Soft X-ray Magnetic Circular Dichroism Research Articles

Direct Evidence of Simultaneous Reversal of Ferrimagnetically Coupled Sm 4f and Mn 3d Angular Momenta in SmMnO3

Using the soft X-ray magnetic circular dichroism (XMCD) technique, we investigated the magnetic states of the Sm 4f and the Mn 3d moments in a Neel N-type ferrimagnet, SmMnO3, which exhibits a striking magnetocapacitive effect around the compensation temperature (Tcomp ≈ 9.4 K). The XMCD results show that the Sm 4f and the Mn 3d moments were always aligned antiparallel to each other and that, upon sweeping a magnetic field, the angular momenta of Sm 4f and Mn 3d were simultaneously reversed at the field where the magnetocapacitive effect was observed. This indicates that the magnetocapacitive effect of SmMnO3 is induced by a simultaneous reversal of Sm 4f and Mn 3d angular momenta, i.e., magnetization reversal. We discuss a plausible origin of the magnetocapacitive effect in terms of the p-d hybridization mechanism.

Magnetic anisotropy in Cr2GeC investigated by X-ray magnetic circular dichroism and ab initio calculations

The magnetism in the inherently nanolaminated ternary MAX-phase Cr2GeC is investigated by element-selective, polarization and temperature-dependent, soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The measurements indicate an antiferro-magnetic Cr-Cr coupling along the c-axis of the hexagonal structure modulated by a ferromagnetic ordering in the nanolaminated ab-basal planes. The weak chromium magnetic moments are an order of magnitude stronger in the nanolaminated planes than along the vertical axis. Theoretically, a small but notable, non-spin-collinear component explains the existence of a non-perfect spin compensation along the c-axis. As shown in this work, this spin distortion generates an overall residual spin moment inside the unit cell resembling that of a ferri-magnet. Due to the different competing magnetic interactions, electron correlations and temperature effects both need to be considered to achieve a correct theoretical description of the Cr2GeC magnetic properties.

Rare-earth-doped Bi2X3 (X = Se, Te) as candidates for magnetic topological insulators

ABSTRACTElectronic structures of rare-earth (R)-doped (; ) single crystals, which are promising candidates for magnetic topological insulators, have been investigated by employing soft X-ray absorption spectroscopy (XAS), soft X-ray magnetic circular dichroism (XMCD), and circular dichroism (CD) angle-resolved photoemission spectroscopy (ARPES). R XAS and XMCD measurements provide evidence that R ions are trivalent and magnetic, confirming that doped R ions have purely magnetic effects. ARPES measurements of show a finite energy gap for x>0, arising from the broken time-reversal symmetry due to magnetic R impurities. CD-ARPES measurements of reveal finite CD effects in the topological surface states. This work demonstrates that () are magnetic topological insulators.

Angle-resolved photoemission spectroscopy study of magnetic topological insulator candidates of Ce-doped Bi2–xCexSe3

The electronic structures of the magnetic topological insulator candidates of Ce-substituted Bi2–xCexSe3 (x ≤ 0.12) have been investigated through angle-resolved photoemission spectroscopy (ARPES), soft X-ray absorption spectroscopy (XAS), and soft X-ray magnetic circular dichroism (XMCD) measurements on high-quality single crystals. Ce 3d XAS and XMCD measurements provide evidence that the substituted Ce ions in Bi2–xCexSe3 are trivalent and magnetic, confirming the purely magnetic effect of the Ce-substitution. ARPES measurements reveal the formation of an energy gap (Δ) for x > 0, indicating that the finite Δ arises from the broken time-reversal symmetry due to the substituted magnetic Ce impurities.

Temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on first-order reversal curve analysis

A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 °C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well confirmed by using the soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well discussed by the temperature dependent local demagnetization field and the saturation field of multidomain state. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.

