X-ray Magnetic Circular Dichroism Signal Research Articles

Magnetic structures of FeTiO3-Fe2O3 solid solution thin films studied by soft X-ray magnetic circular dichroism and ab initio multiplet calculations

The solid solutions between ilmenite (FeTiO3) and hematite (α-Fe2O3) have recently attracted considerable attention as a spintronic material due to their interesting magnetic and electrical properties. In this study, the electronic and magnetic structures of epitaxially grown 0.6FeTiO3·0.4Fe2O3 solid solution thin films were investigated by combining x-ray absorption near-edge structure (XANES), x-ray magnetic circular dichroism (XMCD) for two different crystallographic projections, and first-principles theoretical calculations. The Fe L-edge XANES and XMCD spectra reveal that Fe is in the mixed-valent Fe2+–Fe3+ states while Fe2+ ions are mainly responsible for the magnetization. Moreover, the experimental Fe L-edge XANES and XMCD spectra change depending on the incident x-ray directions, and the theoretical spectra explain such spectral features. We also find a large orbital magnetic moment, which can originate the magnetic anisotropy of this system. On the other hand, although the valence state of Ti was interpreted to be 4+ from the Ti L-edge XANES, XMCD signals indicate that some electrons are present in the Ti-3d orbital, which are coupled antiparallel to the magnetic moment of Fe2+ ions.

Change in magnetic and structural properties of FeRh thin films by gold cluster ion beam irradiation with the energy of 1.67 MeV/atom

The effect of energetic cluster ion beam irradiation on magnetic and structural properties of FeRh thin films have been investigated. The cluster ions used in the present studies consist of a few gold atoms with the energy of 1.67 MeV/gold atom. Saturation magnetization of the sample irradiated with Au3 cluster ion beam (280 emu/cc) is larger than that for the irradiated sample with Au1 ion beam (240 emu/cc) for the same irradiation ion fluence. These results can also be confirmed by the X-ray magnetic circular dichroism (XMCD) measurement; the XMCD signal for Au3 cluster ion irradiation is larger than that for Au1 ion irradiation. Since the ion beam irradiation induced magnetization of FeRh is significantly correlated with the amount of the lattice defects in the samples, cluster ion beam irradiation can be considered to effectively introduce the lattice defects in B2-type FeRh rather than the single ion beam. Consequently, cluster ion irradiation is better than single ion irradiation for the viewpoint of saturation magnetization, even if the same irradiation energy is deposited in the samples.

Magnetic ordering of implanted Mn in HOPG substrates

Currently, significant research efforts are geared towards using carbon-based materials for electronic applications. Here we report the observation of magnetic ordering of implanted Mn in HOPG substrates. Superconducting quantum interference device measurements show higher moments for Mn-doped samples and also the existence of double and inverted hysteresis in both undoped and doped samples. High-resolution transmission electron microscopy shows the presence of nanocrystals in implanted samples. Grazing incidence synchrotron x-ray diffraction studies show the presence of three stable manganese carbides, including antiferromagnetic Mn${}_{23}$C${}_{6}$. X-ray magnetic circular dichroism (XMCD) measurements show ferromagnetic ordering of Mn moments at temperatures below \ensuremath{\sim}100 K. However, a very weak XMCD signal indicates that only about 1$%$ of Mn atoms are ferromagnetically ordered. We conclude that the observed Mn ferromagnetic ordering is caused by uncompensated Mn moments on the surface of antiferromagnetic Mn${}_{23}$C${}_{6}$ nanocrystals that are aligned by the local magnetic field.

X-ray magnetic circular dichroism strongly influenced by non-magnetic cover layers

Total electron yield (TEY) is the dominating measurement mode in soft X-ray absorption spectroscopy (XAS), where the sampling depth is generally assumed to be quite small and constant, and the related self-absorption or saturation phenomena are about to be negligible at normal incidence conditions. From the OK edge to CrL2,3 edge XAS ratio we determined a strong change in the effective electron escape length between an uncovered and a RuO2 covered CrO2 sample. This effect has been explained by a simple electron energy filtering model, providing a semi quantitative description. In addition, this simple model can quantitatively describe the unexpected reduced and positive CrL2,3 X-ray magnetic circular dichroism (XMCD) signal of a RuO2/CrO2 bilayer, while previous results have identified a clear negative Cr magnetization at the RuO2/CrO2 interface. In our case this escape length enhancement has strong impact on the XMCD sum rule results and in general it provides much deeper sampling depth, but also larger self-absorption or saturation effects.

