X-ray Magnetic Circular Dichroism Study Research Articles

Magnetic anisotropy of one-dimensional Co nanostructures

We report a combined scanning tunneling microscopy and x-ray magnetic circular dichroism study to investigate the structural properties and magnetic behavior of a Co ultrathin film composed of dimer nanolines. These Co nanolines, \ensuremath{\sim}6 nm in length, are grown on a Si nanotemplate composed of nanoribbons self-organized on Ag(110). The first two Co layers present a weak magnetic response while upper Co layers exhibit an enhanced magnetization. Orbital and spin moments are experimentally determined. We show that an in-plane magnetization is favored and the magnetic anisotropy energy associated with the directions parallel and perpendicular to the nanolines are measured. The Co ultrathin film is shown to behave as a superparamagnetic system composed of one-dimensional segments containing each \ensuremath{\sim}170 ferromagnetically coupled Co atoms, with a blocking temperature estimated to be between 20 and 40 K. Our set of experiments allows for a comprehensive description of the magnetic behavior of the Co nanoline ultrathin film grown on a functionalized metallic substrate.

4f spin driven ferroelectric-ferromagnetic multiferroicity in PrMn2O5 under a magnetic field

In contrast to all other members of the $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ family with nonzero $4f$ electrons ($R$ = Nd to Lu), ${\mathrm{PrMn}}_{2}{\mathrm{O}}_{5}$ does not show any spin driven ferroelectricity in the magnetically ordered phase. By means of high-field electric polarization measurements up to 45 T, we have found that this exceptional candidate undergoes a spin driven multiferroic phase under magnetic field. X-ray magnetic circular dichroism studies up to 30 T at the Pr ${L}_{2}$ edge show that this ferroelectricity originates from and directly couples to the ferromagnetic component of the ${\mathrm{Pr}}^{3+}$ spins. Experimental observations along with our generalized gradient-approximation $+\phantom{\rule{4pt}{0ex}}U$ calculations reveal that this exotic ferroelectric-ferromagnetic combination stabilizes through the exchange-striction mechanism solely driven by a $3d\ensuremath{-}4f$-type coupling, as opposed to the other $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ members with $3d\ensuremath{-}3d$ driven ferroelectric-antiferromagnetic-type conventional type-II multiferroicity.

Strain healing of spin–orbit coupling:a cause for enhanced magnetic moment in epitaxial SrRuO3 thin films

Enhanced magnetic moment and coercivity in SrRuO3 thin films are significant issues for advanced technological usages and hence are researched extensively in recent times. Most of the previous reports on thin films with enhanced magnetic moment attributed it to the high spin state. Our magnetization results show high magnetic moment of 3.3 μB/Ru ion in the epitaxial thin films grown on LSAT substrate against 1.2 μB/Ru ion observed in bulk compound. Contrary to the previous reports the Ru ions are found to be in low spin state and the orbital moment is shown to be contributing significantly in the enhancement of magnetic moment. We employed x-ray absorption spectroscopy and resonant valance band spectroscopy to probe the spin state and orbital contributions in these films. The existence of strong spin–orbit coupling responsible for the de-quenching of the 4d orbitals is confirmed by the observation of the non-statistical large branching ratio at the Ru M2,3 absorption edges. X-ray magnetic circular dichroism studies performed at the Ru M2,3 edges provided direct evidence of significant contribution of orbital moment in the film grown on LSAT. The relaxation of orbital quenching by strain engineering provides a new tool for enhancing magnetic moment and strain disorder is shown to be an efficient mean to control the spin–orbit coupling.

Magnetization manipulation induced by nonlocal spin injection from a perpendicular spin polarizer: nano-beam X-ray MCD study using an order-sorting-aperture collection method

We present an X-ray magnetic circular dichroism (XMCD) study on a nanoscale nonlocal spin injection structure consisting of a perpendicular polarizer and an in-plane free layer. We perform XMCD measurements while applying an operating current to the sample, and observe that the free layer magnetization tilts to the perpendicular direction owing to injection of perpendicular spin. A photoelectron collection method utilizing an order-sorting-aperture (OSA) is developed for this measurement. This OSA collection method enables XMCD measurement on a sample under current application, and its measurement quality is comparable to that recorded with the conventional total electron yield method.

Evidence for the coexistence of spin-glass and ferrimagnetic phases in BaFe12O19 due to basal plane freezing.

