Nuclear Methods in Studying Thin Films and Heterostructuress

 

 

 

 

 

14-15 November, 2003

KFKI Budapest, Hungary

 

 

 

 

 

 

 

Abstract Booklet

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Francisco ALMEIDA: Characterization of the interlayer coupling in Fe/FeSi/Fe

Electrodeposited magnetic/non-magnetic multilayers: specialties and generalities

 

I. BAKONYI, L. PÉTER, Z. ROLIK+, K. KISS-SZABÓ+, Z. KUPAY+,
E. TÓTH-KÁDÁR, Q.X. LIU, J. TÓTH AND L.F. KISS

 

Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences.
H-1525 Budapest, P.O.B. 49, Hungary

 

Since the first demonstration of the occurrence of the giant magnetoresistance (GMR) in electrodeposited magnetic/non-magnetic multilayers ten years ago, GMR has been reported in several electrodeposited multilayer systems (mainly Ni-Cu/Cu, Ni-Co-Cu/Cu and Co-Cu/Cu) [1]. The presence of some amount of Cu (1 to 20 at.%) in the magnetic layer is due to the applied two-pulse plating technique in which, a high-potential (high-current) pulse is used for depositing the magnetic layer and a low-potential (low-current) pulse for the non-magnetic spacer layer (Cu) from the same bath.

Based on our recent studies [2-6], in this talk we suggest from an analysis of the field and temperature dependence of the magnetic properties and the magnetoresistance of these systems that, besides the usual contribution of ordinarily ferromagnetic (FM) regions, the magnetization and the GMR contains also a contribution that can be ascribed to superparamagnetic (SPM) entities in the multilayer [5,6]. For Ni-Cu/Cu multilayers, a qualitative decomposition of the the FM and SPM contributions will be presented and a quantitative decomposition will be performed for Co-Cu/Cu multilayers.

It will be furthermore discussed to what extent (i) as a specialty, the so-called “exchange” reaction between the less-noble, magnetic metals (Ni and/or Co) and the more noble, non-magnetic metal (Cu) and (ii) as a generality, the nucleation behaviour of the magnetic and non-magnetic elements on each other influence the formation of SPM regions with “loose magnetic moments” and, thus, the GMR of electrodeposited multilayer films.

 

1.     W. Schwarzacher and D.S. Lashmore, IEEE Trans. Magn. 32, 3133 (1996)

2.     L. Péter et al., J. Electrochem. Soc. 148, C168 (2001)

3.     I. Bakonyi et al., J. Electrochem. Soc. 149, C195 (2002)

4.     V. Weihnacht et al., J. Electrochem. Soc. 150, C507 (2003)

5.     G. Nabiyouni et al., J. Magn. Magn. Mater. 253, 77 (2002)

6.     I. Bakonyi et al., J. Magn. Magn. Mater. (in press)

Judit S. BALOGH: Non-equilibrium mixing in FeAg heterostructures

Péter BARNA: Structure formation in co-deposited multicomponent thin films

The role of ions in the synthesis and characterisation of epitaxial structures.

DIRK O. BOERMA

Centro de Microanálisis de Materiales, Universidad Autónoma de Madrid
Cantoblanco, 28049 MADRID

The synthesis and characterisation of (multi-)layers or nano-dots with nanometer dimensions requires a multitude of instruments in an ultra-high vacuum ambient. Ion beams are very important among these instruments, but they should be used in combination with other methods in order to be able to prepare (substrate) surfaces or to produce or to completely analyse thin layer structures. The inclusion of all these instrumentation amounts to setting up a complete surface science laboratory at an accelerator facility, which is in most cases an impossible mission.

In this contribution we show how we intend to solve this problem for the surface and thin layer department at the 5 MV tandem accelerator at the UAM. We also illustrate that the combination of many techniques, in addition to ion-beam based techniques is important. For this we describe a part of the research we did on the growth and characterisation of magnetic epitaxial Fe nitride layers and nano-dots, emphasizing the role of ions. 

