Resonant Magnetic Reflectivity Research Articles

Resonant magnetic reflection coefficients at the Fe2pedge obtained with linearly and circularly polarized soft x rays

A systematic study of the specular reflectivity using circularly and linearly polarized radiation on magnetic materials is presented. Within a frame work based on the complete reflection matrix and the Stokes parameter formalism, the reflectance is modeled as a function of the polarization state of the incoming light for the longitudinal and the transversal magnetization. The nonmagnetic reflection coefficients and their individual magnetic contributions are determined from resonant magnetic reflectivity experiments of polarized soft x rays across the $\mathrm{Fe}\ensuremath{-}2p$ absorption edge on a ferromagnetic Fe/C multilayer. Exploiting tunable undulator radiation the absolute reflectance is investigated as a function of the degree of circular or linear polarization, photon energy, angle of incidence, and magnetic-field direction. As predicted by the developed formalism the corresponding magnetic asymmetry parameters depend nonlinearly on the degree of polarization at large angles of incidence due to the influence of the polarizing power of the sample. In the longitudinal geometry using circularly polarized light, the magnetic contribution is directly related to the magnetic contribution of the optical constants, which have independently been determined by Faraday measurements on an identical transmission sample.

Exchange coupling in Fe/NiO/Co film studied by soft x-ray resonant magnetic reflectivity

A 10 nm Fe/5 nm NiO/10 nm Co polycrystalline trilayer prepared by magnetron sputtering has been investigated by resonant magnetic reflectivity of linearly polarized soft x rays (transversal magneto-optical Kerr effect geometry) at the L 3 edges of Fe, Ni, and Co. The exchange coupling in the trilayer, introduced during synthesis, has been altered by cooling through the Neel temperature T N of NiO at three different cooling fields H F C . We show that the chemical structure at the interfaces, the grain size and the magnetic properties of the trilayer are governed by factors such as successive heat treatment, magnetic field applied during synthesis and cooling through T N of NiO. Clear evidence for oxidation of Fe and Co states has been obtained, suggestive for the formation of mixed oxides at the Fe/NiO and NiO/Co interfaces. Oxidized Fe and Co are found to carry a net magnetic moment, which couples to the moments of the metallic states. We find that the coupling between oxidized and metallic Co spins is ferromagnetic, whereas the coupling between oxidized and metallic Fe spins is antiferromagnetic. We observe the presence of two types of excess Ni spins. A rigid excess Ni magnetic moment is induced at the interface during synthesis as follows from the strong asymmetry of the x-ray Kerr loops of the oxidized Co spins at H F C = 0 Oe and metallic Co spins at H F C =240 Oe. Heat treatment leads to appearence of a soft excess Ni magnetic moment, which is ferromagnetically coupled to the metallic Fe and Co spins and exhibits a conventional hysteresis loop. Competing interactions lead to frustrated magnetization at the NiO/Fe interface and cause a tilt of the Fe spins away from the direction of applied magnetic field. This manifests itself in the presence of characteristic humps in the Fe Kerr loops.

Interplay between chemical and magnetic roughness of Pt in a Pt/Co bilayer investigated with X-ray resonant magnetic reflectometry

Abstract The magnetization depth profiles of Pt have been investigated by X-ray resonant magnetic reflectivity measurements of sputtered Pt/Co bilayer systems with different chemical roughness valu...

Oscillatory Resonant X-Ray Magnetic Bragg-Peak Signs and Gd Moments Induced by the Fe/Gd Interlayer Exchange Interaction

The alternate negative and positive Bragg peaks observed in the resonant X-ray magnetic specular reflectivity profile of a [Fe(3.5 nm)/Gd(5.4 nm)]15 multilayer are due to the large magnetic moments induced in the Gd layers by the Fe/Gd interlayer exchange coupling. The spin polarizations induced on the Gd 5d electrons are localized at the interfaces, which are not extended beyond 1.0 nm. The Bragg-peak signs cannot be explained by the magnetic scattering from the induced Gd moments, but by the interference of the magnetic scattering with the charge scattering from the entire multilayer.

Pt magnetization profile in a Pt/Co bilayer studied by resonant magnetic x-ray reflectometry

The Pt magnetization depth profile at a single buried Pt/Co interface was investigated by x-ray resonant magnetic reflectivity measurements. The asymmetry as function of angle of incidence has been measured in the Pt ${L}_{3}$-near-edge absorption region at two energies. Observed asymmetry ratios in the order of 0.5% are described on the basis of a magnetically modified Parratt algorithm. Excellent agreement between simulations and experiment was achieved for a Pt magnetic moment of $0.21{\ensuremath{\mu}}_{B}$ at the rough interface followed by an exponential decay of the induced polarization within 1 nm.

Soft X-ray resonant magnetic reflectivity from Fe/C multilayers

We report on X-ray resonant magnetic reflectivity (XRMR) at the Fe L 2,3 edges in a series of ex situ grown Fe/C multilayers and a Fe single layer. The reflectivities collected in the T-MOKE and L-MOKE geometries are consistent with a ferromagnetic alignment of the Fe moments. The magnetization decreases with decreasing Fe thickness. According to the X-ray magnetic circular dichroism data the averaged magnetic moment is reduced to 1.21 μ B/Fe in one of the multilayers. The calculated XRMR signals for this multilayer indicate that the Fe moments are reduced in the bulk of the Fe layers and vanish abruptly at the interfaces.