Enhanced ferromagnetic moment in Co-doped BiFeO3 thin films studied by soft x-ray circular dichroism

BiFeO3 (BFO) shows both ferroelectricity and magnetic ordering at room temperature, but its ferromagnetic component, which is due to spin canting, is negligible. Substitution of transition-metal atoms such as Co for Fe is known to enhance the ferromagnetic component in BFO. In order to reveal the origin of such magnetization enhancement, we performed soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD) studies of BiFe1−xCoxO3 (x = 0 to 0.30) (BFCO) thin films grown on LaAlO3(001) substrates. The XAS results indicated that the Fe and Co ions are in the Fe3+ and Co3+ states. The XMCD results showed that the Fe ions show ferromagnetism, while the Co ions are antiferromagnetic at room temperature. The XAS and XMCD measurements also revealed that part of the Fe3+ ions are tetrahedrally co-ordinated by oxygen ions but that the XMCD signals of the octahedrally coordinated Fe3+ ions increase with Co content. The results suggest that an impurity phase such as the ferrimagnetic γ-Fe2O3 which exists at low Co concentration decreases with increasing Co concentration and that the ferromagnetic component of the Fe3+ ion in the octrahedral crystal fields increases with Co concentration, probably reflecting the increased canting of the Fe3+ ions.

Ferromagnetic amorphous oxides in the EuO-TiO2system studied by the Faraday effect in the visible region and the x-ray magnetic circular dichroism at the EuM4,5andL2,3edges

Amorphous Eu${}_{2}$TiO${}_{4}$ and EuTiO${}_{3}$ have been studied by a combination of the Faraday effect in the visible region and polarization-dependent x-ray absorption spectroscopy at the Eu ${M}_{4,5}$ and ${L}_{2,3}$ edges to examine the role of Eu 4$f$-5$d$ exchange interactions on the ferromagnetic behavior. The bulk-sensitive x-ray absorption spectra (XAS) for Eu ${L}_{2,3}$ edges show that most of the europium ions are present as the divalent state in the amorphous Eu${}_{2}$TiO${}_{4}$ and EuTiO${}_{3}$. The Eu ${M}_{4,5}$ edge x-ray magnetic circular dichroism (XMCD) signals, measured for the amorphous Eu${}_{2}$TiO${}_{4}$, dramatically increase upon cooling through the Curie temperature (16 K) determined by a superconducting quantum interference device (SQUID) magnetometer. Sum-rule analysis of the XMCD at Eu ${M}_{4,5}$ edges measured at 10 K yields a 4$f$ spin magnetic moment of 6.6${\ensuremath{\mu}}_{\mathrm{B}}$ per Eu${}^{2+}$ ion. These results confirm that the ferromagnetic properties exclusively arise from 4$f$ spins of Eu${}^{2+}$. In addition, for both the amorphous Eu${}_{2}$TiO${}_{4}$ and EuTiO${}_{3}$, the temperature and magnetic-field dependence of Eu ${L}_{2,3}$ edge XMCD signals can be scaled with the corresponding magnetization measured by SQUID, indicating that the 5$d$ magnetic polarization of Eu${}^{2+}$ is involved in the process to cause the ferromagnetic interaction between Eu${}^{2+}$ ions. We further discuss the origin of ferromagnetism in the amorphous system on the basis of the energy diagram of Eu 4$f$ and 5$d$ levels deduced from the Faraday effect in the visible region. From the wavelength dependence of Faraday rotation angles of the amorphous EuO-TiO${}_{2}$ system in comparison with those of the divalent Eu chalcogenides as reported previously, it is found that the magnitude of crystal-field splitting of Eu 5$d$ levels in the former is on the same order as that in the latter, which explains an enhanced ferromagnetic exchange interaction between Eu 4$f$ and 5$d$ states.

Characterization of Co distribution in ZnO by x-ray magnetic circular dichroism

We analyze the electronic and magnetic properties of the various atomic arrangements of Zn1−xCoxO with x = 10% using K-edge x-ray absorption and magnetic circular dichroism spectra from both measurements and first principle calculations. Significantly, the K-edge spectroscopic features of Co are highly sensitive to the local atomic arrangement, and thus can be used as a powerful tool to investigate structural properties of dilute magnetic semiconductors. We clearly showed that defects such as interstitial Co and O vacancy near to substitutional Co are present in the 10% Co doped ZnO sample. The magnetic ordering of ZnO-based diluted magnetic semiconductors is strongly correlated with the presence of oxygen vacancies. Finally, we elucidated the origin of the X-ray magnetic circular dichroism signals.