Thin Film Studies with Magnetic-Field CEMS Detector:
Mössbauer Polarimetry with Linearly Polarized Source

 

F. TANCZIKÓ, L. DEÁK, D.L. NAGY, L. BOTTYÁN

KFKI Research Institute for Particle and Nuclear Physics, H-1525 Budapest, P. O. B. 49, Hungary

 

Mössbauer spectroscopy (MS) has been very successful in determining the local magnetic and electronic structure in non-isotropic (thin single-crystal, epitaxial film, textured foil and textured powder) absorbers even by using a single line unpolarized source. The full analytical potential of the polarization dependence of the relative intensities can of course be exploited when, beside the absorber the source is split. Source matrix polarization in transverse or longitudinal mag­netic field, sometimes combined with filter techniques has been reported in the literature. These methods are often called ‘Mössbauer polarimetry’ and have been elaborated theoretically and experimentally in details long time ago [e.g. 1, 2, 3].

Magnetic thin films and nanostructures display a richness of magnetic properties not present in bulk materials. Due to the relatively shallow escape depth of the low-energy conversion elec­trons emerging in consequent to the nuclear de-excitation, conversion electron Mössbauer spectroscopy (CEMS) has become an established local probe method in the analysis especially of Fe-containing thin films and surfaces. In thin magnetic films the magnetization, as a rule, due to shape anisotropy, is confined in the film plane. However, strain, local alloying or other effects may result in strong out-of-plane surface and interface anisotropy and a consequent out-of-plane magnetization.

Conventionally, CEMS is per­formed with a single-line (unpolarized) source with g-rays at perpendicular incidence to the film surface. However, such experiments provide no information about the direction of the magnetic hyperfine field within the film plane, which may be vital in various thin film studies. Antiferro­magnetically (AF-) coupled multilayer systems of epitaxial confinement on single crystal sub­strates [4] and exchange spring structures [5, 6] are recent examples to mention.

Here we report on a combined application of the Mössbauer polarimetry and conversion electron Mössbauer spectroscopy. This technique uses a linearly polarized source and perpendicular incidence of the g-rays; however, the polarization direction of the source can finely be adjusted. Technical details of such a polarimeter setup its test and application to study the bulk spin-flop transition [4] in an [57Fe(2.6 nm)/Cr(1.3 nm)]20 epitaxial multilayer is presented here.

 

[1] G.J. Perlow, S.S. Hanna, M. Hamermesch, C. Littlejohn, D.H. Vincent, R.S. Preston and J. Heberle, Phys. Rev. Lett. 4 (1960) 74

2 U. Gonser, R.W. Grant, H. Wiederisch and S. Geller, Appl. Phys Lett. 9 (1966) 18

3 J. Jäschke, H.D. Rüter, E. Gerdau, G.V. Smirnov, W. Sturhahn, J. Pollmann, Nucl. Instr. Meth. B 155 (1999) 189

4 L. Bottyán, L. Deák, J. Dekoster, E. Kunnen, G. Langouche, J. Meersschaut, M. Major, D.L. Nagy, H.D. Rüter, E. Szilágyi, K. Temst, J. Magn. Magn. Mat. 240 (2002) 514

5 R. Röhlsberger, H. Thomas, K. Schlage, E. Burkel, O. Leupold and R. Rüffer Phys. Rev. Lett. 89 (2002) 237201

6 W. Keune, V.E. Kuncser, M. Doi, M. Askin,H. Spies, B. Sahoo, E. Duman, M. Acet, J.S. Jiang, A. Inomata, S.D. Bader, J. Phys. D-Appl. Phys. 35 (2002) 2352

Ion Beam Analysis with Monolayer Depth Resolution

 

H.D. CARSTANJEN

 

MPI for Metals Research, Heisenbergstr. 3, D70569 Stuttgart, Germany

e-mail:   carstanjen@mf.mpg.de

 

