X-ray Magnetic Circular Dichroism Signal Research Articles

Evidence of ZnCO3 interstitial phase in carbon implanted ZnO(002) thin films and room temperature ferromagnetism

High resolution X-ray diffraction (HRXRD) pattern of highly c-oriented (002) thin film provide a direct evidence of the formation of R-3c orthorhombic ZnCO3 structural phase. Room temperature ferromagnetism (RTFM) is measured using bulk vibrating sample magnetometery (VSM). A strong orbital anisotropy has been observed by X-ray linear dichroism (XLD) experiment, which also supports that ZnCO3 phase is structurally non-distortive after carbon-implantation. The absence of X-ray magnetic circular dichroism (XMCD) signal at O K- and C K-edges suggest that bulk VSM ferromagnetism is not related to the magnetic polarization of O 2p/C 2p orbitals hybridized with Zn-states. These experiments clearly indicate that the complex C–ZnO/ZnCO3 phase should be ferromagnetic having out-of-plane/in-plane orbital reconstruction.

Polarity and charge redistribution induced emergent interfacial ferromagnetism in non-magnetic LaNiO3/SrMnO3 superlattices

Artificial heterostructures composed of strongly correlated oxides can host many up-and-coming phenomena. The interfacial ferromagnetism can be observed at the interface of paramagnetic LaNiO3 (LNO) and G-type antiferromagnetic SrMnO3 (SMO). Through the macroscopic magnetic and transport measurements and the microscopic x-ray magnetic circular dichroism signals, the obvious interfacial ferromagnetic behavior exhibited in the thicker superlattices. Using x-ray absorption spectroscopy and x-ray photoelectron spectroscopy measurements, the polar mismatch-induced charge redistribution in Ni and Mn ions accurately demonstrates the dependence on the varying thickness of LNO/SMO superlattices. These results suggest that the ferromagnetic double-exchange interaction of Mn3+–Mn4+ increases and the ferromagnetic super-exchange interaction of Ni2+–Mn4+ decreases with the increasing thickness of LNO/SMO superlattices. Consequently, the obvious ferromagnetic behavior in the thicker superlattices is induced by the double-exchange interaction. For the thinner LNO/SMO superlattice, the interfacial super-exchange ferromagnetic state is offset by the double-exchange ferromagnetism. This is the crucial reason behind the fact that ferromagnetism is hard to exist in the prior LNO/SMO superlattices. Our findings highlight the intrinsic mechanism responsible for the interfacial ferromagnetism and the significance of understanding polar mismatch at the interface of heterostructures.

Bulk and element-specific magnetism of medium-entropy and high-entropy Cantor-Wu alloys

Magnetic Compton scattering, x-ray magnetic circular dichroism spectroscopy and bulk magnetometry measurements are performed on a set of medium (NiFeCo and NiFeCoCr) and high (NiFeCoCrPd and NiFeCoCrMn) entropy Cantor-Wu alloys. The bulk spin momentum densities determined by magnetic Compton scattering are remarkably isotropic, and this is a consequence of the smearing of the electronic structure by disorder scattering of the electron quasiparticles. Non-zero x-ray magnetic circular dichroism signals are observed for every element in every alloy indicating differences in the populations of the majority and minority spin states implying finite magnetic moments. When Cr is included in the solid solution, the Cr spin moment is unambiguously antiparallel to the total magnetic moment, while a vanishingly small magnetic moment is observed for Mn, despite calculations indicating a large moment. Some significant discrepancies are observed between the experimental bulk and surface magnetic moments. Despite the lack of quantitative agreement, the element specific surface magnetic moments seem to be qualitatively reasonable.