Soft x-ray magnetic circular dichroism study of valence and spin states in FeT2O4 (T = V, Cr) spinel oxides

Electronic structures of spinel oxides FeT2O4 (T = V, Cr) have been investigated by employing soft x-ray magnetic circular dichroism (XMCD) and soft x-ray absorption spectroscopy (XAS). XAS reveals that Cr and V ions are trivalent, and that Fe ions are nearly divalent in FeT2O4 (T = V, Cr). Finite XMCD signals are observed in FeV2O4 at T = 80 K, while they are very weak in FeCr2O4. XMCD shows that Fe spins are antiparallel to V and Cr spins, with the V and Cr spins being canted from Fe spins, which suggests a Yafet-Kittel type triangular spin configuration in FeT2O4 (T = V, Cr).

Direct evidence for significant spin-polarization of EuS in Co/EuS multilayers at room temperature

The new era of spintronics promises the development of nanodevices, where the electron spin will be used to store information and charge currents will be replaced by spin currents. For this, ferromagnetic semiconductors at room temperature are needed. We report on significant room-temperature spin polarization of EuS in Co/EuS multilayers recorded by x-ray magnetic circular dichroism (XMCD). The films were found to contain a mixture of divalent and trivalent europium, but only Eu++ is responsible for the ferromagnetic behavior of EuS. The magnetic XMCD signal of Eu at room temperature could unambiguously be assigned to magnetic ordering of EuS and was found to be only one order of magnitude smaller than that at 2.5 K. The room temperature magnetic moment of EuS is as large as the one of bulk ferromagnetic Ni. Our findings pave the path for fabrication of room–temperature spintronic devices using spin polarized EuS layers.

X-ray absorption and magnetic circular dichroism characterization of Fe-doped [formula omitted] thin films

We report x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements of both paramagnetic and ferromagnetic Ti1−xFexO2−δ thin films. Fe L3,2 edge XAS spectra showed that the Fe ions in our samples were in mixed valence states and that the Fe2+/Fe3+ ratio decreased with increasing oxygen partial pressure. Finite XMCD signals were observed only for the Fe2+ state in the ferromagnetic sample. These findings indicate that the observed ferromagnetism in Ti1−xFexO2−δ originates from ferromagnetic interactions between Fe2+ local spins.

Investigation of induced Pt magnetic polarization in Pt/Y3Fe5O12 bilayers

Using X-ray magnetic circular dichroism (XMCD) measurements, we explore the possible existence of induced magnetic moments in thin Pt films deposited onto the ferrimagnetic insulator yttrium iron garnet (Y3Fe5O12). Such a magnetic proximity effect is well established for Pt/ferromagnetic metal heterostructures. Indeed, we observe a clear XMCD signal at the Pt L3 edge in Pt/Fe bilayers, while no such signal can be discerned in XMCD traces of Pt/Y3Fe5O12 bilayers. Integrating the XMCD signals allows to estimate an upper limit for the induced Pt magnetic polarization in Pt/Y3Fe5O12 bilayers.

Proximity effects and exchange bias in Co/MnF2(111) heterostructures studied by x-ray magnetic circular dichroism

Cobalt nano-structured ultrathin films were grown on orthorhombic MnF2 by molecular beam epitaxy on CaF2 epitaxial layers deposited on Si(111) substrates. The Co film was grown at room temperature. It was found to be polycrystalline, forming nano-islands with height≈diameter≤10 nm. X-ray absorption evidences the chemical stability of the Co/MnF2 interface. Remarkably, x-ray magnetic circular dichroism (XMCD) demonstrates that the Co induces a net magnetization on the Mn ions close to the interface. The magnetic moments of these Mn ions couple antiparallel to the Co and rotate upon field reversal following the magnetization of the Co both below and high above the Néel temperature of MnF2 (TN = 67 K). The density of coupled Mn moments is found to be temperature dependent, with an equivalent thickness of ∼1.5 MnF2 monolayers at 20 K, decreasing to about ∼0.5 ML as the temperature is raised to 300 K. Interestingly, the intensity of the Mn XMCD signal appears to be related to the coercivity of the Co layer. This behavior is interpreted in terms of the competition between thermal fluctuations, exchange coupling between Co and Mn at the interface and, at low temperature, the antiferromagnetic order in MnF2.