Observation of orbital angular momentum in the chiral magnet CrNb3S6 by soft x-ray magnetic circular dichroism

The chiral magnet ${\mathrm{CrNb}}_{3}{\mathrm{S}}_{6}$ with its solitonic objects has novel magnetic and transport properties, in which the spin-orbit coupling (SOC) plays a central role. Aiming to address the possible existence of orbital moments driven by SOC, we perform soft x-ray magnetic circular dichroism spectroscopy at the Cr ${L}_{2,3}$ edges with in-plane magnetization. The dichroic signals provide direct experimental evidence that the Cr orbital magnetic moment is not quenched and is coupled antiparallel to the spin counterpart. Application of the orbital sum rule reveals that the magnitude of the Cr orbital moment is about 1% of the total magnetization. These findings are consistent with the first-principles electronic structure calculations that utilize the Cr $2p$ core radial function to define the Cr local $3d$ quantities. The distinct roles of the atomic SOC among the Cr $3d$ and Nb $4d$ states are discussed.

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

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

Angular Momentum Flow During Ultrafast Demagnetization of a Ferrimagnet.

One of the key processes setting the speed of the ultrafast magnetization phenomena is the angular momentum transfer from and into the spin system. However, the way the angular momentum flows during ultrafast demagnetization and magnetization switching phenomena remains elusive so far. We report on time-resolved soft x-ray magnetic circular dichroism measurements of the ferrimagnetic GdFeCo alloy allowing us to record the dynamics of elemental spin and orbital moments at the Fe and Gd sites during femtosecond laser-induced demagnetization. We observe a complete transfer of spin and orbital angular momentum to the lattice during the first hundreds of femtoseconds of the demagnetization process.

Temperature Dependent Magnetization Reversal Process of a Ga-Doped Nd-Fe-B Sintered Magnet Based on First-Order Reversal Curve Analysis

A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well verified using soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well explained by the temperature dependent saturation field of multidomain grains. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.

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.

Observation of Twisted Magnetization at Ferromagnet/Antiferromagnet Interface by Means of Polarized Neutron Reflectivity and Soft X-ray Depth-resolved X-ray Magnetic Circular Dichroism

Observation of Twisted Magnetization at Ferromagnet/Antiferromagnet Interface by Means of Polarized Neutron Reflectivity and Soft X-ray Depth-resolved X-ray Magnetic Circular Dichroism

Observation of the magnetoelectric reversal process of the antiferromagnetic domain

We investigated the switching process of the perpendicular exchange bias, which is driven by the magnetoelectric effect, by conducting magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. Isothermal and simultaneous application of magnetic and electric fields switches the perpendicular exchange bias polarity. The switching process proceeds by the nucleation and growth of reversed domains. The correspondence among the ferromagnetic/antiferromagnetic domains and exchange bias polarity indicates that interfacial antiferromagnetic spin/domain reversal is responsible for the magnetoelectric switching of the perpendicular exchange bias polarity.

Antiferromagnetic domain wall creep driven by magnetoelectric effect

We observed the magnetoelectric induced domain wall propagation in a Pt/Co/Au/Cr2O3/Pt stacked thin film based on magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. The antiferromagnetic (Cr2O3) domain wall velocity was estimated by a quasi-static approach using a pulsed voltage. At a pulse voltage amplitude of −12 V, corresponding to an electric field of −8.0 × 102 kV/cm, the domain wall velocity was very low, at 0.3 m/s. The domain wall velocity increased with increasing voltage amplitude, reaching 22 m/s at −20 V (−1.3 × 103 kV/cm). The change in the domain wall velocity with the applied voltage amplitude indicates the creep motion of the domain wall. Using a phenomenological model, we estimated the domain wall depinning energy, and found that the bulk and interface terms of the magnetic anisotropy affect the effective magnetic field to the same degree, suggesting that the magnetic domain wall motion may be controllable by the antiferromagnetic layer thickness.