This contribution deals with high resolution ion beam analysis by means of the electrostatic spectrometer at the Pelletron of the Max-Planck-Institute for Metals Research in Stuttgart. It reports about recent developments in the application of high resolution RBS (Rutherford backscattering spectroscopy) and ERDA (elastic recoil detection analysis) of the ion beam laboratory in Stuttgart. Besides a short description of the apparatus it in particular comprises: i) Experiments concerned with monolayer depth resolution: e.g. on graphite single crystals up to 7 individual atom layers could be traced out by RBS. ii) Experiments dealing with Stranski-Krastanov growth: here the technique is demonstrated by the analysis of thin ZnO layers on top of organic SAM layers by RBS. iii) The Analysis of H and D by ERDA. The technique is demonstrated with the example of the analysis of a layered structure in polymers. iv) The analysis of oxygen by ERDA. Examples are the anisotropic oxidation of Al surfaces and the tracer diffusion of 18O in nanocrystalline ZrO at low temperatures.

Vibrational spectroscopy of solids with nuclear inelastic scattering

 

ALEKSANDR CHUMAKOV

European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble, France

Nuclear inelastic scattering [1] is a technique to study dynamics of solids [2].  The physical process behind the method is resonant absorption (and/or incoherent scattering) of x rays by nuclei, which involves an excitation of low-energy nuclear transition.  The technique measures a cross section of x-ray interaction with nuclei as a function of x-ray energy in the typical phonon energies range (~100 meV) around the nuclear transition.  This gives the energy dependence of the probability to excite the nucleus with simultaneous creation (or annihilation) of phonons in the lattice.  After reducing the primary experimental data, one arrives to the partial density of phonon states of those particular atoms, which contain the involved resonant nuclei. 

In comparison to other relevant methods as inelastic neutron, x-ray, and Raman scattering, the distinct features of nuclear inelastic scattering are: (i) resonant, (ii) isotope-selective, and (ii) incoherent interaction. 

The resonant nature of interaction brings relatively large cross section of inelastic scattering.  This results in high count rate and, consequently, short measuring time.  Furthermore, in combination with the small size of the synchrotron radiation beam, it gives access to very small (down to ~sub-microgram) samples. 

The isotope selectivity provides the feature of site selectivity, i.e., the access to vibrational properties of few particularly selected sites in the unit cell.  This simplifies the data for complicated (e.g., macromolecular) crystals.  Furthermore, the isotope selectivity allows for various kinds of probe (tracer) investigations. 

The incoherent nature of interaction lifts requirement of necessarily single crystalline samples.  Nuclear inelastic scattering can be carried out with polycrystalline, disordered, and amorphous materials. 

The talk gives an introduction to the technique.  We discuss the physics behind the method, typical experimental setup, instrumentation, and data treatment.  The introduction is illustrated with recent results of its applications, in particularly to thing films, surfaces and interfaces. 

1. M. Seto et al, Phys. Rev. Lett. 74 (1995) 3828. 

2. see, e.g., A. I. Chumakov and W. Sturhahn, Hyp. Interac. 123/124 (1999) 781.

INTERPLAY BETWEEN STRUCTURAL AND MAGNETIC PROPERTIES OF MBE-GROWN Fe/CsCl -57FeSi1-x/Fe SANDWICHES

 

B. CROONENBORGHS, F.M. ALMEIDA, J. MEERSSCHAUT, M. ROTS, A. VANTOMME

 

Katholieke Universiteit Leuven, Instituut voor Kern- en Stralingsfysica, Celestijnenlaan 200D, B-3001 Leuven, Belgium

 

