V. Leiner

Influence of the ionic strength on the structure of polyelectrolyte films at the solid/liquid interface

Abstract The structure of polyelectrolyte multilayers built up by alternate adsorption of polyanions and polycations is investigated by X-ray reflectivity at the solid/air and neutron reflectivity at the solid/liquid interface. The experiments provide detailed information about the density gradient of polyelectrolyte chains across the film and show the influence of the water content of the film on the internal structure. The polyelectrolyte density is determined by the adsorption conditions (e.g. amount of NaCl) and cannot be changed by addition of salt after adsorption. After drying the film thickness is reduced by 30%.

Magneto-optical study of the magnetization reversal process of Fe nanowires

We discuss results of magneto-optical Kerr effect (MOKE) measurements performed on a thin Fe film of 13 nm thickness, which has been patterned into a periodic arrangement of nanowires by means of optical interference lithography. The resulting array of nanowires consist of stripes having a width of 150 nm and a periodicity of 300 nm. MOKE hysteresis loops are measured within magnetic fields which are aligned in different directions, both parallel and perpendicular with respect to the direction of the nanowires as well as for various angles in between. A particular arrangement of the longitudinal Kerr effect measurement allows us to identify both the longitudinal and the transverse component of the magnetization of Fe nanowires. From this both the angle and the magnitude of the magnetization vector are derived. For a non-parallel alignment of the nanowires with respect to the direction of the external magnetic field, the hysteresis loops consist of a plateau region with two coercive fields Hc1 and Hc2, which is discussed as resulting from an anisotropic pinning behaviour of magnetic domains in directions along and perpendicular to the nanowires.

Magnetic induction and domain walls in magnetic thin films at remanence

Magnetic domain walls in thin films can be well analysed using polarized neutron reflectometry. Well defined streaks in the off-specular spin-flip scattering maps are explained by neutron refraction at perpendicular Neel walls. The position of the streaks depends only on the magnetic induction within the domains, whereas the intensity of the off-specular magnetic scattering depends on the spin-flip probability at the domain walls and on the average size of the magnetic domains. This effect is fundamentally different and has to be clearly distinguished from diffuse scattering originating from the size distribution of magnetic domains. Polarized neutron reflectivity experiments were carried out using a 3He gas spin filter with a analysing power as high as 96% and a neutron transmission of approximately 35%. Furthermore, the off-specular magnetic scattering was enhanced by using neutron resonance and neutron standing wave techniques.

Tunability of the interlayer exchange coupling

The effect of the interlayer exchange coupling on the ordering temperature of ultrathin magnetic layers has been explored for two systems: Ho/Y and Fe/V superlattices. In both cases, the exchange coupling can be modified and eventually removed by the introduction of hydrogen into the mediating non-magnetic spacer layer. In case of Ho/Y we observed a drastic drop of the Ho Neel temperature from 105 K for coupled Ho blocks of 10 monolayer thickness to 65 K for isolated or uncoupled Ho blocks. In Fe/V superlattices, the incorporation of hydrogen changes the strength and sign of the exchange coupling constant. Again, we observed that the ordering temperature clearly depends on the strength of the exchange coupling in both the antiferromagnetically and the ferromagnetically coupled range of the phase diagram.

Adjustable magnetic interactions: the use of hydrogen as a tuning agent

Abstract The use of hydrogen to modify the electronic structure in magnetic thin films and heterostructures has opened new routes to tailor magnetic interactions in materials. The presence of hydrogen modifies the electronic structure of the host. Hydrogen can therefore be used to control the strength and character of magnetic interactions. This effect can be used to change the coupling strength in thin films, as well as selectively altering one of the constituents in artificial heterostructures. For example, the switching from antiferromagnetic to ferromagnetic order, and vice versa, has been demonstrated for exchange coupled magnetic superlattices. As the sign of the interlayer exchange coupling J ′ can be switched by the insertion of hydrogen, J ′ arbitrarily close to zero must be accessible. When J ′=0, the exchange interaction between adjacent magnetic layers is completely suppressed. The heterostructure can then be taken to consist of a collection of quasi two-dimensional magnetic sheets, when the ferromagnetic layers are very thin. Consequently, the introduction of hydrogen can be viewed as a route to tune the dimensionality of these structures.