Mo4Ce4Al7C3 : A nanolamellar ferromagnetic Kondo lattice

Herein we show that ${\mathrm{Mo}}_{4}{\mathrm{Ce}}_{4}{\mathrm{Al}}_{7}{\mathrm{C}}_{3}$, a recently discovered nanolamellar compound displaying mixed valence, combines Kondo lattice behavior with ferromagnetism. A sizeable magnetization is carried by $3p$ states of Al as evidenced by a strong x-ray magnetic circular dichroism signal at the $K$ edge of aluminum, whereas no detectable signal was observed at the $K$ edge of carbon and ${L}_{2,3}$ edges of molybdenum. These results point out that the ferromagnetic behavior originates in the Ce atoms with $4{f}^{1}$ electronic configuration lying within the Al planes. The evolution with pressure of the mixed valence of Ce atoms in the Mo-C planes determined via Ce ${L}_{3}$ x-ray-absorption spectra along with the magnetoresistance measurements across the ferromagnetic transition unambiguously reveal a Kondo behavior. Angle-resolved photoemission spectroscopy and density functional theory confirm a certain degree of Ce $4f$ electron delocalization. More generally, conduction electrons are not restricted to lie in the MoC planes but are also delocalized in the Al planes.

A versatile X-ray phase retarder for lock-in XMCD measurements.

X-ray magnetic circular dichroism (XMCD) is a technique commonly used to probe magnetic properties of materials with element and orbital selectivity, which requires the use of circularly polarized (CP) X-rays. It is possible to accomplish XMCD experiments with fixed CP and alternating the magnetic fieldorientation, but most reliable data are obtained when alternating the magnetization orientation and the polarization between right and left helicities. A versatile strategy has been developed to perform XMCD experiments using a hard X-ray quarter-wave plate, at both polychromatic dispersive and conventional monochromatic optics, in combination with synchronous data acquisition. The switching frequency waveform is fed into a lock-in amplifier to detect andamplify the XMCD signal. The results on a reference sample demonstrate an improvement in data quality and acquisition time. The instrumentation successfully generated 98% of CP X-rays switching the beam helicity at 13 Hz, with the possibility of faster helicity switching once it is installed at the new Brazilian fourth-generation source, SIRIUS.

Spin and orbital magnetic moments in perpendicularly magnetized Ni1−xCo2+yO4−z epitaxial thin films: Effects of site-dependent cation valence states

We carried out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy and investigated cation valence states and spin and orbital magnetic moments in the inverse-spinel ferrimagnet Ni$_{1-x}$Co$_{2+y}$O$_{4-z}$ (NCO) epitaxial films with the perpendicular magnetic anisotropy. We show that the oxygen pressure P$_{O2}$ during the film growth by pulsed laser deposition influences not only the cation stoichiometry (site-occupation) but also the cation valence state. Our XAS results show that the Ni in the O$_{h}$-site is in the intermediate valence state between +2 and +3, Ni$^{(2+\delta)+}$ (0<$\delta$<1), whose nominal valence state (the $\delta$ value) varies depending on P$_{O2}$. On the other hand, the Co in the octahedral (O$_{h}$) and tetrahedral (T$_{d}$) sites respectively have the valence state close to +3 and +2. We also find that the XMCD signals originate mainly from the T$_{d}$-site Co$^{2+}$ (Co$_{Td}$) and O$_{h}$-site Ni$^{(2+\delta)+}$ (Ni$_{Oh}$), indicating that these cation valence states are the key in determining the magnetic and transport properties of NCO films. Interestingly, the valence state of Ni$^{(2+\delta)+}$ that gives rise to the XMCD signal remains unchanged independent of P$_{O2}$. The electronic structure of Ni$^{(2+\delta)+}$ that is responsible for the magnetic moment and electrical conduction differs from those of Ni$^{2+}$ and Ni$^{3+}$. In addition, the orbital magnetic moment originating from Co$_{Td}$ is as large as 0.14 $\mu_{B}/Co_{Td}$ and parallel to the magnetization while the Ni$_{Oh}$ orbital moment is as small as 0.07 $\mu_{B}/Ni_{Oh}$ and is rather isotropic. The Co$_{Td}$ therefore plays the key role in the perpendicular magnetic anisotropy of the films. Our results demonstrate the significance of the site-dependent cations valence states for the magnetic and transport properties of NCO films.

