Depth-resolved X-ray Magnetic Circular Dichroism Research Articles

Determination of anisotropy in magnetic moments at the interface of Au/Co/Au thin film without a thickness-dependent experiment

Anisotropy in the magnetic moments at the interface of a Co thin film sandwiched by Au layers is determined without thickness-dependent experiment, by using the depth-resolved x-ray magnetic circular dichroism (XMCD) technique, in which the fluorescence soft x rays are collected at different detection angles. By combining the depth-resolved XMCD technique with incidence angle-dependent XMCD sum rules, the spin magnetic moment, perpendicular and in-plane orbital magnetic moments, and perpendicular and in-plane magnetic dipole moments are separately estimated at the interface and inner layers of the Co thin film. It is revealed that the perpendicular orbital magnetic moment is significantly larger than the in-plane one at the interface, while the inner Co layers exhibit a small opposite tendency. It is verified that the interfacial Co has large interfacial anisotropy in the orbital magnetic moment even in the relatively thick-film region.

Electron Correlation Enhances Orbital Polarization at a Ferromagnetic Metal/Insulator Interface: Depth-Resolved X-ray Magnetic Circular Dichroism and First-Principles Study

The Fe(CoB)/MgO interface is vital to spintronics as it exhibits the tunneling magnetoresistance (TMR) effect and interfacial perpendicular magnetic anisotropy (PMA) simultaneously. To further enhance TMR and PMA for the development of high-density magnetoresistive random access memory, it is essential to clarify the behavior of Fe atoms interfaced with MgO. This study reveals that the spin and orbital magnetic moments of Fe are enhanced at the Fe/MgO interface. The enhancement in the orbital magnetic moment is much more significant than that predicted by the standard density functional theory. Theoretical calculations based on the orbital polarization correction reproduce this enhancement, the origin of which is attributed to the electron–electron correlation resulting from electron localization at the Fe/MgO interface. The present findings highlight the importance of electron–electron correlation at ferromagnet/oxide interfaces, which has often been disregarded in spintronics.

Unveiling a Chemisorbed Crystallographically Heterogeneous Graphene/L10-FePd Interface with a Robust and Perpendicular Orbital Moment.

A crystallographically heterogeneous interface was fabricated by growing hexagonal graphene (Gr) using chemical vapor deposition (CVD) on a tetragonal FePd epitaxial film grown by magnetron sputtering. FePd was alternately arranged with Fe and Pd in the vertical direction, and the outermost surface atom was identified primarily as Fe rather than Pd. This means that FePd has a high degree of L10-ordering, and the outermost Fe bonds to the carbon of Gr at the interface. When Gr is grown by CVD, the crystal orientation of hexagonal Gr toward tetragonal L10-FePd selects an energetically stable structure based on the van der Waals (vdW) force. The atomic relationship of Gr/L10-FePd, which is an energetically stable interface, was unveiled theoretically and experimentally. The Gr armchair axis was parallel to FePd [100]L10, where Gr was under a small strain by chemical bonding. Focusing on the interatomic distance between the Gr and FePd layers, the distance was theoretically and experimentally determined to be approximately 0.2 nm. This shorter distance (≈0.2 nm) can be explained by the chemisorption-type vdW force of strong orbital hybridization, rather than the longer distance (≈0.38 nm) of the physisorption-type vdW force. Notably, depth-resolved X-ray magnetic circular dichroism analyses revealed that the orbital magnetic moment (Ml) of Fe in FePd emerged at the Gr/FePd interface (@inner FePd: Ml = 0.16 μB → @Gr/FePd interface: Ml = 0.32 μB). This interfacially enhanced Ml showed obvious anisotropy in the perpendicular direction, which contributed to interfacial perpendicular magnetic anisotropy (IPMA). Moreover, the interfacially enhanced Ml and interfacially enhanced electron density exhibited robustness. It is considered that the shortening of the interatomic distance produces a robust high electron density at the interface, resulting in a chemisorption-type vdW force and orbital hybridization. Eventually, the robust interfacial anisotropic Ml emerged at the crystallographically heterogeneous Gr/L10-FePd interface. From a practical viewpoint, IPMA is useful because it can be incorporated into the large bulk perpendicular magnetic anisotropy (PMA) of L10-FePd. A micromagnetic simulation assuming both PMA and IPMA predicted that perpendicularly magnetized magnetic tunnel junctions (p-MTJs) using Gr/L10-FePd could realize 10-year data retention in a small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively. We unveiled the energetically stable atomic structure in the crystallographically heterogeneous interface, discovered the emergence of the robust IPMA, and predicted that the Gr/L10-FePd p-MTJ is significant for high-density X nm generation magnetic random-access memory (MRAM) applications.