We present here the results of low-temperature magnetization and X-ray magnetic circular dichroism studies on the single crystals of BaFe12O19 which reveal for the first time the emergence of a spin glass phase, in coexistence with a long-range ordered ferrimagnetic phase, due to the freezing of the basal plane spin component.

Giant perpendicular magnetic anisotropy in Ir/Co/Pt multilayers

We have studied the magnetic properties of multilayers composed of ferromagnetic metal Co and heavy metals with strong spin orbit coupling (Pt and Ir). Multilayers with symmetric (ABA stacking) and asymmetric (ABC stacking) structures are grown to study the effect of broken structural inversion symmetry. We compare the perpendicular magnetic anisotropy (PMA) energy of symmetric Pt/Co/Pt, Ir/Co/Ir multilayers and asymmetric Pt/Co/Ir, Ir/Co/Pt multilayers. First, the interface contribution to the PMA is studied using the Co layer thickness dependence of the effective PMA energy. Comparison of the interfacial PMA between the Ir/Co/Pt, Pt/Co/Ir asymmetric structures and Pt/Co/Pt, Ir/Co/Ir symmetric structures indicate that the broken structural inversion symmetry induced PMA is small compared to the overall interfacial PMA. Second, we find the magnetic anisotropy field is significantly increased in multilayers when the ferromagnetic layers are antiferromagnetically coupled via interlayer exchange coupling (IEC). Macrospin model calculations can qualitatively account for the relation between the anisotropy field and the IEC. Among the structures studied, the IEC is the largest for the asymmetric Ir/Co/Pt multilayers: the exchange coupling field exceeds 3 T and consequently, the anisotropy field approaches 10 T. Third, comparing the asymmetric Ir/Co/Pt and Pt/Co/Ir structures, we find the IEC and, to some extent, the interface PMA are stronger for the former than the latter. X-ray magnetic circular dichroism (XMCD) studies suggest that the proximity-induced magnetization in Pt is larger for the Ir/Co/Pt multilayers than the inverted structure (Pt/Co/Ir), which may partly account for the difference in the magnetic properties. These results show the intricate relation between PMA, IEC, and the proximity-induced magnetization that can be exploited to design artificial structures with unique magnetic characteristics.

Structural and magnetic properties of FePt-Tb alloy thin films

We have studied structural and magnetic properties of ${(\mathrm{FePt})}_{1\ensuremath{-}x}\mathrm{T}{\mathrm{b}}_{x}$ thin films grown at elevated temperatures on MgO(100) substrates. The incorporation of Tb into the chemically ordered $\mathrm{L}{1}_{0}$ FePt lattice gives rise to considerable changes in the structural and magnetic properties, depending on the Tb content and the deposition temperature. When grown at $770{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, pronounced $L{1}_{0}$ ordering of the initial FePt phase is present. It vanishes gradually with Tb addition, thereby forming additional crystalline Tb-Pt phases at higher Tb content. These structural changes are accompanied by a strong reduction of the perpendicular magnetic anisotropy with increasing Tb content, finally resulting in a soft magnetic material with in-plane easy-axis magnetization. Extended x-ray absorption fine-structure measurements were performed to examine the possible incorporation of Tb into the FePt lattice. Based on the number of Pt first neighbors, the transformation from the initial tetragonal $L{1}_{0}$ structure to the chemically disordered fcc ($A$1) phase with increasing Tb content could be firmly concluded. In a further sample series, FePt was grown at a lower temperature of 530 \ifmmode^\circ\else\textdegree\fi{}C, which leads to reduced initial $L{1}_{0}$ chemical ordering. Adding Tb results in strong elastic stress in the lattice, eventually causing full amorphization of the film. Interestingly, in this case, a spin-reorientation transition from in-plane to out-of-plane magnetic easy axis is found at low temperatures, which is associated with an anisotropic chemical short-range order present in the amorphous phase. Furthermore, x-ray magnetic circular dichroism studies at the $\mathrm{Fe}\phantom{\rule{0.28em}{0ex}}{L}_{3,2}$ and Tb ${M}_{5,4}$ edges for all samples reveal strong antiferromagnetic coupling between Fe and Tb, resulting in a reduction of the net magnetization of the film.

