X-ray Resonant Magnetic Reflectivity Research Articles

Interface Fe magnetic moment enhancement in MgO/Fe/MgO trilayers

We model room temperature soft x-ray resonant magnetic reflectivity to determine a 24% increase of the Fe magnetic moment of the 2–3 monolayers next to both MgO interfaces in a MgO(3 nm)/Fe(12 nm)/MgO(001) heterostructure. This direct measurement of such enhanced interface magnetic moments for buried interfaces confirms theoretical predictions and highlights the importance of considering inhomogeneous in-depth magnetic profile in Fe/MgO based magnetic tunnel junctions.

Static Magnetic Proximity Effect in Pt/NiFe2O4 and Pt/Fe Bilayers Investigated by X-Ray Resonant Magnetic Reflectivity.

The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ratio of the reflectivity is measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be (0.6±0.1) μB for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of 0.02 μB per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect.

Noncollinearity of the canted spins across ultrathin Fe films on vicinal Ag surfaces

We investigate the depth dependence of the canted magnetization in a bcc Fe ultrathin film grown on a vicinal Ag substrate. This study is performed for different Fe thicknesses, in the vicinity of the spin reorientation transition, by soft x-ray resonant magnetic reflectivity (SXRMR). Above 5.5 monolayers, the high spatial resolution of SXRMR allows us to describe a gradient of the canted magnetization. The tilt angle is larger near the Ag interface and decreases towards the surface. This noncollinearity is ascribed to a different balance of various anisotropies across the layer thickness. Below that value, a large increase of the tilt angle is observed and ascribed to the spin reorientation transition. Unlike the previous situation, the tilt angle is found to be homogeneous throughout the film. This can be related either to the out-of-plane spin reorientation associated to a reduced number of monolayers or to the $q$-space limitation of the SXRMR technique at the Fe ${L}_{3}$ edge.

Element-specific depth profile of magnetism and stoichiometry at theLa0.67Sr0.33MnO3/BiFeO3interface

Depth-sensitive magnetic, structural and chemical characterization is important in the understanding and optimization of novel physical phenomena emerging at interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant X-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic La0.67Sr0.33MnO3 / multiferroic BiFeO3 bi-layers with sub-nanometer resolution. In particular, the X-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element specific depth profile of the ferromagnetic moments in both the La0.67Sr0.33MnO3 and BiFeO3 layers. Our measurements indicate a magnetically diluted interface layer within the La0.67Sr0.33MnO3 layer, in contrast to previous observations on inversely deposited layers. Additional resonant X-ray reflection measurements indicate a region of an altered Mn- and O-content at the interface, with a thickness matching that of the magnetic diluted layer, as origin of the reduction of the magnetic moment.

Charge and Induced Magnetic Structures of Au Layers in Fe/Au Bilayer and Fe/Au/Fe Trilayer Films by Resonant X-ray Magnetic Reflectivity at the Au L3Absorption Edge

It is known that spin polarization induced in the Au layers of Fe/Au multilayers which show interlayer exchange coupling consists of an interface-concentrated component and a uniformly extended interior component. To investigate the origin of the interior component, magnetic depth profiles of spin polarization induced in the Au layers of Fe (15 A)/Au (50 A) bilayer and Fe (15 A)/Au (50 A)/Fe (15 A) trilayer films were investigated with resonant X-ray magnetic reflectivity (RXMR) measurements at the Au L3 absorption edge. After the determination of charge structures, the spin attenuation length for the interior component of the bilayer sample was evaluated to be longer than 15 A from the analysis of the RXMR pattern. The RXMR pattern of the trilayer sample was not sufficiently reproduced if the magnetic structure of the trilayer sample was assumed to be a simple sum of the magnetic structure obtained from the bilayer sample. Results were discussed based on the quantum well model.

Altered magnetism and new electronic length scales in magneto-electric La2/3Sr1/3MnO3–BiFeO3 heterointerface

We map out the charge-spin density profile of magneto-electric La2/3Sr1/3MnO3 (LSMO)–BiFeO3 (BFO) heterostructure using soft x-ray resonant magnetic reflectivity. We show that the spatial extent of interface orbitals can extend over a few lattice periods even for an atomically sharp interface. While LSMO magnetization is depleted at the interface, BFO does develop a weak magnetic moment mostly near the interface, probably due to a proximity-induced charge-transfer process. Our study reveals that simultaneous control of electronic and magnetic interfaces is essential in realizing the potential of oxide devices.

Magnetization profile across Au-covered bcc Fe films grown on a vicinal surface of Ag(001) as seen by x-ray resonant magnetic reflectivity

We report on the local magnetic properties of ultrathin bcc Fe films grown on a Ag(1,1,6) stepped surface and covered by a Au layer. We determined the structure of the multilayer stack and the depth-resolved magnetic profile for 5.9 and 13.5 monolayer (ML) thick films. The experiments were carried out on magnetically saturated samples by soft x-ray resonant magnetic reflectivity using both circularly and linearly polarized x rays and two different acquisition configurations. Our results indicate a 20$%$ enhancement of the magnetic moment in the first 2--3 ML of Fe at both interfaces. A comparison of the magnetization profile for Fe films grown on a flat Ag(001) surface indicates a very similar effect and does not reveal any difference regarding the method's accuracy.