Graphene/Half-Metallic Heusler Alloy: A Novel Heterostructure toward High-Performance Graphene Spintronic Devices.

Graphene-based vertical spin valves (SVs) are expected to offer a large magnetoresistance effect without impairing the electrical conductivity, which can pave the way for the next generation of high-speed and low-power-consumption storage and memory technologies. However, the graphene-based vertical SV has failed to prove its competence due to the lack of a graphene/ferromagnet heterostructure, which can provide highly efficient spin transport. Herein, the synthesis and spin-dependent electronic properties of a novel heterostructure consisting of single-layer graphene (SLG) and a half-metallic Co2 Fe(Ge0.5 Ga0.5 ) (CFGG) Heusler alloy ferromagnet are reported. The growth of high-quality SLG with complete coverage by ultrahigh-vacuum chemical vapor deposition on a magnetron-sputtered single-crystalline CFGG thin film is demonstrated. The quasi-free-standing nature of SLG and robust magnetism of CFGG at the SLG/CFGG interface are revealed through depth-resolved X-ray magnetic circular dichroism spectroscopy. Density functional theory (DFT) calculation results indicate that the inherent electronic properties of SLG and CFGG such as the linear Dirac band and half-metallic band structure are preserved in the vicinity of the interface. These exciting findings suggest that the SLG/CFGG heterostructure possesses distinctive advantages over other reported graphene/ferromagnet heterostructures, for realizing effective transport of highly spin-polarized electrons in graphene-based vertical SV and other advanced spintronic devices.

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 an electric field-induced interface redox reaction and magnetic modification in GdOx/Co thin film by means of depth-resolved X-ray absorption spectroscopy.

We study an electric field-induced redox reaction at the interface of GdOx/Co thin film, by means of soft X-ray absorption spectroscopy (XAS). The fluorescence-yield depth-resolved XAS analysis reveals that the interfacial Co layer at GdOx/Co is composed of ∼50% Co oxide when the negative field is applied, while metallic Co is dominant in the case of the positive field. We suppose that the interfacial layer is oxidized by oxygen migration from the GdOx layer with the negative field while the interfacial layer shows the metallic state with the positive field, which means that the redox reaction is induced by the electric field. In addition, it is found from the X-ray magnetic circular dichroism (XMCD) measurement that the orbital magnetic moment of Co is larger when the negative electric field is applied to the film, as compared to the positive field. Moreover, the depth-resolved XMCD analysis reveals that the interfacial Co layer shows no or little magnetization regardless of the electric field, while for the inner layer, an increase of the orbital magnetic moment is suggested with the negative field. The field-induced magnetic modification could be attributed to the change of the orbital moment in the inner Co layer due to interfacial modifications. We thus succeed in the direct observation of the redox reaction-induced change in the interface magnetic state.

Depth-resolved X-ray magnetic circular dichroism measurement by a multi-anode microchannel plate detector combined with polarization switching

A depth-resolved X-ray magnetic circular dichroism (XMCD) technique has been developed in the soft X-ray region by combining a multi-anode microchannel plate detector with polarization switching between right and left circular polarizations. A simultaneous measurement for 30-channel anodes has been achieved with a continuous data acquisition rate of 1 kHz, which is fast enough for 10 Hz polarization switching. This enables us to realize the layer-resolved XMCD analysis with a high signal-to-noise ratio.