Chemical and magnetization profile study of Ce in [CeLaCe/Fe] and [LaCeLa/Fe] multilayers by resonant X-ray reflectivity

The distribution of the induced 5d magnetic moments across the Ce layers in [CeLaCe/Fe] 57 and [LaCeLa/Fe] 57 multilayers has been investigated by resonant X-ray magnetic reflectivity. The quantitative analysis of the energy dependence of the asymmetry ratios measured on top of several multilayer Bragg peaks required the determination of the composition profile across the period of the multilayer. In order to separate out the structural parameters of the Ce and La layers, resonant X-ray reflectivity measurements were carried out at the La and Ce L-edges as well as at the Fe K-edge. The induced Ce 5d magnetic profile which is derived from the XRMS spectra is found to oscillate in both samples, with however, a different depth dependence probably related to the influence of the different interfaces.

Magnetic properties of bctFexMn1−xthin-film alloys investigated by linearly polarized soft-x-ray resonant magnetic reflectivity

The magnetic properties of ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}/\mathrm{Ir}(001)$ superlattices (SL's) have been studied using linearly polarized soft x-ray resonant magnetic reflectivity (XRMS) at the Fe ${L}_{2,3}$ absorption edges. Previous superconducting quantum interference device magnetometry measurements have shown that the SL's exhibit a ferromagnetic behavior for $x>0.75$ and a nonmagnetic or antiferromagnetic one for $x<0.75.$ XRMS measurements were performed on two SL's with compositions on each side of $x=0.75,$ as well as on a thin layer of Fe-rich alloy $(x=0.9).$ The Fe magnetic moments in these alloys were determined by comparing experimental and calculated energy-dependent asymmetry ratios at different angles in the reflectivity curves. Fe atoms are found in a high-spin ferromagnetic state for $x=0.9,$ while they are in a low-spin ferromagnetic state for $x=0.7.$ Our results emphasize the role of the tetragonalization parameter $c/a$ on the occurrence of the different magnetic states. Measurements, at the Mn ${L}_{2,3}$ edges in a bct ${\mathrm{Fe}}_{0.9}{\mathrm{Mn}}_{0.1}$ ultrathin layer reveal a net magnetic moment per Mn atom of about $1.7{\ensuremath{\mu}}_{B}$ coupled antiparallel to the Fe one.

FexMn1−xalloy phases strained in thin films: Depth-dependent investigation ofFexMn1−x/Ir(001)multilayers by x-ray diffraction

X-ray-diffraction measurements have been performed on ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}(001)$ thin films strained in ${\mathrm{I}\mathrm{r}/\mathrm{F}\mathrm{e}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}/\mathrm{Ir}(001)$ sandwiches and in $[{\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}/\mathrm{Ir}{]}_{20}$ superlattices with $x=0.7$ and $x=0.9.$ These concentrations were chosen with regard to the magnetic properties of the alloys. Superconducting quantum interference device measurements showed a ferromagnetic state above $x\ensuremath{\approx}0.75$ and an antiferromagnetic or a nonmagnetic one below. The samples have been investigated by using the atomic contrast available through anomalous diffraction and by recording reciprocal space maps (RSM). The (111) RSM, sensitive to both in-plane and out-of-plane parameters, has been studied as a function of the grazing incidence angle. It has revealed that the $[{\mathrm{Fe}}_{0.9}{\mathrm{Mn}}_{0.1}/\mathrm{Ir}{]}_{20}$ superlattice has a complex structure with two phases stacked along the growth axis whose origin stems from the existence of the two cubic structures (bcc and fcc) of the bulk alloys. In each case, we have shown that the ${\mathrm{Fe}}_{x}{\mathrm{Mn}}_{1\ensuremath{-}x}$ alloys are strained in a bct structure. This tetragonalization allows us to investigate the magnetic properties throughout a continuous transformation from a bcc phase $(c/a=1)$ to a fcc one $(c/a=\sqrt{2}).$ Together with x-ray resonant magnetic reflectivity measurements, our results show that the tetragonalization plays a dominant role on the magnetic state observed for the investigated samples. For $c/a$ in the range 1.2--1.3, the Fe atoms are found to be in a ferromagnetic low-spin state.

Soft x-ray resonant magnetic reflectivity study of thin films and multilayers

Soft x-ray resonant magnetic reflectivity measurements on thin films and multilayers in a transverse geometry using linear polarized photons are presented. Magneto-optic calculations taking into account the layer roughness allows us to reproduce all the experimental features of the angular and energy reflectivity curves as well as the asymmetry ratio in both cases. Application to FexMn1−x alloy films epitaxially grown on Ir(001) brings more insights on the magnetic transition occurring at x=0.75.

Magnetic effects on the resonant X-ray reflectivity: circular dichroism at the 2p edges of Ni

We measured the X-ray reflectivity of a Ni(110) crystal over the photon energy range including the 2p absorption edges of Ni. Using circularly polarized X-rays, we observed large variations in the resonant reflectivity according to the relative orientation between sample magnetization and photon helicity. The angular dependence of the dichroism and the use of resonant magnetic reflectivity for drawing hysteresis curves are also discussed.