Study of magneto capacitance effect, exchange bias, XMCD and XAS in La0.8Bi0.2Fe0.7Mn0.3O3/LaNiO3/LaAlO3 multiferroic thin film

Magneto capacitance (MC) property of La0.8Bi0.2Fe0.7Mn0.3O3/LaNiO3/LaAlO3 thin film has been studied at an applied magnetic field of 0 T and 5 T. Variation of capacitance versus frequency at different temperatures (80 K, 100 K,180 K and 200 K) with the application of magnetic field depicts the unification of ferroic (ferroelectric and ferromagnetic) properties. This is a significant aspect for a system to be multiferroic, a choice for better device application. Exchange bias effect is observed on meticulous study of hysteresis loop taken at 5 K and 10 K upto ~1 Tesla (while cooling thin film in 5 kOe of applied magnetic field) in the form of systematic shift in loops on positive side of applied magnetic field. Hysteresis loop shifts accordingly on negative side on changing the polarity of field (−5 kOe) in field cooled mode, confirming the intrinsic nature of exchange bias effect in thin film. The NEXAFS technique has been used in finding the electronic structure, charge state and local symmetry of ions present in thin film; therefore the mechanism(s) responsible for its major properties. Magnetic character of thin film is studied using x-ray magnetic circular dichroism (XMCD) measurements. NEXAFS data of Mn L3,2- edge spectra indicates the presence of Mn2+ /Mn3+ valence states with contribution from Mn4+ ions in the system, which corroborates the ferrimagnetic nature revealed by thin film. L3,2- edge of Fe along with reference compound (Fe2O3) reveals that these ions are present in trivalent state (Fe3+) in octahedral symmetry. O K-edge features compliment the magneto-electric (ME) properties of the thin film. Charge transfer multiplet calculations confirm that Mn ions exist in different charge states (Mn2+,3,+4+) whereas Fe ions exhibit single (Fe3+) valent state in thin film.

Probing proximity effects in the ferromagnetic semiconductor EuO

Abstract Ferromagnetic insulators are widely employed to induce magnetic phenomena in adjacent layers via proximity effect. This approach could make non-magnetic materials (ranging from silicon to graphene) available for spintronic applications. Eu chalcogenides, EuO in particular, are highly efficient spin generators but suffer from low Curie temperatures. Here, experiments aimed at TC increase in EuO by its integration with the ferromagnetic metal Gd are reported. The epitaxial bilayers Gd/EuO are synthesized on different substrates and characterized by a combination of diffraction and microscopy techniques. Their magnetic structure – established with magnetization and transport measurements as well as element-selective X-ray magnetic circular dichroism study – comprises coupled magnetic orders of EuO and Gd. EuO is robust against proximity effects – its TC is still low, increased at most by a few tens of K. Nevertheless, the results encourage further studies of proximity-enhanced ferromagnetism to extend the range of applications of ultrathin layers of EuO in spintronics.

Simultaneous Observation of the Er- and Fe-Sublattice Magnetization of Ferrimagnetic Er_{3}Fe_{5}O_{12} in High Magnetic Fields Using X-Ray Magnetic Circular Dichroism at the Er L_{2,3} Edges.

X-ray magnetic circular dichroism (XMCD) studies at the Er L_{2,3} edges of Er_{3}Fe_{5}O_{12} exhibit a change of the spectral shape as a function of temperature and magnetic field. Using singular value decomposition, this variation is understood as a linear combination of two components. The dominating component is associated with the Er magnetization, while the second contribution is identified as an induced signal from the Fe sites. XMCD at either of the L edges in Er_{3}Fe_{5}O_{12} provides information on the net magnetization of both sublattices. Their evolution in fields up to 30T reveals details of the ferrimagnetic interactions on two very different scales.

Electronic states and possible origin of the orbital-glass state in a nearly metallic spinel cobalt vanadate: An x-ray magnetic circular dichroism study

We have investigated the orbital states of the orbital-glassy (short-range orbital ordered) spinel vanadate Co$_{1.21}$V$_{1.79}$O$_{4}$ using x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and subsequent configuration-interaction cluster-model calculation. From the sign of the XMCD spectra, it was found that the spin magnetic moment of the Co ion is aligned parallel to the applied magnetic field and that of the V ion anti-parallel to it, consistent with neutron scattering studies. It was revealed that the excess Co ions at the octahedral site take the trivalent low-spin state, and induce a random potential to the V sublattice. The orbital magnetic moment of the V ion is small although finite, suggesting that the ordered orbitals mainly consists of real-number orbitals.