Quantum states of neutrons in magnetic thin films

We have studied experimentally and theoretically the interaction of polarized neutrons with magnetic thin films and magnetic multilayers. In particular, we have analyzed the behavior of the critical edges for total external reflection in both cases. For a single film we have observed experimentally and theoretically a simple behavior: the critical edges remain fixed and the intensity varies according to the angle between the polarization axis and the magnetization vector inside the film. For the multilayer case we find that the critical edges for spin-up and spin-down polarized neutrons move toward each other as a function of the angle between the magnetization vectors in adjacent ferromagnetic films. Although the results for multilayers and single thick layers appear to be different, in fact, the same spinor method explains both results. An interpretation of the critical edges behavior for the multilyers as a superposition of ferromagnetic and antifferomagnetic states is given.

Magnetization reversal studies of magnetic wire arrays by polarized neutron scattering

Polarized neutron reflectivity has been used to investigate the peak pattern and the magnetization reversal process of laterally structured Fe- and CoFe-films. The peak pattern of the wire arrays was analyzed in the specular as well as in the off-specular regime. The intensities of the different cross-sections at the off-specular first-order peak were investigated as a function of external magnetic field. Magnetization reversals are discussed and compared to MOKE studies of the same system.

Antiferromagnetic dipolar ordering in [Co{sub 2}MnGe/V]{sub N} multilayers

We have studied [Co{sub 2}MnGe/V]{sub N} multilayers with a thickness of the V layers t{sub V} between 1.5 and 10 nm and a fixed thickness of the Heusler layer t{sub Co{sub 2}}{sub MnGe}=3 nm by x-ray scattering, neutron reflectivity, and magnetization measurements. In the thickness range t{sub V}{ =}4 nm the multilayers undergo a cluster glass transition at T{sub f}{approx_equal}150 K. At high temperatures above T{sub N} or T{sub f} the mutilayers are superparamagnetic with a huge cluster magnetic moment {mu}{sub c}{>=}10{sup 5}{mu}{sub B}.The halides CaCl{sub 2},CaBr{sub 2}, and CrCl{sub 2} all adopt, at room temperature, the same distorted rutile structure, in orthorhombic space group Pnnm, known as the calcium chloride structure. Upon heating, CaCl{sub 2} and CaBr{sub 2} each undergoes a continuous transformation to the true tetragonal rutile structure, in space group P4{sub 2}/mnm, the transition temperatures being 235 and 553 deg. C, respectively. By contrast, the structural change in CrCl{sub 2} upon heating is just further elongation of octahedra already lengthened by Jahn-Teller effects, and no phase transition occurs. The orthorhombic structure is maintained by a strong and temperature-dependent geometrical coupling of the orthorhombic strain to the order parameter, represented by the tilt angle of the CrCl{sub 6} octahedron.The author has studied the influence of fermion-boson conversion on Mott states near Feshbach resonances. It is demonstrated that Mott states are unstable with respect to fermion-boson conversion. A branch of collective modes in superfluids has been found, which involve antisymmetric phase oscillations in fermionic and bosonic channels and are always gapped. The low-energy quantum dynamics of a Fermi-Bose superfluid can be fully characterized by either an effective coupled U(1)xU(1) quantum rotor Hamiltonian or a coupled XXZxXXZ spin model.Anisotropic Pr-Fe-B thin films with perpendicular texture have been prepared by magnetron sputtering and subsequent heat treatment. After crystallization at 873 K, the films deposited at 773 K show a perpendicular anisotropic texture, due to the existence of a weak anisotropy in the as-deposited films. The deposition rate has been proved to be an important parameter for the control of microstructure, morphology, and coercivity. The coercivity mechanism of the anisotropic Pr-Fe-B films has been studied by analyzing the temperature dependence of coercivity from 5 to 300 K, based on the micromagnetic model. Nucleation of reversed domains, taking place preferentially at the grain surface where the magnetic anisotropy is reduced and the local demagnetization field is the highest, is determined to be the leading mechanism in controlling the magnetization reversal processes of the anisotropic Pr-Fe-B films. Though the grains in the