Cu doped ZnO pellets: study of structure and Cu specific magnetic properties

Cu doped ZnO polycrystalline pellets were synthesized with Cu concentrations varying from 2 to 10 wt% by a solid state reaction route (mixing of ZnO and CuO powders). Global magnetization measurements showed that all the samples were paramagnetic. Fitting the temperature-dependence of the magnetization to the Curie–Weiss law revealed the presence of an antiferromagnetic interaction between magnetic moments. Structural characterizations were carried out by x-ray diffraction and x-ray absorption spectroscopy (XAS) at the Cu K-edge. By analyzing the XAS data, we found that at low Cu content most of the Cu atoms substitute for Zn inside the ZnO wurtzite lattice, while for higher Cu concentrations some unreacted CuO remains segregated from the Zn1−xCuxO solid solution. Element-specific magnetic measurements were carried out by x-ray magnetic circular dichroism (XMCD) and compared to the results of ab initio calculations. The XMCD signal at the Cu K-edge originates from magnetic moments localized at Cu sites and, by monitoring the magnetic field dependence, we concur that these moments are associated with a paramagnetic state.

Breakdown of Hund's third rule in amorphous Co-W nanoparticles and crystalline Co3W alloys

We present x-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) measurements performed at the Co $K$ and W ${L}_{2,3}$ edges, on amorphous Co-W alloy nanoparticles, and a comparison with those on a bulk Co${}_{3}$W alloy. A strong hybridization between the $4p$ and $3d$ orbitals in Co and the $5d$ band in W are observed, resulting in an induced magnetic moment in the W atoms. The orbital to spin moment ratio in W of all these Co-W systems is positive, suggesting a parallel orientation of the two moments. This is opposite to the expected antiparallel coupling for atoms with a less-than-half-filled $5d$ band, according to Hund's third rule. These findings are supported by calculations of the electronic density of states projected at the Co $3d$ and W $5d$ orbitals, as well as XANES spectra and XMCD signals at the Co $K$ and W ${L}_{2,3}$ edges in a Co${}_{3}$W system.

Pressure effects on the magnetic properties of FeCuZr studied by x-ray magnetic circular dichroism: Evidence of weakening of ferromagnetism in FeCuZr alloys

The room temperature changes of the magnetic behavior under pressure of an invar alloy of nominal composition (Fe0.5Cu0.5)87Zr13 (at. %) has been studied by K-edge x-ray magnetic circular dichroism (XMCD) and x-ray absorption near edge spectroscopy (XANES). The Curie temperature, as determined from low field magnetization measurements, is (255 ± 15)K. However XMCD shows a non negligible signal above this temperature. In addition, the XMCD signal decreases upon increasing pressure indicating a pressure-induced collapse of the magnetic moment. These results evidence the itinerant character of FeCuZr alloys as well as the occurrence of magnetovolume effects characterized by a strong dependence of the 3d band on the Fe-Fe nearest neighbor distances.

Element-specific characterization of heterogeneous magnetism in (Ga,Fe)N films

We employ x-ray spectroscopy to characterize the distribution and magnetism of particular alloy constituents in (Ga,Fe)N films grown by metal organic vapor phase epitaxy. Furthermore, photoelectron microscopy gives direct evidence for the aggregation of Fe ions, leading to the formation of Fe-rich nanoregions adjacent to the samples surface. A sizable x-ray magnetic circular dichroism (XMCD) signal at the Fe L-edges in remanence and at moderate magnetic fields at 300 K links the high temperature ferromagnetism with the Fe(3d) states. The XMCD response at the N K-edge highlights that the N(2p) states carry considerable spin polarization. We conclude that FeN{\delta} nanocrystals, with \delta > 0.25, stabilize the ferromagnetic response of the films.

Examining the chemistry and magnetism of magnetotactic bacterium Candidatus Magnetovibrio blakemorei strain MV-1 using scanning transmission X-ray microscopy

Scanning transmission X-ray microscopy (STXM) measurements at the CK, O K and Fe L3 edges were used to study the magnetotactic bacterium Candidatus Magnetovibrio blakemorei strain MV-1 on an individual cell basis. Improved data acquisition methodologies resulting in higher quality results are presented. Visualization of magnetosomes from their O K-edge signal is demonstrated. The Fe L3 X-ray magnetic circular dichroism (XMCD) signal was used to characterize the magnetic properties of several MV-1 cells on an individual magnetosome basis. The absolute magnetic moment from two cells was evaluated and found to be 0.93(6) of that of saturated abiotic magnetite, or 3.6(2) μB. Previously observed excess Fe (II) was not found in this study, suggesting the prior observation (Lam et al., Chem. Geol 270 (2010) 110) may have been a radiation damage artifact.