Thermally modulated hydrogenation in FexPd1−x alloy films: Temperature-driven peculiar variation of magnetism

The sensitive hydrogen effect on spintronic materials has been recently demonstrated to have high application potential. However, the correlation between hydrogen pressure (PH2), temperature, and magnetic properties still remains unclear. In this study, the magnetic moment of Fe in an Fe–Pd alloy thin film was increased through hydrogen absorption, as evidenced by the enhanced x-ray magnetic circular dichroism signal of Fe. Hydrogen absorption and desorption hysteresis loops in the magnetic coercivity Hc-PH2 diagram revealed that most hydrogen was absorbed when PH2 was above 10 mbar and desorbed when PH2 was approximately 10–6 mbar. The hydrogenation effect on the magnetism of an Fe–Pd alloy film was eliminated at an annealing temperature of 360 K without considerable hydrogen desorption. The annealing-driven cyclic enhancement of Hc was demonstrated because of the competition between thermal activation and H bonding. These results clearly reveal the critical temperature dependence and provide applicable knowledge of the hydrogenation effect on magnetic Pd-alloys.

Magnetic reversal in rare-earth free Mn4 − xNixN epitaxial films below and above Ni composition needed for magnetic compensation around room temperature

Ferrimagnets close to the magnetic compensation are excellent candidates to spin-torque-based spintronic applications, as their small magnetizations allow lowering switching currents. Here, we studied the magnetic compensation of Mn4 − xNixN epitaxial films by performing x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the L2,3 absorption edges of Mn and Ni atoms and compared them with those of Ni3FeN films. The XAS spectrum of the Ni3FeN films exhibits shoulders at approximately 2 eV above the Ni L2,3 main peaks, originating from orbitals hybridization between Ni 3d at face-centered (II) sites and N 2p at body-centered sites. However, such shoulders are not observed at the Ni L2,3 edges of the Mn4 − xNixN films (x = 0.1 and 0.25). These results indicate that the orbitals of Ni atoms do not hybridize with those of N atoms. Hence, Ni atoms preferentially occupy corner (I) sites, where hybridization is weak because of the relatively long distance between Ni at I sites and N atoms. The XMCD signals of Mn and Ni atoms reverse sign between x = 0.1 and 0.25. This shows that the directions of the magnetic moments carried by Mn and Ni atoms are reversed, indicating that the magnetic compensation occurs in the range 0.1 &amp;lt; x &amp;lt; 0.25. In addition, the signs of Mn(I) XMCD signals are opposite to those of Mn(II) and Ni for each composition. Thus, the magnetic moments of Ni atoms are aligned parallel to those of Mn(II) regardless of whether x is below or above the compensation point.

Interfacial ferromagnetism and atomic structures in high-temperature grown Fe3O4/Pt/Fe3O4 epitaxial trilayers

Induced Pt ferromagnetism in Fe3O4/Pt/Fe3O4 epitaxial trilayer films has been investigated by means of X-ray magnetic circular dichroism (XMCD) at the Pt L3,2-edges at various temperatures from 300K to 12K, including the metal-insulator transition temperature of Fe3O4 (TV∼114K). At all the temperatures, we observed clear XMCD signals due to Pt ferromagnetism, the amplitude of which was determined to be 0.39μB at 300K and 0.52μB at 12K for the sample with the Pt thickness of ∼2nm. Interestingly, these values are comparable to or even greater than those in Pt/3d-ferromagnetic-metal (Fe, Ni, Co, and Ni81Fe19) junction systems. The results can be interpreted in terms of a possible Fe interdiffusion into the Pt layer and also possible Fe-Pt alloying due to its high-temperature deposition.

