Nuclear Resonance Reflectivity Research Articles

Interface sharpening in miscible and isotopic multilayers: Role of short-circuit diffusion

Atomic diffusion at nanometer length scale may differ significantly from bulk diffusion, and may sometimes even exhibit counterintuitive behavior. In the present work, taking Cu/Ni as a model system, a general phenomenon is reported which results in sharpening of interfaces upon thermal annealing, even in miscible systems. Anomalous x-ray reflectivity from a Cu/Ni multilayer has been used to study the evolution of interfaces with thermal annealing. Annealing at 423 K results in sharpening of interfaces by about 38%. This is the temperature at which no asymmetry exists in the inter-diffusivities of Ni and Cu. Thus, the effect is very general in nature, and is different from the one reported in the literature, which requires a large asymmetry in the diffusivities of the two constituents [Z. Erd\'elyi et al., Science 306, 1913 (2004).]. General nature of the effect is conclusively demonstrated using isotopic multilayers of 57Fe/naturalFe, in which evolution of isotopic interfaces has been observed using nuclear resonance reflectivity. It is found that annealing at suitably low temperature (e.g. 523 K) results in sharpening of the isotopic interfaces. Since chemically it is a single Fe layer, any effect associated with concentration dependent diffusivity can be ruled out. The results can be understood in terms of fast diffusion along short-circuit paths like triple junctions, which results in an effective sharpening of the interfaces at relatively low temperatures.

Influence of the Interface State on the Magnetoresistive Properties of Co/Cu Superlattices

The influence of the interface structure on the magnetoresistive properties of Co/Cu superlattices has been studied using the techniques of nuclear magnetic resonance and X-ray reflectivity. It has been found that the roughness of interfaces and the number of cobalt atoms involved in their formation increase with annealing temperature, while the fraction of highly perfect interfaces decreases at higher temperatures. It has been demonstrated that the probability of the scattering of conduction electrons by interfaces increases with the width of these boundaries.

Nuclear resonance reflectivity from a [57Fe/Cr]30 multilayer with the Synchrotron Mössbauer Source.

Mössbauer reflectivity spectra and nuclear resonance reflectivity (NRR) curves have been measured using the Synchrotron Mössbauer Source (SMS) for a [57Fe/Cr]30 periodic multilayer, characterized by the antiferromagnetic interlayer coupling between adjacent 57Fe layers. Specific features of the Mössbauer reflectivity spectra measured with π-polarized radiation of the SMS near the critical angle and at the `magnetic' maximum on the NRR curve are analyzed. The variation of the ratio of lines in the Mössbauer reflectivity spectra and the change of the intensity of the `magnetic' maximum under an applied external field has been used to reveal the transformation of the magnetic alignment in the investigated multilayer.

Nuclear Resonance Reflectivity of Dy/Gd Superlattices

Nuclear resonance reflectivity from a [Dy19Gd19] × 20 superlattice has been measured utilizing the 25.652 keV nuclear level of 161Dy. The measured time spectra of nuclear resonance reflectivity make it possible to reveal the variation of the hyperfine magnetic field Bhf on dysprosium nuclei in the temperature range of 4–110 K and to determine the relaxation time of the hyperfine field, using the decay speed-up of the excited state of 161Dy nuclei.

Double-spiral magnetic structure of the Fe/Cr multilayer revealed by nuclear resonance reflectivity

We have studied the magnetization depth profiles in a [57Fe(dFe)/Cr(dCr)]x30 multilayer with ultrathin Fe layers and nominal thickness of the chromium spacers dCr 2.0 nm using nuclear resonance scattering of synchrotron radiation. The presence of a broad pure-magnetic half-order (1/2) Bragg reflection has been detected at zero external field. The joint fit of the reflectivity curves and Mossbauer spectra of reflectivity measured near the critical angle and at the "magnetic" peak reveals that the magnetic structure of the multilayer is formed by two spirals, one in the odd and another one in the even iron layers, with the opposite signs of rotation. The double-spiral structure starts from the surface with the almost antiferromagnetic alignment of the adjacent Fe layers. The rotation of the two spirals leads to nearly ferromagnetic alignment of the two magnetic subsystems at some depth, where the sudden turn of the magnetic vectors by ~180 deg (spin-flop) appears, and both spirals start to rotate in opposite directions. The observation of this unusual double-spiral magnetic structure suggests that the unique properties of giant magneto-resistance devices can be further tailored using ultrathin magnetic layers.