The interest in multilayers consisting of Fe layers separated by Si or FeSi layers is driven by the challenge to extend the well-established picture of antiferromagnetic coupling in metal/metal systems (e.g. Co/Cu, Fe/Au) to coupling across more exotic spacer materials which exhibit semiconducting or half-metallic properties.  In the present work Fe(4nm)/ 57FeSi1-y (x nm)/Fe(8nm) (0.8£ x £ 3; y=0 – 0.25) sandwiches were grown with MBE on polished MgO(001) substrates held at 150°C during deposition.  The 57FeSi layer was deposited by co-evaporation of 57Fe and Si.  Epitaxial growth was monitored by in-situ RHEED and further explored by ex-situ ion-beam channeling experiments.  Both low- and high angle x-ray diffraction measurements show that the trilayers are artificially modulated.  We show that CEMS experiments give valuable extra information about the microscopic properties of the 57FeSi spacer layer. In accordance to single crystal CsCl FeSi CEMS measurements [1] all spectra contain a narrow single line component with isomer shift d@ 0.26 mm/s (with respect to a-Fe).  This shows the preference of Fe and Si to form a well-defined metastable phase with a CsCl structure when coevaporated during the growth of the trilayer.  In-plane magnetic hysteresis profiles were taken between 10K and RT.  Both very strong bilinear and biquadratic interlayer coupling were observed.  Type and strength of the interlayer coupling appear to be critically dependent on the spacer thickness and stoichiometry of the silicide.  In case of a stoichiometric silicide the total coupling strength decays in a monotonous way with the 57FeSi spacer layer thickness.   No second antiferromagnetic peak was found in the investigated thickness regime.  Furthermore,  the coupling strength shows strong temperature dependence.  For an 57Fe-rich spacer a metallic-like coupling behavior was observed.  We discuss our results within the framework of recent publications by de Vries [2] and Gareev [3].

 

[1]  M. Fanciulli et al. Phys. Rev. B 59 (1999) 3675

[2]  J.J. de Vries et al. Phys. Rev. Lett. 78 (1997) 3023

[3]  R.R. Gareev et al. Phys. Rev. Lett. 87 (2001) 157202

László DEÁK: Application of PNR and SMR for laterally inhomogeneous structures

Zoltán ERDÉLYI : Diffusion on nanoscale (thin films and multilayers)

Bruno GILLES : Pseudomorphism And Metastability In MBE Growth

Maria Luisa FERNANDEZ-GUBIEDA: Study of Fe/Ag granular alloys prepared by laser deposition

Wave field structure of polarized neutrons in an exchange-spring layer system with spiral magnetic structure and wave-guide enhancement of spin-flip scattering

 

YU. N. KHAIDUKOV1,2)*, M.A. ANDREEVA1)

 

1) Department of Physics, M.V. Lomonosov Moscow State University, Moscow, Russia

2) Frank Laboratory of Neutron Physics, Joint Institute For Nuclear Research, Dubna, Russia

 

Polarized neutron reflectometry (PNR) is a unique method for investigation of magnetization of individual layers and interfaces in multilayered structures. If the direction of magnetization changes with depth – as it takes place in bilayers containing both soft and hard magnetic layers when exposed to an external magnetic field – the spin-flip reflecting process of polarized neutrons takes place. Spin-flip reflectivity is the most informative channel for determination of the magnetization direction profile. Under usual conditions the spin-flip reflectivity is small. One method to increase this component is the so-called wave-guide mode creation in the soft layer by special choice of the parameters of the structure.

 

The algorithm of 4x4 propagation matrixes is used for model calculations. Generalized Parrat formalism (with 2x2 reflectivity and transmittance matrices) is also used for testing the results. Spin-flip reflectivities for the different models of magnetization rotation with depth are compared in the wave-guide structures and the high sensitivity of the method for exchange-spring layer magnetization is demonstrated.

 

The work was supported by the Russian Foundation for Basic Research (project № 01-02-17541).

Conduction Electron Spin Polarization in the spacer layer of
Fe/Ag/Fe and Fe/FeAl/Fe trilayers

 

JÓZEF KORECKI

 

Department of Solid State Physics, Faculty of Physic and Nuclear Techniques

AGH University of Science and Technology, 30-059 Kraków;

Institute of Catalysis and Surface Chemistry

Polish Academy of Science, 30-239 Kraków, Poland

 

Using nuclear probes, the spacer magnetism of magnetic multilayers can be investigated on a local scale. Examples are given of using the low energy muon spin rotation (LE-mSR) and the conversion electron Mössbauer spectroscopy (CEMS).