A brief discussion of the magnetocaloric effect in thin films of manganite doped with chromium

In this work we report on the magnetocaloric effect of La2/3Ca1/3MnO3 (LCMO) and La2/3Ca1/3Mn0.94Cr0.06O3 (LCMCrO) manganite thin films grown by DC magnetron sputtering on LaAlO3 (100) substrates. X-ray diffraction shows that both doped and undoped films crystallize in the orthorhombic structure. Magnetic measurements show a decrease in both the Curie temperature, TC, and the saturation magnetization, MS, for the LCMCrO sample. The change in the magnetic entropy (ΔSm) was extracted from hysteresis loops at different temperatures around the ferromagnetic to paramagnetic transition, displaying a maximum of entropy change (ΔSm)max near TC in both films. Moreover, a shift in (ΔSm)max toward temperatures above TCwith increasing magnetic field and a broadening of the entropy change curve were observed. Results of refrigeration cooling power show a lower efficiency for LCMCrO. In order to obtain a local insight into the magnetic interactions of these films, measurements of X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) were performed. XMCD suggests that an antiferromagnetic coupling between Mn4+ - Mn3+ is favored with Cr3+ incorporation, which reduces the Mn L2,3 XMCD signal and results in a decrease of MS and (ΔSm)max in LCMCrO films.

High-pressure magnetism of the double perovskite Sr2FeOsO6 studied by synchrotron Fe57 Mössbauer spectroscopy

The pressure dependence of the magnetism of the double perovskite ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ was investigated by temperature- and magnetic-field-dependent synchrotron $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy in the energy domain up to $\ensuremath{\sim}50$ GPa. ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ is known to feature antiferromagnetic ordering below ${T}_{\mathrm{N}}\ensuremath{\sim}140$ K and a change in spin structure from AF1 to AF2 near 70 K at ambient pressure. Previous Os ${L}_{2,3}$ x-ray magnetic circular dichroism (XMCD) spectra [Veiga et al., Phys. Rev. B 91, 235135 (2015)] indicated a pressure-driven change from the antiferromagnetic to the ferrimagnetic state. Raman spectra of ${\mathrm{Sr}}_{2}{\mathrm{FeOsO}}_{6}$ collected at room temperature and up to 34 GPa are in agreement with previous x-ray-diffraction studies which verified that the tetragonal ambient pressure crystal structure is retained at high pressure. The M\ossbauer investigations show that the Fe ions remain in the +3 high-spin $({{t}_{2g}}^{3}{e}_{g}^{2})$ state over the whole pressure range. The magnetic ordering is strongly stabilized by high pressure and for $pg30$ GPa the ordering temperature is increased to above room temperature. Broad magnetic hyperfine patterns as well as coexistence of magnetic and paramagnetic signals at high pressures indicate the formation of inhomogeneous magnetic states. The shapes of applied-field M\ossbauer spectra at low temperature are similar at 2 and 33 GPa and do not allow resolution of distinct antiferromagnetic and ferrimagnetic components. Pressure-induced spin canting in a basically antiferromagnetic spin structure or magnetic phase separation involving a pressure-induced minority ferrimagnetic phase may be the origin for the ferromagnetic XMCD signal.

Evolution of electronic and magnetic properties of nominal magnetite nanoparticles at high pressure probed by x-ray absorption and emission techniques

We present a study of electronic and magnetic properties of nominal magnetite nanoparticles (NPs) (~6 nm) at high pressure in the presence of silicon pressure medium using x-ray absorption near edge structure (XANES), x-ray magnetic circular dichroism (XMCD) and non-resonant x-ray emission spectroscopy (XES). XANES data show a reduction of Fe charge state, a change in local environment around Fe at tetrahedral sites, and a reduced occupation of Fe 4p orbitals, not seen in previous pressure studies of bulk magnetite. XMCD data show a continuous magnetic moment reduction of ~50% between ambient pressure and 20 GPa, similar to what was observed in previous bulk magnetite studies. XES spectra of NPs indicate a gradual change in spin configuration away from the high-spin state consistent with a postulated charge transfer from Fe 4p to 3d states and the observed reduction in XMCD signal. Taken together, the results point to substantial differences in the response of electronic and magnetic properties of the nano-counterparts of bulk magnetite at high pressure.