The influence of surface roughness in X-ray resonant magnetic reflectivity experiments

We present simulations of X-ray resonant magnetic reflectivity (XRMR) spectra of the surface magnetic dead layer in La1−x Sr x MnO3 (LSMO) films that take in account the effect of different forms of roughness that can be encountered experimentally. The results demonstrate a method to distinguish between surface (morphological) roughness, and two generic kinds of magnetic roughness at the buried interface between the surface dead layer and the fully magnetic bulk part of the film. We show that the XRMR technique can distinguish between different types of magnetic roughness at the dead layer/bulk interface only if the sample surface is nearly atomically flat (the morphological roughness is one unit cell or less). Furthermore, to distinguish between the two types of magnetic roughness, the simulations show that fitting of XRMR spectra out to very high incidence angles must be performed. In the specific case of LSMO films with a dead layer with average thickness of 4 unit cells, this corresponds to an incidence angle > 50∘.

Depth-resolved magnetization distribution in ultra thin films by soft X-ray resonant magnetic reflectivity

The analysis of complex magnetic profiles throughout an ultrathin magnetic films by soft X-ray resonant magnetic reflectivity is discussed. Subnanometer resolution can be achieved allowing the separation of interface and inner layer magnetic contributions as well as the determination of antiferromagnetic and non-collinear spin structures. Reflectivity measurements are carried out up to large scattering angles allowing the determination of the depth-resolved profiles of the out-of-plane magnetic component.

Non-collinear magnetic profile in (Rh/Fe1−xCox)2/Rh(001) bilayer probed by polarized soft x-ray resonant magnetic reflectivity

Magnetization configuration in Fe/Rh/Fe0.5Co0.5/Rh(001) can be non-orthogonal due to competition between the anisotropy and the exchange coupling. Soft x-ray resonant magnetic reflectivity allows us to probe the magnetic profile along the growth axis with in-plane and perpendicular magnetization components. By using different modes of acquisition, we show that it is possible to determine separately the magnetic profile of both magnetization components. Measurements at the Co L3-edge enable us to investigate the magnetism of the alloy layer independently from the layer of Fe.

Direct in-depth determination of a complex magnetic configuration in an exchange-coupled bilayer with perpendicular and in-plane anisotropy

By means of soft x-ray resonant magnetic reflectivity, we show that the utilization of different acquisition modes allows the separate analysis of in- and out-of-plane components of a complex magnetic configuration. This technique reveals high sensitivity towards magnetic inhomogeneity, in particular at interfaces. We found that Fe moments (in the layer with in-plane anisotropy) can be locally antiferromagnetically coupled to the layer of perpendicular anisotropy, when the layers are ferromagnetically exchange coupled.

X-ray resonant magnetic reflectivity of stratified magnetic structures: Eigenwave formalism and application to a W/Fe/W trilayer

A formalism for the reflectivity of electromagnetic waves by magnetic materials is presented with an application. It is applied to retrieve the magnetic moment density along the depth of magnetic materials with arbitrary magnetic moment direction using matricial algebra, including roughness between layers. The reflectivity is derived following a classical description with Maxwell equations and a permittivity built from the quantum scattering amplitude. Approximations on the relative power of the Thomson scattering and the magnetic terms are trackable in order to evaluate the validity of the formalism case-by-case, from the optical light regime up to soft and hard X-rays. Eigenwaves are used throughout the whole formalism. In order to illustrate the methodology, we present an application to a W/Fe/W trilayer performed at the Fe L-edge, in the soft X-ray regime.

Uncompensated spins in trilayer CoFe/IrMn/NiFe exchange bias: soft x-ray resonantmagnetic scattering study

We investigated an uncompensated (UC) layer in a magnetic trilayer CoFe/IrMn/NiFeusing soft x-ray resonant magnetic reflectivity. Under a carefully controlled magnetic field,we resolved two UC spins in the single antiferromagnetic (AFM) layer. We found thatthe unpinned spins formed at both sides within the 50 Å thick IrMn AFM layer:tUC1 = 17.8 Å forCoFe/IrMn(1) andtUC2 = 6.4 Å forIrMn(2)/NiFe. Moreover, we noted their different blocking temperatures, namelyTB = 500 K at the UC1 layer and 470 K at the UC2 layer. There might be no noticeable magneticinteraction of the pinned spins of the two separated UC layers. Our results verify thedistinct spatial separation of the two UC layers, despite the limited thickness of the singleAFM layer.

The temperature-dependent magnetization profile across an epitaxial bilayer of ferromagnetic La2/3Ca1/3MnO3 and superconducting YBa2Cu3O7−δ

Epitaxial bilayers of ferromagnetic (FM) La2/3Ca1/3MnO3 (LCMO) and superconducting YBa2Cu3O7−δ (YBCO) have been grown on single-crystalline SrTiO3 (STO) substrates by pulsed laser deposition. The manganese magnetization profile across the FM layer has been determined with high spatial resolution at low temperatures by x-ray resonant magnetic reflectivity (XRMR) performed at the BESSY II synchrotron light source of the Helmholtz Zentrum Berlin. It is found that not only the adjacent superconductor but also the substrate underneath influences the magnetization of the LCMO film at the interface at low temperatures. Both effects can be investigated individually by XRMR.