In the Ag spacer of a 4nmFe/20 nmAg/4nm Fe epitaxial trilayer grown on MgO(001) the spatially oscillating electron spin polarization has been determined by means of LE-mSR [1]. It oscillates with the same period as the interlayer exchange coupling in Fe/Ag/Fe systems but shows a much weaker attenuation at large distances x from the interface. The measured magnetization profile from the inner 14 nm of the spacer is described by an oscillating polarization decaying as x-0.8. This unusual behavior may arise from a full confinement of electron states within the spacer.

In Fe/FeAl/Fe trilayers, the spin polarization of the spacer layer could be directly correlated with the coupling character. The FeAl spacers were grown by alternate deposition of n Al and Fe monolayers on a Fe(001) buffer layer deposited on cleaved MgO(001). LEED and CEMS measurements proved that FeAl ordered alloy (CsCl-type) was formed [2]. For the coupled Fe/FeAl/Fe systems, the magnetic hyperfine field at the nuclei of a 57Fe probe layer situated in the center of the nonmagnetic spacer, being a measure of the spin polarization from the ferromagnetic Fe layers, was investigated as a function of the total spacer thickness. A decaying oscillatory dependence of the polarization effect vs. the spacer thickness was found. Correlation of the spin polarization and hysteresis loops, revealing an oscillatory coupling behavior, was clearly identified.

Financial support from The Foundation for Polish Science (FNP) is kindly acknowledged.

[1] H. Luetkens et al. Phys. Rev. Lett. 91 (2003) 017204

[2] Slezak, M.Kubik, J.Korecki, JMMM, 198-199 (1999) 405

Hyperfine Spectroscopy with Nuclear Resonance Scattering of Synchrotron Radiation

 

OLAF LEUPOLD

European Synchrotron Radiation Facility

F-38043 Grenoble, France

 

Since its observation in 1985 Nuclear Resonance Scattering (NRS) of synchrotron radiation has become an excellent spectroscopic tool with its main branches: Nuclear Inelastic Scattering (NIS), Nuclear Forward Scattering (NFS), and, more recently, Synchrotron Radiation based Perturbed Angular Correlation of gamma radiation (SRPAC). The NIS technique enables one to study dynamical effects (phonons) in solids. NFS and SRPAC probe hyperfine interactions and slow dynamics, like diffusion and relaxation processes.

 

NFS as a time differential method has proven to be complementary to Mossbauer spectroscopy, an energy differential method. In NFS the coherent excitation by the synchrotron radiation pulse yields a collective excited nuclear state, the nuclear exciton. The time behaviour of the reemitted photons following the nuclear decay exhibits characteristic modulations, bearing information on the splitting of nuclear levels by the hyperfine fields. Due to the polarization of the synchrotron radiation this quantum beat structure gives more information on the direction of the hyperfine fields than is usually obtained by classical Mossbauer spectroscopy.

 

SRPAC is a time differential method like conventional TDPAC using radioactive sources, or like NFS. As compared to TDPAC the intermediate nuclear level is directly excited from the ground state via the synchrotron radiation pulse. There is no need for a radioactive parent nucleus or a gamma-gamma cascade. As compared to NFS, the splitting of nuclear levels leads to quantum beats, as well, however, a single nucleus is excited, there is no spatially coherent excitation or reemission, which makes the scattering process independent of the Lamb-Mossbauer factor.

 

We will first give a short introduction to the NFS and SRPAC techniques and the corresponding experimental setups. Then we will present examples on experiments which can especially benefit from the outstanding properties of the synchrotron radiation, i.e. the small beam size and divergence on 3rd generation synchrotron radiation sources. These are NFS experiments under high pressure and in grazing incidence geometry, as well as SRPAC in soft condensed matter and/or at high temperatures and/or high gamma-energies, where NFS and Mossbauer spectroscopy cannot be performed.