Depth-resolved magnetic and structural analysis of relaxing epitaxialSr2CrReO6

Structural relaxation in a ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$ epitaxial film, which exhibits strong spin-orbit coupling, leads to depth-dependent magnetism. We combine two depth-resolved synchrotron x-ray techniques---two-dimensional reciprocal space mapping and x-ray magnetic circular dichroism---to quantitatively determine this effect. An 800-nm-thick film of ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$, grown with tensile epitaxial strain on ${\mathrm{SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_{3}(225\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/{\text{(LaAlO}}_{3}{)}_{0.3}({\text{Sr}}_{2}{\mathrm{AlTaO}}_{6}{)}_{0.7}$, relaxes away from the ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}{\text{/SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_{3}$ interface to its bulk lattice parameters, with much of the film being fully relaxed. Grazing incidence x-ray diffraction of the film elucidates the in-plane strain relaxation near the film-substrate interface, while depth-resolved x-ray magnetic circular dichroism at the Re L edge reveals the magnetic contributions of the Re site. The smooth relaxation of the film near the interface correlates with changes in the magnetic anisotropy. This provides a systematic and powerful way to probe the depth-varying structural and magnetic properties of a complex oxide with synchrotron-source x-ray techniques.

Twisted magnetic structure in ferromagnetic ultrathin Ni films induced by magnetic anisotropy interaction with antiferromagnetic FeMn

A twisted magnetic structure in Ni ultrathin films attached to antiferromagnetic FeMn is revealed by a combination of the depth-resolved x-ray magnetic circular dichroism (XMCD) and the polarized neutron reflectivity (PNR) techniques. The depth-resolved XMCD at remanent magnetization shows that the perpendicular magnetization component in the Ni film decreases around the interface to FeMn when the film exhibits perpendicular magnetization, whereas the in-plane component is kept constant through the whole film in the case of in-plane magnetization. Moreover, the PNR data shows that when a weak in-plane magnetic field is applied to a film, which exhibits perpendicular magnetization at the remanent state, an in-plane magnetization component is induced in Ni around the interface to FeMn. These results are reasonably interpreted by assuming that the magnetic moment in the Ni layer is twisted from the perpendicular to the in-plane directions towards the interface to FeMn. Such a magnetic structure is supposed to be induced by a magnetic anisotropy interaction at the interface between ferromagnetic Ni and antiferromagnetic FeMn, which makes the Ni magnetic moment around the interface rotate towards the in-plane direction.

Temperature dependence of remanent magnetization of thin films at the interface to a nonmagnetic material: Cu/Ni/Cu(100)

Temperature dependence of remanent magnetization of Ni thin films at the interface to Cu is investigated by means of the depth-resolved x-ray magnetic circular dichroism technique. Although magnetization at the interface is smaller than that in the inner layers, no detectable difference in Curie temperature, at which remanent magnetization vanishes, is found between the interface and the inner layers. In contrast, the magnetization-temperature curves at the interface and in the inner layers show a small difference, which cannot be explained by using a simple mean-field theory if the interface layer is assumed to have the same exchange-coupling constant as that in the inner layers. It is also shown that the critical exponent for the in-plane magnetized film is significantly larger than that for the perpendicularly magnetized film due to the difference in the degree of freedom for the magnetic moment.

Observation of magnetic moments at the interface region in magnetic tunnel junctions using depth-resolved x-ray magnetic circular dichroism

The depth-resolved x-ray magnetic circular dichroism (XMCD) technique was applied to observe the interfacial and inner magnetic moments in CoFe/MgO and Co${}_{2}$MnSi(CMS)/MgO. The magnetic moments of the real interface region (two monolayers from the interface) and the inner layers were separately analyzed using the XMCD sum rules. The observed interfacial moments of Co at the CMS/MgO interface show a remarkable reduction from that in the inner layers compared with the CoFe/MgO structure, suggesting small exchange stiffness in the CMS at the MgO interface. The weak exchange stiffness of Co is a possible reason for the large temperature dependence of tunnel magnetoresistance in CMS-based magnetic tunnel junctions.

