spin structure is retained on surfaces.

Magnetic States in 3d Transition Metal Hydrides Probed by X-ray Magnetic Circular Dichroism

X-ray magnetic circular dichroism (XMCD) study of 3d transition metal hydrides FeH, CoH, and NiH is presented. The hydrogen effects on the electronic structure and magnetic states have been investigated. This study reveals that ferromagnetic Ni transforms to nonmagnetic NiH, whereas FeH and CoH remain ferromagnetic after the hydrogenation. The XMCD profiles indicate that the electronic states of 3d electron near the Fermi energy are modified as a result of the hydrogenation. In this article, the stability of the ferromagnetic state in FeH and CoH is also discussed.

XAS and XMCD studies of amorphous FeCo-based ribbons

This paper reports x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) studies at Fe L 3,2 and Co L 3,2-edges to investigate the electronic structure of (Fe 100 − x Co x) 78Si 9Nb 3B 9Cu 1 (0 < x < 95) alloys. The influence of controlled Co addition on electronic structural and magnetic properties of (Fe 100 − x Co x) 78Si 9Nb 3B 9Cu 1 (x = 0, 20, 40, and 60) alloys has been investigated and it has been observed that Co exists as Co 2+/Co 3+, while Fe exists as a mixture of Fe 0 (metallic) and Fe 2+. The XMCD studies confirm these results and reveal that Co-ions are responsible for the room temperature ferromagnetism (RTFM) in the system, while at Fe L 3,2-edge it shows a diamagnetic behavior.

Absence of abrupt pressure-induced magnetic transitions in magnetite

The structural properties of magnetite ${\text{Fe}}_{3}{\text{O}}_{4}$ under pressure have been extensively studied and are well established. This is not the case for its electronic and magnetic properties. We report a high-pressure x-ray magnetic circular dichroism study of magnetite up to 41 GPa. We observe a continuous evolution of the magnetic moment in the whole pressure range, in agreement with a band-structure description of the electronic state of magnetite. We highlight distinct regimes of magnetism. The x-ray absorption spectra do not suggest any major modification in the local structure in this pressure range.