XMCD and PES study of a compensated-ferrimagnetic half-metal Mn3Ga

By employing soft X-ray magnetic circular dichroism (XMCD), soft X-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES), we have investigated the electronic structure of the candidate zero-moment half-metallic Mn3Ga. We have studied the ball-milled and annealed Mn3Ga powder samples that exhibit nearly zero magnetization. Mn 2p XAS revealed that Mn ions in Mn3Ga are nearly divalent for both of the Mn ions having the locally octahedral symmetry and those having the locally tetrahedral symmetry. The measured Mn 2p XMCD spectrum of Mn3Ga is very similar to that of ferrimagnetic MnFe2O4 having divalent Mn ions. The sum-rule analysis of the Mn 2p XMCD spectrum shows that both the spin and orbital magnetic moments of Mn ions in Mn3Ga are negligibly small, in agreement with the nearly compensated-ferrimagnetic ground state of Mn3Ga. The valence-band PES spectrum of Mn3Ga agrees well with the calculated density of states, supporting the half-metallic electronic structure of Mn3Ga.

Experimental evidence for mixed-valent Cr ions in half-metallic CrO2: Temperature-dependent XMCD study

By employing soft X-ray magnetic circular dichroism (XMCD) and soft X-ray absorption spectroscopy (XAS), we have investigated the temperature (T)-dependent valence and spin states of Cr ions in half-metallic ferromagnet CrO2 nanorod powder. The lineshape-analysis of both the Cr 2p XAS and XMCD spectra of CrO2 reveals that the Cr ions in bulk CrO2 have both Cr4+t2g2↑ and Cr3+t2g3↑ valence configurations. This work provides the direct experimental evidence for the Cr3+-Cr4+ mixed-valent states in bulk CrO2(Cr3.8+), demonstrating that the half-metallic ferromagnetism in CrO2 originates from the double-exchange interaction. The T-dependent intensity variation of the Cr 2p XMCD spectrum agrees well with the bulk magnetization M(T) and the Curie temperature, indicating that the measured Cr 2p XMCD data represent the intrinsic bulk ferromagnetism in CrO2, and support the above finding of the XMCD lineshape analysis.

Iron L2,3-Edge X-ray Absorption and X-ray Magnetic Circular Dichroism Studies of Molecular Iron Complexes with Relevance to the FeMoco and FeVco Active Sites of Nitrogenase.

Herein, a systematic study of a series of molecular iron model complexes has been carried out using Fe L2,3-edge X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopies. This series spans iron complexes of increasing complexity, starting from ferric and ferrous tetrachlorides ([FeCl4]−/2–), to ferric and ferrous tetrathiolates ([Fe(SR)4]−/2–), to diferric and mixed-valent iron–sulfur complexes [Fe2S2R4]2–/3–. This test set of compounds is used to evaluate the sensitivity of both Fe L2,3-edge XAS and XMCD spectroscopy to oxidation state and ligation changes. It is demonstrated that the energy shift and intensity of the L2,3-edge XAS spectra depends on both the oxidation state and covalency of the system; however, the quantitative information that can be extracted from these data is limited. On the other hand, analysis of the Fe XMCD shows distinct changes in the intensity at both L3 and L2 edges, depending on the oxidation state of the system. It is also demonstrated that the XMCD intensity is modulated by the covalency of the system. For mononuclear systems, the experimental data are correlated with atomic multiplet calculations in order to provide insights into the experimental observations. Finally, XMCD is applied to the tetranuclear heterometal–iron–sulfur clusters [MFe3S4]3+/2+ (M = Mo, V), which serve as structural analogues of the FeMoco and FeVco active sites of nitrogenase. It is demonstrated that the XMCD data can be utilized to obtain information on the oxidation state distribution in complex clusters that is not readily accessible for the Fe L2,3-edge XAS data alone. The advantages of XMCD relative to standard K-edge and L2,3-edge XAS are highlighted. This study provides an important foundation for future XMCD studies on complex (bio)inorganic systems.

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.

The Nanocluster Trap endstation at BESSY II

The Nanocluster Trap endstation at BESSY II combines a cryogenic linear radio-frequency ion trap with an applied magnetic field for x-ray magnetic circular dichroism studies of cold and size-selected trapped ions. Applications include atomic, molecular, and cluster ions as well as ionic complexes.