Applications of Field-reversal and Angle-dependent XMCD Techniques to Mn-based Diluted Magnetic Materials

Recent progress of the soft X-ray magnetic circular dichroism (XMCD) techniques and relevant applications at beamline 4B7B in Beijing Synchrotron Radiation Facility are reported here. The key progress of the XMCD techniques include i) improvements in the accuracy and sensitivity of XMCD measurements by fast-reversing magnetic field with electromagnet, and ii) establishment an angle-dependent experimental method for obtaining the magnetic anisotropy information. These techniques have been applied to investigate the interface ferromagnetism and magnetic anisotropy of two Mn-based materials, i.e. Fe/(Ga,Mn)As and La2/3Sr1/3MnO3/SrTiO3 bilayer heterostructures. An enhanced XMCD signal has been observed at the Fe L2,3-edges whereas a relative small but unambiguous Mn XMCD signal with opposite sign has been detected which indicates an antiferromagnetic coupling at Fe/(Ga,Mn)As interfaces. A comparative study of the stoichiometric and nonstoichiometric La2/3Sr1/3MnO3/SrTiO3 bilayers clearly demonstrates that the oxygen vacancies degrade the magnetic properties of the perovskite manganese oxide film. These achievements benefit from the established field-reversal and angle-dependent XMCD techniques, which will make it possible to extend the research field of our devices from ferromagnetic to paramagnetic or diluted magnetic semiconductor system.

Iron Phthalocyanine and Ferromagnetic Thin Films: Magnetic Behavior of Single and Double Interfaces.

Metal-phthalocyanines are quasi-planar heterocyclic macrocycle molecules with a highly conjugated structure. They can be engineered at the molecular scale (central atom, ligand) to tailor new properties for organic spintronics devices. In this study, we evaluated the magnetic behavior of FePc in a ∼1 nm molecular film sandwiched between two ferromagnetic films: cobalt (bottom) and nickel (top). In the single interface, FePc in contact with a Co film is magnetically coupled with the inorganic film magnetization, though the relatively small Fe(Pc) X-ray magnetic circular dichroism (XMCD) signal in remanence, with respect to that observed in applied field of 6 T, suggests that a fraction of molecules in the organometallic film have their magnetic moment not aligned or antiparallel with respect to Co. When in contact with two interfaces, Fe(Pc) XMCD doubles, indicating that part of the Fe(Pc) are now aligned with the Ni topmost layer, saturated at 1 T. We discussed the relevance of the finding in terms of understanding and developing hybrid organic/inorganic spin devices.

First results from the XMCD facility at the Energy-Dispersive EXAFS beamline of the Indus-2 synchrotron source.

Setting up of the X-ray Magnetic Circular Dichroism (XMCD) measurement facility with hard X-rays at the Energy-Dispersive EXAFS beamline (BL-08) at the Indus-2 synchrotron source is reported. This includes the design and development of a water-cooled electromagnet having a highest magnetic field of 2 T in a good field volume of 125 mm3 and having a 10 mm hole throughout for passage of the synchrotron beam. This also includes the development of an (X-Z-θ) motion stage for the heavy electromagnet for aligning its axis and the beam hole along the synchrotron beam direction. Along with the above developments, also reported is the first XMCD signal measured on a thick Gd film in the above set-up which shows good agreement with the reported results. This is the first facility to carry out XMCD measurement with hard X-rays inIndia.

Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator.

We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M5,4 edge XMCD, orbital and spin magnetic moments of (-0.1 ± 0.3)μB and (+7.0 ± 0.6)μB, respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower.