Photoemission and soft X-ray absorption spectroscopy study of Gd-substituted thermoelectric and topological Bi2Te3

The electronic structures of Gd-substituted Bi2−xGdxTe3 single crystals have been investigated by employing synchrotron-radiation spectroscopies, such as soft X-ray absorption spectroscopy (XAS), soft X-ray magnetic circular dichroism (XMCD), photoemission spectroscopy (PES), and scanning photoelectron microscopy (SPEM). SPEM measurements confirm that Gd ions are substituted homogeneously in Bi2−xGdxTe3 crystals. The measured Gd 3d XAS and XMCD spectra provide evidence that the substituted Gd ions are trivalent and magnetic. The hν-dependence of the valence-band (VB) PES spectra are described well by the hν-dependent contributions of the Te 5p and Bi 6p states. A good agreement is found between the measured VB PES spectra of Bi2−xGdxTe3 and the calculated partial densities of states, revealing that the Te 5p states will play an important role in determining the thermoelectric and topological properties of Bi2−xGdxTe3.

Cation distribution and magnetic properties in ultrathin(Ni1–xCox)Fe2O4(x=0–1)layers on Si(111) studied by soft x-ray magnetic circular dichroism

We study the electronic structure and the magnetic properties of epitaxial (Ni1-xCox)Fe2O4(111) layers (x = 0 - 1) with thicknesses d = 1.7 - 5.2 nm grown on Al2O3(111)/Si(111) structures, to achieve a high value of inversion parameter y, which is the inverse-to-normal spinel-structure ratio, and hence to obtain good magnetic properties even when the thickness is thin enough for electron tunneling as a spin filter. We revealed the crystallographic (octahedral Oh or tetrahedral Td) sites and the valences of the Fe, Co, and Ni cations using experimental soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra and configuration-interaction cluster-model calculation. In all the (Ni1-xCox)Fe2O4 layers with d = about 4 nm, all Ni cations occupy the Ni2+ (Oh) site, whereas Co cations occupy the three different Co2+ (Oh), Co2+ (Td), and Co3+ (Oh) sites with constant occupancies. According to these features, the occupancy of the Fe3+ (Oh) cations decreases and that of the Fe3+ (Td) cations increases with decreasing x. Consequently, we obtained a systematic increase of y with decreasing x and achieved the highest y value of 0.91 for the NiFe2O4 layer with d = 3.5 nm. From the d dependences of y and magnetization in the d range of 1.7 - 5.2 nm, a magnetically dead layer is present near the NiFe2O4/Al2O3 interface, but its influence on the magnetization was significantly suppressed compared with the case of CoFe2O4 layers reported previously [Y. K. Wakabayasi et al., Phys. Rev. B 96, 104410 (2017)], due to the high site selectivity of the Ni cations. Since our epitaxial NiFe2O4 layer with d = 3.5 nm has a high y values (0.91) and a reasonably large magnetization (180 emu/cc), it is expected to exhibit a strong spin filter effect, which can be used for efficient spin injection into Si.

Ultralow-temperature device dedicated to soft X-ray magnetic circular dichroism experiments.

A new ultralow-temperature setup dedicated to soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) experiments is described. Two experiments, performed on the DEIMOS beamline (SOLEIL synchrotron), demonstrate the outstanding performance of this new platform in terms of the lowest achievable temperature under X-ray irradiation (T = 220 mK), the precision in controlling the temperature during measurements as well as the speed of the cooling-down and warming-up procedures. Moreover, owing to the new design of the setup, the eddy-current power is strongly reduced, allowing fast scanning of the magnetic field in XMCD experiments; these performances lead to a powerful device for X-ray spectroscopies on synchrotron-radiation beamlines facilities.