Sub-nm resolution depth profiling of the chemical state and magnetic structure of thin films by a depth-resolved X-ray absorption spectroscopy technique

Development and recent progress of a depth-resolved X-ray absorption spectroscopy (XAS) technique are presented, together with future prospects. The technique has been developed by controlling the probing depth of the electron-yield XAS data, which depends on the electron emission angle. This novel technique enables us to achieve depth profiling of the magnetic structure of thin films with a sub-nm depth resolution by using X-ray magnetic circular dichroism (XMCD) in X-ray absorption, which provides quantitative information on the element-specific spin and orbital magnetic moments. The chemical state and electronic structure at the surface and interface are also investigated by depth-resolved XAS analysis. As for future prospects, a three-dimensional micro XAS technique is being developed by combining an X-ray microbeam with depth-resolved XAS. Moreover, it is expected to manipulate magnetic anisotropy by using element-specific and depth-resolved magnetic anisotropy energies obtained from the depth-resolved XMCD to design thin films and multilayers with proper elements and proper thicknesses. The observation of the spin dynamics at the interface will be also possible in future by adopting the pump-probe method.

Sub-nm resolution depth profiling of the magnetic structure of thin films by the depth-resolved x-ray magnetic circular dichroism technique

The principle and applications of the depth-resolved x-ray magnetic circular dichroism (XMCD) technique are demonstrated, by which depth profiling of the magnetic structure of thin films is achieved with sub-nm resolution. The formation of a NiO-like layer at the surface of a Ni/Cu(1 0 0) thin film is revealed by this technique. Moreover, a small uncompensated magnetic moment in the surface layer is suggested, which shows an antiparallel coupling to that in the inner Ni layers. Development of a three-dimensional XMCD technique is also demonstrated, in which an x-ray microbeam is combined with the depth-resolved XMCD. A preliminary study on the magnetic anisotropy of Fe/Ni/Cu(1 0 0) is shown using this new technique.

Depth profiling of magnetic and atomic structures of ultrathin films by depth-resolved XMCD and XAFS techniques with a sub-nm depth resolution

Recent progresses in the depth-resolved X-ray magnetic circular dichroism (XMCD) and X-ray absorption fine structure (XAFS) techniques are presented. The depth-resolved atomic and magnetic structures of Co films grown on Ru(0001) was investigated by using the depth-resolved XMCD and extended X-ray absorption fine structure EXAFS. The three-dimensional magnetic structure of a micro-patterned Fe/Ni/Cu(100) film was studied by combining the depth-resolved XMCD with the X-ray microbeam. The technique was also applied to Co thin films covered with a relatively thick Mo overlayer, in order to demonstrate the possibility to investigate ex-situ samples with a protection layer.

Depth-Resolved XMCD Study of Ultrathin Mo/Co/Au Films

The effect of Mo overlayer on the magnetism of ultrathin Co films grown on Au(111) was studied. The structure and magnetism at the Mo/Co interface were studied by means of depth-resolved X-ray magnetic circular dichroism. The magnetic moment of Co near the Mo/Co interface was reduced in comparison to the inner part of the film for the samples which have in-plane anisotropy. This effect is more prominent in the case of Mo overlayer with higher roughness, and can be attributed to Co–Mo intermixing at the interface.

Spin reorientation transitions of [formula omitted] studied by using the depth-resolved X-ray magnetic circular dichroism technique

The spin reorientation transition (SRT) of Ni / Cu ( 0 0 1 ) induced by Fe deposition was investigated using the X-ray magnetic circular dichroism (XMCD) method. In-plane magnetized Ni films ( ⩽ 9 ML ) undergo the SRT twice; first, a small amount ( < 1 ML ) of Fe deposition causes a transition to perpendicular magnetization. Second, further Fe deposition (1–2 ML in total) causes a return to in-plane magnetization. Perpendicularly magnetized Ni films ( ⩾ 10 ML ) also exhibit a transition to in-plane by 1–2 ML Fe deposition. A precise magnetic anisotropy phase diagram was obtained using a combination of wedge-shaped Ni samples and stepwise Fe deposition. Magnetic anisotropy energies in the bulk, surface and interface layers of Ni films were separately determined using the depth-resolved XMCD technique, while values in the 1 ML and 2 ML portions of the Fe films were obtained from the conventional XMCD measurements. The origin of the SRTs is successfully explained with a simple phenomenological layer model using the obtained magnetic anisotropy energies.