Vector Magnetization Processes of Individual Magnetic Layers in TM/Ru/TM (TM=Co or Fe) Trilayers with Antiferromagnetic Interlayer Exchange Coupling by Resonant X-ray Magnetic Reflectivity

Magnetization processes of both parallel and orthogonal components in upper and lower magnetic layers are separately investigated in antiferromagnetically coupled Co/Ru/Co, Fe/Ru/Fe, and Fe/Ru/Co trilayers by hysteresis measurements of resonant x-ray magnetic reflectivity at Co and Fe K absorption edges. From the measurements, it is found that antiferromagnetically coupled magnetizations of upper and lower magnetic layers rotate to the opposite directions in the magnetization process. In the case of the Co/Ru/Co and Fe/Ru/Fe trilayers, the dominant rotation direction of the upper magnetic layer is counterclockwise, while in the case of Fe/Ru/Co trilayer, each magnetic layer contains the clockwise and counterclockwise rotation domains in equal probability.

Determination of the magnetic contribution to the YFe2 susceptibility by means of X-ray resonant magnetic reflectivity

The reflectometry curves of an Nb(4 nm)/YFe2(40 nm, 〈110〉)/Fe(1.5 nm)/Nb(50 nm) sample measured in the synchrotron radiation of right- and left-handed circular polarization for a set of wavelengths near the Y L2,3 absorption edges have been used to determine the energy dependences of the component magnitudes for the X-ray susceptibility tensor of YFe2 near the yttrium absorption edges, including off-diagonal magnetic additives. Our result is in good agreement with the normalized experimental absorption and circular dichroism spectra and their Kramers-Kronig transformations.

Soft x-ray resonant magnetic reflectivity studies for in-and out-of-plane magnetization profile in ultra thin films

The possibility to investigate complex magnetic profiles throughout an ultrathin magnetic film or an interface by soft x-ray resonant magnetic reflectivity is presented. The determination of in- and out-of-plane magnetic profile is shown to be possible with a subnanometer resolution by measuring the reflectivity over a wide angular range. The technique is applied to a granular magnetic multilayer and to a perpendicular exchange bias coupled system.

Longitudinal and transverse magnetization components in thin films: A resonant magnetic reflectivity investigation using circularly polarized soft x-rays

An in-plane vectorial analysis of the magnetization of thin magnetic films is presented. Longitudinal soft x-ray resonant magnetic reflectivity curves display characteristic nodes where the longitudinal scattering component is suppressed by x-ray interference. The transverse magnetic component can be effectively retrieved at these nodal points, despite the use of circular polarization and longitudinal scattering geometry. Using a single geometric configuration, transverse and longitudinal magnetic hysteresis loops can be clearly separated. Calculations based on a Stoner–Wohlfarth model satisfactorily describe both loops. Therefore, this method presents a viable alternative to standard vectorial analysis techniques, with the additional benefit of element specificity.

Study of Spin Distribution in the MnIr/CoFe Exchange Bias System for the Magnetic Storage Devices by X-ray Resonant Magnetic Reflectivity and Polarized Neutron Reflectivity

反強磁性層と強磁性層の積層界面に働く交換結合力は，強磁性層の磁化曲線に，交換バイアスと呼ばれるシフトを生じさせる。新しい材料であるMnIr/CoFe積層膜は，薄い膜厚でも大きい交換バイアスを発生することができるため，磁気記録デバイス用として注目されている。筆者らは，この積層膜の磁気構造を，SPring-8におけるX線共鳴磁気反射率測定及び，日本原子力研究開発機構(JAEA)の中性子施設JRR-3における偏極中性子反射率測定により調べた。これらの測定からは，界面でのスピンの固着成分と回転成分の元素選択的な評価と，強磁性層の磁化ベクトルの回転角が得られた。これらの実験結果から，MnIr/CoFe交換結合膜中でのスピンの分布と，交換バイアスとの関係を議論する。

Absence of element specific ferromagnetism in Co doped ZnO investigated by soft X-ray resonant reflectivity

On the quest for the intrinsic origin of ferromagnetism (FM) in ZnO doped with a few percent transition metal, we show detailed X-ray resonant magnetic reflectivity (XRMR) measurements, performed at the Co L2,3 and the O K edges of pulsed laser deposition (PLD) prepared samples. These samples show ferromagnetism at room temperature (RT) (QUID: about 2μB /Co). But in contrast to the QUID measurements, element specific reflection measurements as a function of angle (θ-2θ scans) and energy (const. qz) do not show any sign of ferromagnetism. Therefore, we can exclude without doubt Co as a possible origin for FM in this system. Our results are in perfect agreement with earlier published XMCD data and strongly support the new proposed grain boundary based model for oxygen vacancies related RT-FM.