films are strongly magnetically coupled, the magnetization reversal processes in the Pr-Fe-B thin films are not realized by uniform rotations of the magnetic moments.Electron spin resonance was applied on samples of Gd{sub 5.09}Ge{sub 2.03}Si{sub 1.88}. The results are discussed under the scope of magnetization measurements, optical metallography, and wavelength dispersive spectroscopy. Polycrystalline arc-melted samples submitted to different heat treatments were investigated. The correlation of the electron spin resonance and magnetization results permitted a characterization of the present phases and their transitions. Two coexisting phases in the temperature range between two phase transitions have been identified and associated to distinct crystallographic phases. Additionally, the magnetic moment at high temperatures has been estimated from the measured effective g factor. A peak value of 21.5 J/kg K for the magnetocaloric effect was obtained for a sample heat treated at 1500 deg. C for 16 h.Geometrical models of neutral single vacancy-arsenic complexes are determined from first principles and used for atomistic simulation of Rutherford backscattering channeling (RBS-C) spectra in heavily As-doped crystalline silicon, both with and without compensating Si self-interstitials. The goal is to investigate whether the relaxation patterns of complexes containing different numbers (from 1 to 4) of As atoms can be used as a fingerprint in structural analysis by conventional RBS-C. Simulation of RBS-C spectra in million-atoms supercells containing a population of As{sub m}V, show the off-lattice displacement of the Si atoms neighboring the vacancy, due to Jahn-Teller effect. On the other side, As displacement is found to be similar in all clusters investigated. The present results suggest that in the case of samples equilibrated at high temperature, the lack of any significant disorder of Si atoms is consistent with the hypothesis of electrically inactive As being in the form of either As{sub 3}V or As{sub 4}V complexes.The ternary germanide La{sub 3}Pd{sub 4}Ge{sub 4} has been prepared by arc melting. This compound takes a body-centered lattice with an orthorhombic unit cell with the lattice parameters of a=4.2200(3) A, b=4.3850(3) A, and c=25.003(2) A. The crystal structure of La{sub 3}Pd{sub 4}Ge{sub 4} is U{sub 3}Ni{sub 4}Si{sub 4}-type with the space group of Immm, consisting of the combination of structural units of AlB{sub 2}-type and BaAl{sub 4}-type layers. This compound is a type-II superconductor with a critical temperature (T{sub c}) of 2.75 K. The lower critical field H{sub c1}(0) is estimated to be 54 Oe. The upper critical field H{sub c2}(0) estimated by linear extrapolation of the H{sub c2}(T) curves is about 4.0 kOe, whereas the Werthamer-Hefland-Hohemberg theory gives H{sub c2}(0){sup WHH}=3.0 kOe. This is an interesting observation of superconductivity in the compounds with U{sub 3}Ni{sub 4}Si{sub 4}-type structure. The coherence length {xi}(0) of 330 A and the penetration depth {lambda}(0) of 2480 A are derived.

Magnetization Reversal of Stripe Arrays

Magnetic stripes with different aspect ratios provide control over the remanent domain state, the coercivity, and over different types of reversal mechanisms. We discuss two methods for evaluating the magnetization vector during reversal: vector magneto-optical Kerr effect and polarized neutron reflectivity. Both methods are compared and critically discussed.

Neutron waveguides: a new neutron optical device for the production of submicrometer neutron beams

Planar neutron waveguide structures can be used as resonant beam coupling devices to produce a coherent and divergent neutron line source with cross-sections in the sub-micrometer range. This article reveiws our recent work on the theoretical model, the fabrication and the characterization of these devices, demonstratign cleraly the good agreement between the theoretical model and the feasability of using neutron waveguides for various applications. As one major result, the farfield-pattern of the first three modes of a neutron waveguide was measured, yielding a 17 times enhanced flux throughput compared to a pair of slits corresponding to the thickness of the guiding layer. Additionally we have generalized the principle of neutron resonant beam coupling to waveguides containing multiple guiding layers, where several beams iwth a width of the order of 10 to 100 nm can be extracted at the end leading to an typical farfield interference pattern.