Field-temperature evolution of the magnetic state of [Fe(1.2 Å)/Cr(10.5 Å)]30 structure by Mössbauer reflectometry with synchrotron radiation

Nanostructured epitaxial [57Fe(1.2Å)/Cr(10.5Å)]30 multilayer with ultrathin Fe layers, demonstrated Kondo-like behavior, has been investigated by means of the nuclear resonance reflectivity at low temperatures (down to 4K) in magnetic fields up to 4T. We detected the beginning of the magnetic phase transition at 150K which correlates with the minimum on the electrical resistivity curve. The determined magnetic hyperfine field distribution P(Bhf) indicates an inhomogeneity of the iron environments. When we apply the very high external magnetic field up to 4T, it aligns Bhf(i) along the direction of the external field only partially. Also it has been noticed that Bhf(i) orientations depend on the magnetic pre-history.

Nuclear resonance reflection of synchrotron radiation from thin dysprosium films with different types of magnetic ordering

Epitaxial thin films of dysprosium have been successfully synthesized by the method of high-vacuum magnetron sputtering and their structure and magnetic properties have been investigated. The opportunity of the nuclear resonance scattering for the investigation of nanostructures containing 161Dy has been considered; the specific features of the spectra of nuclear resonance reflectivity from the films have been analyzed on the energy and time scales at different orientations of the magnetic hyperfine field. The simulation of the angular dependences of nuclear resonance reflectivity for the case of spiral ordering in periodic structures containing 161Dy has been carried out. It has been shown that these dependences make it possible to uniquely determine the period of magnetic ordering.

In situsmall-angle x-ray and nuclear resonant scattering study of the evolution of structural and magnetic properties of an Fe thin film on MgO (001)

Growth of magnetron sputtered Fe films on clean single crystalline MgO (001) substrate has been studied using in situ grazing incidence small angle x-ray scattering (GISAXS) and grazing incidence nuclear resonant scattering (GINRS) measurements. While GISAXS provides information about morphological changes, GINRS provides information about structural and magnetic properties, thus making it possible to correlate the evolution of magnetic properties with that of morphology and structure of the film. The film exhibits a Volmer-Weber type growth, with percolation transition occurring around 2 nm film thickness. Presence of a finite quadrupole splitting, as seen in GINRS measurements, suggests a significant distortion from cubic symmetry up to a film thickness of 3.5 nm, which can be attributed to hybridization between Fe $3d$ and O $2p$ orbitals at the interface as well as in-plane tensile strain induced as a result of coalescence of islands. Initially Fe islands exhibit superparamagnetic relaxation, while finite magnetic moment appears upon formation of macroscopic percolation islands. The film exhibits a weak perpendicular magnetic anisotropy (PMA), which vanishes concurrently with disappearance of structural distortion, suggesting that the observed PMA at least partly originates from inherent strain in the film. No presence of any known oxide of Fe was detected at the interface. More precise information about topological and magnetic structure of the interfaces between Fe and MgO layers is obtained using combined x-ray reflectivity and nuclear resonance reflectivity measurements on a $^{57}\mathrm{Fe}/\mathrm{MgO}$ multilayer. Measurements show that about two monolayers of Fe at the interface have a reduced hyperfine field, providing evidence for hybridization with O atoms, as predicted by theory.