 

Peter LIEB : Ion-Induced Magnetic Texturing of Thin Ni, Fe and Co Films

Spin Echo Resolved Grazing Incidence Scattering (SERGIS):
a New Method for Surface Studies

 

J. MAJOR (1), H. DOSCH (1), G. P. FELCHER (2), K. HABICHT (3), T. KELLER (4), A. VOROBIEV (1), M. WAHL (1), and S. G. E. TE VELTHUIS (2)

 

                   (1) Max-Planck-Institut für Metallforschung,

                       Heisenbergstr. 3, D-70569 Stuttgart, Germany

                   (2) Argonne National Laboratory, Argonne, IL, USA

                   (3) Hahn-Meitner-Institut, BENSC,

                       Glienickerstr. 100, D-14109 Berlin, Germany

                   (4) Max-Planck-Institut für Festkörperforschung,

                       Heisenbergstr. 1, D-70569  Stuttgart, Germany

 

In neutron reflectivity experiments, due to the limited beam brightness, the beam is usually defined only by two pairs of slits, parallel to the sample surface. The application of neutron-spin echo (NSE) in such experiments makes the determination of the otherwise inaccessible out-of-the-reflexion-plane (transverse) component of the momentum transfer possible. For the NSE encoding of this component, well defined flat, parallel magnetic-field borders, which are tilted (not perpendicular to the mean direction of the beam), are necessary in the equipment.

This novel method, which can use the intrinsically limited beam intensity of the neutron sources on a more effective way, is dubbed as spin-echo resolved grazing incidence scattering (SERGIS). The main experimental difficulty at the construction of such a set-up is the realization of the precise magnetic-field borders. For the solution of this problem, we propose the application of the neutron resonance spin echo (NRSE) method in combination with a traditional neutron reflectometer. Prototype experiments proved the feasibility of such a combination. At present, a dedicated SERGIS-NRSE set-up is under construction.

 

Such instrument will make possible the exploration of the true 2D in-plane correlation structure of the studied surfaces. Although the transverse divergence of the beam is large, due to the NSE encoding, the transverse correlation range of the neutron wave can be larger than 1000 nm. By the application of different geometries for the precession-field regions of the spin-echo devices, different components of the momentum transfer can be resolved. If the magnetic-field borders are perpendicular to the mean beam direction, the SERGIS facility also gives access to inelastic grazing-incidence scattering.

Márton MAJOR : The new MBE installation of KFKI RMKI

Johan MEERSSCHAUT : Local probe studies of magnetic structure in thin films

Thin film analysis by AES depth profiling.

 

M. MENYHARD

 

Research Institute for Technical Physics and Materials Sciences, P.O. Box 49, H-1525

 

The chemical analysis of thin films (in the range of  nm-s) which are buried by thicker structures can practically only be carried out by means of destructive depth profiling. Using this technique the specimen is sectioned by ion sputtering. In the case of AES depth profiling the instantaneous surface is analyzed by AES. We will show that using proper sputtering conditions (low ion energy, grazing angle of incidence, rotated specimen) the ion bombardment induced damage is in the range of 1-2 nm. The reconstruction of the original in-depth distribution from the measured depth profile (which is distorted by the ion bombardment induced defects) is possible by applying a trial and error method, which is based on the simulation of the ion removal process (e.g. TRIM). Some examples will be given to demonstrate the capability of this method.

 

Formation and transformation of antiferromagnetic domains in an Fe/Cr multilayer

D.L. NAGY, L. BOTTYÁN, L. DEÁK, M. MAJOR, E. SZILÁGYI, F TANCZIKÓ

KFKI Research Institute for Particle and Nuclear Physics, P.O.B. 49, H–1525 Budapest, Hungary

Antiferromagnetically (AF) coupled metallic multilayers have received much attention in recent years due to their relevance in fundamental science and magnetic recording technology alike. Their magnetisation and transport behaviour is usually described in terms of the layer–layer coupling and little role is attributed to the in-plane magnetocrystalline anisotropy. We shall demonstrate that the magnetocrystalline anisotropy may lead to spectacular effects which can be most efficiently stud­ied by two closely related nuclear scattering techniques, viz. synchrotron Mössbauer reflectometry (SMR) and polarised neutron reflectometry (PNR).