XMCD study of the local magnetic and structural properties of microcrystalline NdFeB-based alloys

X-ray Magnetic Circular Dichroism (XMCD) technique was used to investigate local magnetic properties of microcrystalline Nd10.4Zr4.0Fe79.2B6.4 samples, oriented along either easy or hard magnetization direction. The Nd L 2,3 and Fe K edge XMCD spectra were measured at room temperature under a magnetic field of T. A very strong dependence of XMCD spectra on the sample orientation has been observed at the NdL 2,3-edges, whereas the Fe K-edge XMCD spectra are found to be practically isotropic. This result indicates that magnetic anisotropy of NdFeB-based alloys originates from the Nd sublattice. In addition, element selective magnetization curves have been recorded by measuring the intensity of XMCD signals as a function of an applied magnetic field up to T. To find a correlation between local and macroscopic magnetic properties of studied samples we compared these data with magnetization curves, measured by vibrating sample magnetometer up to T. Results are important for understanding the origin of high-coercivity state in NdFeB-based intermetallic compounds.

Magnetic and XMCD studies of Pr1−xSrxMnO3 manganite films

Magnetic properties of the Pr0.8Sr0.2MnO3 and Pr0.6Sr0.4MnO3 polycrystalline films have been studied using temperature and magnetic field dependences of the static magnetization and X-ray magnetic circular dichroism (XMCD) spectroscopy. For the both compositions, the difference between the temperature dependences of magnetization obtained in the zero fields cooling (ZFC) and field cooling (FC) modes has been revealed. The ZFC curves demonstrate a pronounced maximum at temperature Tm. It is shown that the Tm value dependence on the magnetic field follows the Almeida-Thouless line typical for the classic spin glass, what allows us to assume the possible spin-glass behavior of the films. Effect of the disorder in a direction of the crystallites easy-axis on the difference between FC and ZFC curves has been discussed also. Magnetic field dependences of the sample magnetization are presented by the hysteresis loops with the shape changing upon temperature variation. This behavior has been attributed to the effect of crystallographic anisotropy and Pr ions. The spectra and magnetic field dependences of XMCD at the Pr L2- and Mn K-edges have been studied at 90K. The magnetic field dependences of the XMCD at the Pr L2-edge had shown Van Vleck paramagnetism from Pr3+ ions.

X-ray magnetic circular dichroism study of Dy-doped Bi2Te3 topological insulator thin films

Magnetic doping of topological insulators (TIs) is crucial for unlocking novel quantum phenomena, paving the way for spintronics applications. Recently, we have shown that doping with rare earth ions introduces large magnetic moments and allows for high doping concentrations without the loss of crystal quality, however no long range magnetic order was observed. In Dy-doped Bi2Te3 we found a band gap opening above a critical doping concentration, despite the paramagnetic bulk behavior. Here, we present a surface-sensitive x-ray magnetic circular dichroism (XMCD) study of an in situ cleaved film in the cleanest possible environment. The Dy M4,5 absorption spectra measured with circularly polarized x-rays are fitted using multiplet calculations to obtain the effective magnetic moment. Arrott–Noakes plots, measured by the Dy M5 XMCD as a function of field at low temperatures, give a negative transition temperature. The evaporation of a ferromagnetic Co thin film did not introduce ferromagnetic ordering of the Dy dopants either; instead a lowering of the transition temperature was observed, pointing towards an antiferromagnetic ordering scenario. This result shows that there is a competition between the magnetic exchange interaction and the Zeeman interaction. The latter favors the Co and Dy magnetic moments to be both aligned along the direction of the applied magnetic field, while the exchange interaction is minimized if the Dy and Co atoms are antiferromagnetically coupled, as in zero applied field.

An X-ray magnetic circular dichroism (XMCD) study of Fe ordering in a synthetic MgAl2O4-Fe3O4(spinel-magnetite) solid-solution series: Implications for magnetic properties and cation site ordering

We thank Richard Pattrick and Vicky Coker for help in collecting XMCD on these samples at the Daresbury SRS and subsequently at the Advanced Light Source (ALS), Berkeley. The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences (OBES) of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231 and we thank Elke Arenholz for her assistance. RJH acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 320750. KMR gratefully acknowledges support from the DOE OBES Chemical Sciences, Geosciences, and Biosciences Division, through the Geosciences Program at Pacific Northwest National Laboratory. We also thank Gerrit van der Laan and Nick Telling for help with XMCD data analysis; David Plant carried out the electron microprobe analyses at Manchester and Paul Schofield provided information on the natural magnesian spinel. We also thank two anonymous referees for constructive comments.