Current-induced domain wall oscillations in a nanowire imaged by time-resolved photoemission electron microscopy

Abstract We study reversible domain wall motion in half-ring Ni 80 Fe 20 nanowires on a nanosecond (ns) timescale in a truly current-induced pump-probe experiment using an energy filtered, aberration-corrected photoemission electron microscope. The X-ray magnetic circular dichroism signal is probed at different time delays before, during and after the current pulse in a stroboscopic mode with circularly polarized synchrotron radiation in the energy range of the Fe L 3 -edge (707 eV). We observe lateral domain wall oscillations with a frequency of ∼ 0.4 GHz. Comparing the results to a proposed string model, we find that the domain wall oscillations can be described as string-like asymmetric oscillations.

Ultra-fast annealing manipulated spinodal nano-decomposition in Mn-implanted Ge

In the present work, millisecond-range flash lamp annealing is used to recrystallize Mn-implanted Ge. Through systematic investigations of structural and magnetic properties, we find that the flash lamp annealing produces a phase mixture consisting of spinodally decomposed Mn-rich ferromagnetic clusters within a paramagnetic-like matrix with randomly distributed Mn atoms. Increasing the annealing energy density from 46, via 50, to 56 J cm−2 causes the segregation of Mn atoms into clusters, as proven by transmission electron microscopy analysis and quantitatively confirmed by magnetization measurements. According to x-ray absorption spectroscopy, the dilute Mn ions within Ge are in d5 electronic configuration. This Mn-doped Ge shows paramagnetism, as evidenced by the unsaturated magnetic-field-dependent x-ray magnetic circular dichroism signal. Our study reveals how spinodal decomposition occurs and influences the formation of ferromagnetic Mn-rich Ge–Mn nanoclusters.

Electric field effect on magnetism in a MgO/Pd/Co system with a solid-state capacitor structure

The electric field effect on the magnetism in a MgO/Pd/Co system, in which a magnetic moment is induced in the Pd layer owing to the ferromagnetic proximity effect, has been investigated using various experimental methods. An electric field was applied to the surface of the Pd layer through a solid-state HfO2/MgO dielectric bilayer by applying a gate voltage with a back-gating configuration. Changes in the magnetic properties of the system as a result of gate voltage application were detected using magnetization and polar-Kerr effect measurements as well as X-ray absorption and X-ray magnetic circular dichroism (XMCD) spectroscopies. A systematic change in the magnetic moment of the system by the application of a gate voltage is observed. The magnetic hysteresis loops obtained by the polar-Kerr effect measurement and the element-specific XMCD signal at the Co L3-edge clearly show a reproducible change in the coercivity that is dependent on the gate voltage.

X-ray magnetic circular dichroism and near-edge X-ray absorption fine structure of buried interfacial magnetism measured by using a scanning tunneling microscope tip

Magnetism at buried interfaces plays a crucial role in many emerging phenomena, but detection of interfacial magnetism in close proximity to a surface with elemental and chemical sensitivity is a challenging task. Here, we use low temperature synchrotron x-ray scanning tunneling microscopy to investigate x-ray magnetic circular dichroism and the near edge x-ray absorption fine structure of La0.67Sr0.33MnO3-LaNiO3 superlattices. In stark contrast to the weak magnetic signal of Mn when the La0.67Sr0.33MnO3 layers are located on top, a robust x-ray magnetic circular dichroism signal is detected when they are buried underneath the LaNiO3 layers. The near edge x-ray absorption fine structure reveals the valence states of manganese, while the oxygen K-edge x-ray absorption spectra show an increase in hole formation, indicating a cogent charge transfer at the LaNiO3/La0.67Sr0.33MnO3 interface. This work demonstrates that scanning tunneling microscopy can be extended to the synchrotron X-ray study of buried interfaces by controlling the tip-sample separation in the nanometer regime.