Spin-reorientation transition ofNi∕Cu(100)andCO∕Ni∕Cu(100): Separation of the surface and bulk components of the x-ray magnetic circular dichroism spectrum

The spin-reorientation transition of $\mathrm{Ni}∕\mathrm{Cu}(100)$ and $\mathrm{CO}∕\mathrm{Ni}∕\mathrm{Cu}(100)$ films was investigated with the depth-resolved x-ray magnetic circular dichroism (XMCD) technique. The XMCD spectra from the surface and inner layers were separately extracted. As for the bare Ni films, the in-plane orbital magnetic moment in the surface layer is significantly enhanced. In contrast, the inner layers exhibit larger perpendicular orbital magnetic moment than the in-plane one. Upon CO adsorption, the surface magnetization is drastically reduced, while the inner layers are unaffected. These results directly explain the spin-reorientation transition in $\mathrm{Ni}∕\mathrm{Cu}(100)$ and $\mathrm{CO}∕\mathrm{Ni}∕\mathrm{Cu}(100)$.

Direct observation of magnetic depth profile with a depth-resolved x-ray magnetic dichroism technique

We have established a high-precision depth-resolved x-ray magnetic circular dichroism (XMCD) technique, in which the probing depth of the partial electron yield XMCD spectra is controlled by changing the detection angle of the emitted electrons. The data quality has been significantly improved by using an imaging-type microchannel plate detector. We applied this new technique to Fe ultrathin films grown on Cu(100), and directly confirmed that the surface two layers of the 7 ML Fe film are ferromagnetically coupled, while the inner layers are in the spin density wave (SDW) state at 130 K. Then we separately extracted the XMCD spectra for the surface ferromagnetic (FM) and inner SDW layers. It was indicated that the SDW magnitude is almost the same as the magnetic moment of the surface FM layers, and that the orbital moment of the surface layers is larger than that of the SDW ones. Moreover, the extracted spectrum for the SDW layers exhibited a bulk-like feature, while the surface spectrum was similar to that from the 2 ML film, in which no bulk component should be included.

Development of a Depth-resolved XMCD Method and Its Application to the Study of Surface and Interface Magnetic Properties of Metal Thin Films

X-ray magnetic circular dichroism (XMCD) is a powerful method to study the magnetic properties of magnetic thin films. However it provides depth-averaged information. Recently, we have developed a simple but effective depth-resolved technique in XMCD by using an imaging type microchannel plate detector. The performance of this method is demonstrated by applying to the depth-resolved analyses of Ni/Cu(100) and the effect of CO adsorption on Ni/Cu(100), Fe/Cu(100) and Fe/Ni/Cu(100) magnetic thin films. For Ni/Cu(100), magnetic orbital moments of the surface and internal layers are extracted to give deep insight for the spin reorientation transitions. For 7 ML Fe/Cu(100), the surface two layers are ferromagnetically coupled, while the inner layers are non-magnetic at 200 K and are in the spin density wave (SDW) state at 130 K. For Fe/Ni/Cu(100), magnetically live surface layers and some thickness-dependent magnetic coupling between the Fe surface and Ni film have been observed.

Direct observation of oscillatory behavior in the surface magnetization ofFethin films grown on aNi∕Cu(100)film

The surface and interface magnetic structures of Fe thin films (4-11 ML) grown on a 6 ML Ni/Cu(100) film have been directly investigated with the depth-resolved x-ray magnetic circular dichroism technique. An oscillatory behavior in the Fe surface magnetization as a function of Fe thickness is found. The spectra were analyzed with a simple magnetic model, in which only surface two layers and interface (bottom) single layer are magnetically active. The analysis clearly shows existence of an oscillatory or rotational interlayer magnetic coupling between the surface and interface of the Fe film via the inner layers with the interface magnetization essentially unchanged. Temperature dependent experiments showed that the surface magnetization vanishes with increasing temperature, while the interface one stays unchanged.