Field-induced spin reorientation in[Fe/Cr]nmultilayers studied by nuclear resonance reflectivity

We present depth-resolved nuclear resonance reflectivity studies of the magnetization evolution in ${[^{57}\mathrm{Fe}(3\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}(1.2\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{10}$ multilayer under applied external field. The measurements have been performed at the station BL09XU of SPring-8 at different values of the external field (0--1500 Oe). We apply the joint fit of the delayed reflectivity curves and the time spectra of the nuclear resonance reflectivity measured at different grazing angles for enhancement of the depth resolution and reliability of results. We show that the azimuth angle, which is used in all papers devoted to the magnetization profile determination, has a more complicated physical sense due to the partially coherent averaging of the scattering amplitudes from magnetic lateral domains. We describe how to select the true azimuth angle from the determined ``effective azimuth angle.'' Finally we obtain the noncollinear twisted magnetization depth profiles where the spin-flop state appears sequentially in different $^{57}\mathrm{Fe}$ layers at increasing applied field.

Mechanism of stress relaxation in nanocrystalline Fe-N thin films

The mechanism of stress relaxation in nanocrystalline Fe-N thin film has been studied. The as-deposited film possesses a strong in-plane compressive stress which relaxes with thermal annealing. Precise diffusion measurements using nuclear resonance reflectivity show that stress relaxation does not involve any long-range diffusion of Fe atoms. Rather, a redistribution of nitrogen atoms at various interstitial sites, as evidenced by conversion electron M\"ossbauer spectroscopy, is responsible for the relaxation of internal stresses. On the other hand, formation of the ${\ensuremath{\gamma}}^{\ensuremath{'}}$-Fe${}_{4}$N phase at temperatures above 523 K involves long-range rearrangement of Fe atoms. The activation energy for Fe self-diffusion is found to be $0.38\ifmmode\pm\else\textpm\fi{}0.04$ eV.

Swift heavy ions for controlled modification of soft magnetic properties ofFe0.85N0.15 thin film

It is shown that the soft magnetic properties of amorphousFe0.85N0.15 film can be modified in a controlled manner by irradiation with 120 MeV Ag ions.Irradiation causes relaxation of short range as well as long range stresses, resulting in animprovement in the soft magnetic properties. Nuclear resonance reflectivity has beenused to measure the diffusion length of Fe atoms as a function of irradiationfluence. It is found that the atomic motion associated with irradiation is notexpected to modify significantly the interfaces with substrate or other layers.

Nanoscale diffusion in amorphousFe75Zr25films

Self-diffusion of Fe in amorphous ${\text{Fe}}_{75}{\text{Zr}}_{5}$ films has been studied over a wide temperature range by combining secondary-ion-mass spectrometry and nuclear-resonance reflectivity measurements. Subnanometer accuracy of nuclear-resonance reflectivity in diffusion length measurement allows quantitative determination of time-dependent diffusivity of Fe during structural relaxation. A clear correlation between diffusivity and different types of structural relaxations is observed. It is found that in both structurally relaxed and unrelaxed states, diffusive jumps occur via a collective motion of a group of atoms. However, the presence of excess free volume in unrelaxed amorphous films causes the activation energy as well as the diffusion entropy to decrease, suggesting that the average number of atoms participating in a diffusive jump is significantly less as compared to that in the fully relaxed state. The typical diffusion length involved in annihilation of free volume is 0.7 nm, which agrees with the length scale of structural fluctuations as seen in neutron- and x-ray-scattering experiments.

Fe diffusion in amorphous and nanocrystalline alloys studied using nuclear resonance reflectivity