Theoretical analysis of X-ray magnetic circular dichroism at the Yb L2, 3 absorption edges of YbInCu4 in high magnetic fields around the field-induced valence transition

High-magnetic-field X-ray absorption spectra (XAS) and its X-ray magnetic circular dichroism (XMCD) at the Yb L2, 3 edges of YbInCu4 are calculated around the field-induced valence transition at about 30 T. The calculations are made by using a new theoretical framework with an extended single impurity Anderson model (SIAM) developed recently by the present author. Two parameters in SIAM, the 4f level and the hybridization strength, are taken as different values in low- and high-magnetic-field phases of the field-induced valence transition. The calculated results are compared with recent experimental data measured by Matsuda et al. by utilizing a miniature pulsed magnet up to 40 T. The field-dependence of the calculated XMCD spectra is explained in detail on the basis of the field-dependence of the Yb 4f wavefunctions in the ground state. Some possibilities are discussed on the negative XMCD signal observed experimentally at the L2 edge.

방사광을 이용한 FeV2O4스피넬 산화물의 덩치상태와 분말상태의 전자구조 차이 연구

연 x선 광흡수 분광법(soft x-ray absorption spectroscopy: XAS)과 연 x선 자기 원편광 이색성(soft x-ray magnetic circular dichroism: XMCD)을 이용하여 스피넬 준강자성 산화물인 <TEX>$FeV_2O_4$</TEX>의 전자 구조를 연구하였다. Fe 2p XAS와 V 2p XAS 측정으로 부터 <TEX>$FeV_2O_4$</TEX>에서 Fe 이온과 V이온의 고유한 원자가는 각각 약 <TEX>$Fe^{2.3+}$</TEX>의 혼합원자가 상태와 약 <TEX>$V^{3+}$</TEX>의 상태임을 알 수 있었다. 실험적으로 측정된 Fe 2p XMCD의 신호는 거의 <TEX>$Fe^{2+}$</TEX> 상태에서 기인하였으며, <TEX>$Fe^{3+}$</TEX> 상태는 Fe 2p XMCD의 신호에 거의 기여하지 않는다는 사실이 발견되었다. 그러므로 <TEX>$FeV_2O_4$</TEX>의 자성 특성을 결정함에 있어서 자기적으로 정렬된 <TEX>$Fe^{2+}$</TEX> 상태 이온들이 중요한 역할을 한다고 생각된다. The electronic structure of ferrimagnetic spinel oxide of <TEX>$FeV_2O_4$</TEX> has been investigated by employing soft x-ray absorption spectroscopy (XAS) and soft x-ray magnetic circular dichroism (XMCD). The Fe 2p and V 2p XAS spectra show that the valence states of Fe and V ions are <TEX>${\sim}Fe^{2.3+}$</TEX> mixed-valent states and <TEX>${\sim}V^{3+}$</TEX> states, respectively. In Fe 2p XMCD spectra, finite XMCD signals are observed for divalent <TEX>$Fe^{2+}$</TEX> states only, but not for <TEX>$Fe^{3+}$</TEX> states. This finding indicates that the magnetic moments of <TEX>$Fe^{2+}$</TEX> ions are ordered ferromagnetically but that those of <TEX>$Fe^{3+}$</TEX> ions are cancelled, implying that <TEX>$Fe^{2+}$</TEX> ions play an important role in determining magnetic properties of <TEX>$FeV_2O_4$</TEX>.

Precise Chemical, Electronic, and Magnetic Structure of Binuclear Complexes Studied by Means of X-ray Spectroscopies and Theoretical Methods

We investigate two planar complexes MnNi and CoNi (see Scheme 1) by X-ray photoelectron spectroscopy (XPS) and ultralow-temperature X-ray magnetic circular dichroism (XMCD). In this way the valence states as well as the presence of uncompensated magnetic moments are obtained. The magnetism has been probed at a temperature of 0.6 K in order to reveal the magnetic ground state properties. We find that divalent Ni ions are in a diamagnetic low spin ground state in both complexes; however, in MnNi a small fraction of divalent nickel high-spin ions leads to a residual XMCD signal, indicating parallel spin alignment with the Mn spins. Mn and Co are found to be in a divalent high-spin configuration in both compounds. Theoretically, we address the energetic ordering of the different possible spin states of the binuclear complexes using (zeroth-order) relativistic approximation density functional calculations and a triple-ζ quality basis set. These results show that intermediate-spin states are often favored over ...