First-order reversal curve analysis of a Nd-Fe-B sintered magnet with soft X-ray magnetic circular dichroism microscopy

First-order reversal curve (FORC) diagram, which visualizes the variation of magnetic susceptibility on a field plane, has been applied to a Nd-Fe-B sintered magnet. The FORC diagram exhibits the characteristic behavior of two remarkable spots in low-field and high-field regions. The high-field FORC spot corresponds to the irreversible magnetization reversal at a coercive field, whereas the low-field FORC spot indicates the appearance of a large magnetic susceptibility state during the demagnetization process. Moreover, this low-field FORC spot becomes dominant at high temperature, accompanied by a significant reduction in coercivity. These results suggest that the low-field FORC spot has a strong correlation with the degradation of magnetic properties of a Nd-Fe-B sintered magnet. To clarify the actual magnetization reversal processes corresponding to these two FORC spots, soft X-ray magnetic circular dichroism (XMCD) microscopy observation was employed with similar field sequences of the FORC measurements. Consequently, the low-field FORC spot is mainly attributed to the domain wall motion in multi-domain grains, whereas the high-field FORC spot corresponds to the magnetization reversal of single-domain grains. These indicate that a FORC diagram is a powerful evaluation method for the magnetization reversal processes of permanent magnets.

Magnetic and electronic properties of the ferroelectric-photovoltaic ordered double perovskiteCaMnTi2O6investigated by x-ray absorption spectroscopies

The ferroelectric and magnetic phases of the double perovskite $\mathrm{CaMnT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ with $A$-site order have been investigated by soft x-ray absorption and magnetic circular dichroism. All spectra point to a very ionic state of divalent Mn and tetravalent Ti atoms. The effects of the crystal field produced by O ligands around tetravalent titanium and the dissimilar Mn1 and Mn2 sites were investigated. Both the so-called square-planar and the octahedrally coordinated Mn sites spectroscopically contribute in a rather similar way, with little influence by the oxygen environment. Multiplet calculations suggest a small $O\phantom{\rule{0.16em}{0ex}}2p\text{\ensuremath{-}}\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}3d$ charge-transfer component in the FE phase. Magnetic symmetry calculations were performed to determine probable configurations of Mn spins compatible with the acentric $P{4}_{2}mc$ structure and, in combination with the computational magnetic results in Inorg. Chem. 56, 11854 (2017), we have identified the $P{4}_{2}^{\ensuremath{'}}{m}^{\ensuremath{'}}c$ as the most likely magnetic space group keeping invariant the unit cell below ${T}_{\mathrm{N}}$. This symmetry forces the sign of the magnetic coupling along the Mn columns parallel to $c$ to reverse with respect to the coupling between neighboring columns. Below ${T}_{\mathrm{N}}$, the dichroic magnetization loops at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{3}$ edge confirm the absence of spontaneous ferromagnetism, although a very small field-induced spin polarization was detected in the sample.

Element-specific observation of the ferromagnetic ordering process in UCoAl via soft x-ray magnetic circular dichroism

We have performed soft x-ray magnetic circular dichroism (XMCD) experiments on the itinerant-electron metamagnet UCoAl at the U $4d\ensuremath{-}5f({N}_{4,5})$ and Co $2p\ensuremath{-}3d({L}_{2,3})$ absorption edges in order to investigate the magnetic properties of the U $5f$ and Co $3d$ electrons separately. From the line shape of the XMCD spectrum, it is deduced that the orbital magnetic moment of the Co $3d$ electrons is unusually large. Through the systematic temperature $(T)$- and magnetic field $(H)$-dependent XMCD measurements, we have obtained two types of the magnetization curve as a function of $H$ and $T$ (M-H curve and M-T curve, respectively). The metamagnetic transition from a paramagnetic state to a field-induced ferromagnetic state was clearly observed under 15 K at ${H}_{\mathrm{M}}$. The value of the ${H}_{\mathrm{M}}$ and its $T$ dependence agree well between the U and Co sites, and the bulk magnetization. Whereas, we have discovered the remarkable differences in the M-H and M-T curves between the U and Co sites. The present findings clearly show that the role of the Co $3d$ electrons should be considered more carefully in order to understand the origin of the magnetic ordering in UCoAl.

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.