The electronically forbidden AF reflections in specular SMR fully appear or completely disappear during the spin-flop transition. In case of specular PNR the AF reflection moves from the spin-flip to the non-spin-flip channel on spin-flop transition or vice versa.

The off-specular SMR and PNR techniques are sensitive to the in-plane correlation length x of the layer magnetisation direction and are, thereby, able to map the size distribution of the AF domains in multilayers.

Sub-micrometer domains are formed in a strongly AF-coupled epitaxial Fe/Cr multilayer upon leaving magnetic saturation. The correlation length is spontaneously and irreversibly increasing to about 1 mm when approaching remanence. At the same time, the shape of the autocorrelation func­tion of the magnetisation is also changing abruptly (“domain ripening”).

A spin-flop transition takes place in the multilayer of fourfold in-plane anisotropy when a moder­ate magnetic field is applied along the easy axis in which the layer magnetisations actually lay. A dramatic increase of x from about 1 mm to at least 10 mm, i.e., an explosion-like “coarsening” of the AF domains was observed in the same multilayer when it passed the spin-flop transition pro­vided that the external magnetic field was previously decreased from magnetic saturation to zero.

Domain correlation length both after ripening and coarsening becomes small again only after the multilayer is exposed to a field significantly higher than the saturation field belonging to the Fe layer magnetisations.

The presented SMR and PNR experiments were performed at the nuclear resonant scattering beamline ID18 of the European Synchrotron Radiation Facility, Grenoble and at the SPN-1 polar­ised neutron reflectometer (alias REMUR) of the Frank Laboratory of Neutron Physics, Joint In­stitute for Nuclear Research, Dubna, respectively.

György RADNÓCZI : CNxNi nanocomposite layers: structure and mechanical properties

Interface mixing induced by high energy ion bombardment

 

GYÖRGY SZENES

 

Department of General Physics, Eötvös University Budapest, Hungary

 

Interface mixing is analyzed when the inelastic energy deposition of ions considerably exceeds the elastic one. Systematic experiments of Bolse et al. on Cu, CuO, Cu2O, ZnO, NiO layers on SiO2 substrates [1] are reviewed. In the experiments the variation of the concentration profile was followed by RBS method and it was characterized by the variance of the Gauss error integral Δσ2. The mixing rate k was defined from the equation  Δσ2=kF, where F is the fluence. A correlation between k and the electronic stopping power Se was observed. Mixing was detected only above a threshold value of Se which was different for various systems. It was assumed that diffusion proceeds in the ion induced melts.

 

The thermal spike model elaborated for track formation [2] has been applied to electronic mixing [3]. A Gaussian temperature distribution is assumed in the model and simple equations are proposed for the melt radius r(t) and for the threshold of melt formation Set. An expression is derived for mixing in the melts predicting k=A[(Se/Set)2-2ln(Se/Set)-1] with A=161(Dd/DT) nm2 (DT – thermal diffusivity, Dd – diffusion coefficient). In the conditions of the spike Dd was estimated for the first time and 0.053, 0.03 and 0.004 cm2/s were obtained for Zn, Ni and Cu, respectively. Good agreement with the experimental results is obtained, when Set is calculated from the macroscopic properties according to the model. The peak temperatures Tp for irradiation with 350 MeV Au ions was 11000 K for the NiO, CuO and Cu2O layers, 6500 K and 9000 K for the ZnO and SiO2 layers, respectively. In a single track the concentration of Ni or Cu ions is reduced to 10% at a depth of 7.9 and 3.6 nm, respectively.

 

[1] W. Bolse and B. Schattat, Nucl. Instr. Meth. B209, 32 (2003).