It is demonstrated that nuclear resonance reflectivity from isotopic multilayers can be used as a sensitive technique to study self-diffusion of a M\ossbauer isotope ($^{57}\mathrm{Fe}$ in the present case). In the case of isotopic multilayers, in which alternate layers have the same chemical composition and differ only in the abundance of $^{57}\mathrm{Fe}$, nuclear resonance scattering causes x-ray scattering contrast between adjacent layers, resulting in the appearance of Bragg peaks corresponding to the bilayer periodicity. Diffusion of $^{57}\mathrm{Fe}$ across the isotopic interface results in a decrease in the scattering contrast and thus a decrease in the intensity of the Bragg peak, making it possible to measure diffusion lengths of the order of $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in chemically homogeneous films. The technique has been used to study self-diffusion of Fe in amorphous FeZr and nanocrystalline FeN alloys. In $\mathrm{a}\text{\ensuremath{-}}\mathrm{Fe}\mathrm{Zr}$, measurements yield activation energy for Fe diffusion $E=0.42\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and the pre-exponent factor ${D}_{0}=\mathrm{exp}(\ensuremath{-}39\ifmmode\pm\else\textpm\fi{}1)\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}∕\mathrm{s}$. In nanocrystalline ${\mathrm{Fe}}_{60}{\mathrm{N}}_{40}$, variation in diffusivity due to structural relaxation at temperatures as low as $393\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ could be observed. Measurements in the structurally relaxed state yield $E=0.8\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and ${D}_{0}=\mathrm{exp}(\ensuremath{-}28\ifmmode\pm\else\textpm\fi{}4)\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}∕\mathrm{s}$.

Self-Diffusion in Chemically Homogeneous Multilayers Using Neutron and Nuclear Resonance Reflectivity

ABSTRACTNeutron reflectivity is a well-established technique for studying self-diffusion in chemically homogeneous system as neutron scattering lengths are different for isotopes of an element. For x-ray there is no contrast for a multilayer with isotopic abundance. However, placing Mössbauer active nuclei in a chemically homogeneous system, self-diffusion of the constituents can be probed using nuclear resonance scattering of Mössbauer active nuclei. In the present work, we have applied the neutron reflectivity technique for studying the self-diffusion of iron and nitrogen in nano crystalline multilayers of FeN/57FeN and FeN/Fe15N and nuclear resonance reflectivity techniques for studying iron self diffusion in FeNZr/57FeNZr. Both the techniques are complementary to each other and give a unique depth resolution of the order of 0.1 nm. As compared with conventional techniques used for probing self-diffusion, neutron and nuclear resonance reflectivity techniques can be applied at significantly lower temperatures. On the basis of the obtained results the diffusion mechanism in chemically homogeneous multilayers is discussed in the present work.

Standing waves and reflectivity from an ultrathin layer

We have analytically found that the reflectivity amplitude from one thin layer in the presence of the other reflecting layers is modulated by the squared amplitude of the radiation field but not by the first-order amplitude, as was supposed in the paper of R. Rohlsberger (Hyperfine Interactions 123/124, 455 (1999)), for delayed nuclear resonance reflectivity. Hence, a larger sensitivity to standing waves is expected for the reflectivity signal from selected groups of atoms or nuclei compared with the secondary radiation emission.

Magnetic Properties of Monocrystal 57 Fe/V Multilayers Investigated by CEMS, Nuclear Resonance Reflectivity in the Time Domain and Polarized Neutron Scattering

We have performed a depth selective investigation of monocrystalline samples with composition [57Fe(10 ML)/V(5 ML)]20 and [57Fe(7 ML)/V(10 ML)]29 by means of synchrotron radiation Mossbauer reflectometry in time domain. A bilayer depth scan was achieved by angular variation in vicinity of the superstructure Bragg peak. Simultaneous fits of the different time spectra for each sample reveal that the interfaces are not symmetrical and that the higher hyperfine magnetic fields are concentrated near the V-on-Fe interface. Polarized neutron reflectivity data were recorded for comparison.

57Fe/Co Multilayers Investigated by CEMS and Nuclear Resonance Reflectivity Time Spectra Using Grazing Incident SR

[Fe6Co3] and [Fe5Co5] superlattices have been studied by CEMS and time dependent nuclear resonance scattering in grazing incidence Bragg reflection geometry. CEMS shows hyperfine fields (Bhf) between 33 T and 37 T. The time spectra of reflectivity allows us to separate contributions from different 57Fe monolayers in a bilayer. Contrary to our expectations the largest field is in the center of the Fe layers. The spontaneous orientation of Bhf is along [1 1 0] in the layer plane.