[2] G. Szenes, Phys. Rev. B51, 8026 (1995).

[3] G. Szenes, Appl. Phys. Lett. 81, 4622 (2002).

The influence of the deposition energy on the growth of thin films

 

K. VANORMELINGEN

 

Katholieke Universiteit Leuven, Instituut voor Kern- en Stralingsfysica, Celestijnenlaan 200D, B-3001 Leuven, Belgium

 

Properties of thin films often depend on the deposition method used. Frequently this dependence is due to a difference in the energy of the deposited atoms, e.g. Chemical Vapor Deposition (absorption), Molecular Beam Epitaxy (10-100 meV) or sputtering (10 eV). We studied the energy dependence systematically, with special interest in the initial layer growth. We compared the growth of an ultra-thin Co film on a Si(111) substrate for two techniques: Low Energy Ion Deposition (LEID) and MBE. In LEID, an isotopically pure ion beam with energy of 50 keV is produced. This beam can be decelerated in an electrostatic lens to well defined energies between 0 and 200 eV. Prior to Co deposition, the samples were cleaned with a two-step silicon flux method to achieve the Si(111) 7x7 reconstruction. After deposition the samples were transported through vacuum to an in situ scanning tunneling microscope. The early stages of the layer formation are expected to influence the growth of thicker films. For a coverage of 2 ML we investigated the influence of the deposition energy on the surface morphology. Depositions with LEID show a higher cluster density and a lower surface roughness compared to MBE depositions, where the size of the clusters is of the order of half the size of a 7x7 unit cell. The surface roughness shows a minimum for a deposition energy of 25 eV. Plausible explanations for this behavior will be discussed.

Magnetization reversal of stripe arrays: comparison of magneto-optical and polarized neutron reflectivity results

 

HARTMUT ZABEL

 

Institut für Experimentalphysik / Festkoerperphysik,
Ruhr-Universitaet, D 44780 Bochum, Germany

 

Artificially patterned magnetic films are becoming of high interest thanks to advances in lithographic techniques and to applications in magnetic device applications. It is therefore of utmost importance to properly characterize the magnetization processes of these new systems with different experimental techniques. A number of new and powerful experimental tools have become available for analyzing their properties in real space and in reciprocal space. Lateral structures often show a variety of domain configurations. A method, which is very suitable for studying different magnetization processes is polarized neutron reflectivity (PNR) because it can distinguish between reversal via coherent rotation or domain nucleation and wall movement [1]. Furthermore, by applying PNR to lateral structures it is possible to perform measurements at Bragg peaks from the artificial periodicity and therefore filter out correlation effects between individual magnetic units. We studied the magnetization reversal of laterally structured CoFe films by PNR and compared the results to longitudinal magneto-optical Kerr effect with the field applied in longitudinal and transverse orientation (vector MOKE) [2-4]. Kerr-microscopy was used for visualizing the domain state. The parameters of the CoFe stripes were chosen such that a strong two-fold shape anisotropy is present in the polycrystalline stripes. By varying the width of the stripes the anisotropy changes. Consequently we observed more complex domain configurations for stripes with a larger width compared to those with smaller width. From PNR we also observe inhomogeneous stray fields most probably due to dipole-dipole interaction between the stripes.

 

We acknowledge founding by DFG, SFB 491 and BMBF 032AE8BO.

 

1. H. Zabel and K. Theis-Bröhl, J. Phys.: Condens. Matter 15, S505 (2003)

2. K. Theis-Bröhl, T. Schmitte, V. Leiner, H. Zabel,  K. Rott, H. Brückl, and J. McCord, Phys. Rev. B 67 (2003)

3. Till Schmitte, Kurt Westerholt, Hartmut Zabel, J. Appl. Phys. 92 (8), 4524 (2002)

4. T. Schmitte, K. Theis-Bröhl, V. Leiner, H. Zabel, S. Kirsch, and A. Carl, J. Phys.: Condens. Matter 14, 7525 (2002)