Resistivity, rho(T), of the amorphous Re x Si1-x thin films with x = 0.285-0.351 is investigated in the interval of T similar to 300-0.03 K. At x = 0.285-0.324 the activated behavior of rho(T) is governed by the Mott and the Shklovskii-Efros variable-range hopping (VRH) conduction mechanisms in different temperature intervals and the three-dimensional regime of the hopping. Between x = 0.328 and 0.351 the activationless dependence of.(T) takes place. The critical behavior of the characteristic VRH temperatures and of the Coulomb gap, Delta, pertinent to proximity to the metal-insulator transition at the critical value of x(c) approximate to 0.327, is observed. The analysis of the critical behavior of Delta yields directly the critical exponent of the dielectric permittivity, eta = 2.1 +/- 0.2, in agreement with the theoretical prediction, eta = 2. On the other hand, the values of the critical exponent of the correlation length nu similar to 0.8-1.1 close to the expected value of unity can be obtained from the analysis of the critical behavior of the VRH characteristic temperatures under an additional assumption of a strong underbarrier scattering of hopping charge carriers in conditions, when the concentration of scattering centers considerably exceeds the concentration of sites involved in the hopping.

Using the spin switch design F1/F2/S theoretically proposed by Oh et al., [Appl. Phys. Lett. 71, 2376 (1997)], that comprises a ferromagnetic bilayer as a ferromagnetic component, and an ordinary superconductor as the second interface component, we have realized a full spin switch effect for the superconducting current. An experimental realization of this spin switch construction was achieved for the CoOx/Fe1/Cu/Fe2/In multilayer.

C. D. Cao,1,2 R. Klingeler,1,3 H. Vinzelberg,1 N. Leps,1 W. Loser,1 G. Behr,1 F. Muranyi,1,4 V. Kataev,1 and B. Buchner1 1Leibniz-Institut fur Festkorperund Werkstoffforschung (IFW) Dresden, Postfach 270116, D-01171 Dresden, Germany 2Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China 3Kirchhoff Institute for Physics, University of Heidelberg, INF 227, D-69120 Heidelberg, Germany 4Physics Institute, University of Zurich, Winterthurerstr. 190, 8057 Zurich, Switzerland Received 19 November 2009; revised manuscript received 19 July 2010; published 27 October 2010

An experimental study of magnetic and superconducting properties of a trilayer Ni/V/Ni thin film system grown on single-crystalline MgO(001) substrate is reported. The field dependence of the superconducting transition temperature T c for samples comprising Ni layers with similar values of the coercive field H c reveals no anomalies. However, in samples with different thicknesses of the nickel layers the difference in H c amounts up to Δ H c ∼ 1.8 kOe, thus enabling to manipulate the relative orientations of the layers’ magnetization by an external magnetic field. Surprisingly, for these samples the T c for the parallel orientation of the magnetizations of the Ni layers is higher, in a certain magnetic field range, than for the antiparallel one, at odds with theoretical predictions. Possible reasons of this contradiction are discussed.

This paper presents results on the preparation, structural, electrical and magnetic properties of Fe(3)Si films as a representative for a Heusler alloy-like compound which are known as half-metallic materials with ferromagnetic behaviour. The films have been prepared by means of ultra-high vacuum (UHV) electron beam evaporation with the aim of achieving epitaxial growth on GaAs(100) substrates. The main focus of this work is the structural characterization of the Fe(3)Si films grown on GaAs by means of high resolution transmission electron microscopy (TEM) to confirm the epitaxial growth. For Fe(3)Si with a composition in the vicinity of stoichiometry an almost lattice-matched growth on GaAs(001) has been observed characterized by a high crystalline quality and a good interface perfection. Besides the studies on cross-sectional samples by TEM data from reflection high energy electron diffraction (RHEED) and x-ray diffraction (XRD) were also included into the discussion. The electrical and magnetic parameters of the films studied are found to be in good agreement with data reported for the best Fe(3)Si molecular beam epitaxy (MBE) layers. As evidenced by x-ray diffraction, transmission electron microscopy, resistivity and magnetic measurements, we find an optimum growth temperature of 280-350 degrees C to obtain ferromagnetic layers with high crystal and interface perfection as well as a high degree of atomic ordering.

The paper presents resistivity and magnetization measurements on nearly stoichiometric Fe3Si films epitaxially grown on GaAs substrates by electron-beam evaporation in an ultrahigh vacuum chamber. In the low-temperature resistivity a T3 term was found in all samples. A term like that is known to describe the anomalous single-magnon scattering processes in half-metallic materials and confirms so for our samples the hypothesis of half-metallic ferromagnetism in Fe3Si. The films show an anisotropic magnetoresistance in low magnetic fields. In high magnetic fields a negative longitudinal and transverse magnetoresistance (MR) has been observed linearly depending on the field strength. In the vicinity of 200 K the MR shows maximum absolute values up to 1.5% at magnetic fields of about 8 T. From the magnetization measurements a magnetic moment of 0.86μB/atom was obtained, which is close to that of bulk Fe3Si.

The static and dynamic magnetic properties, electrical resistivity, specific heat, and magnetoresistance have been studied in EuCu(2)Si(2) single crystals grown by a floating zone method. The magnetic susceptibility exhibits a considerable anisotropy and a steep rise below 10 K for external fields parallel to the c axis but with no evident magnetic ordering in the temperature range of 2-350 K. The data imply a gradual change in the Eu valence as a function of temperature. Electron spin resonance (ESR) measurements reveal a sizeable fraction of stable Eu(2+) magnetic moments that interact with conduction electrons and develop quasistatic antiferromagnetic correlations on the ESR timescale. The electrical resistivity and specific heat demonstrate the presence of spin fluctuations and Kondo-like behavior, which apparently competes with the antiferromagnetic order. The analysis of experimental data enables to conclude that the remarkable diversity of the physical properties of EuCu(2)Si(2) results from the variation of lattice parameters as well as of local crystal chemistry as a consequence of the particular preparation route employed for the growth of single crystals and polycrystals.

Single crystal and polycrystalline near-stoichiometric RuAl alloys were grown with the magnetic field controlled RF-floating zone technique. The resistivity measured from the room temperature to 4.2K decreases linearly up to 70K. The reciprocal residual resistance ratio is very sensitive to changes in composition and shows a maximum value of 6.7 close to the stoichiometric composition. However, the Vickers hardness decreases to a minimum where the reciprocal residual resistance ratio is maximum.

The article gives a report on resistivity measurements on Mn-doped p-type FeSi2 single crystals and analyzes the data within the framework of different hopping conductivity models. Both the Mott [N. Mott and E. A. Davies, Electron Processes in Non-Crystalline Materials (Clarendon, Oxford, 1979)] and the Shklovskii–Efros [B. I. Shklovskii and A. L. Efros, Electronic Properties o0066 Doped Semiconductors (Springer, Berlin, 1984)] regimes of the variable-range hopping (VRH) conductivity are observed. It is shown that the temperature dependence of the resistivity of Mn-doped β-FeSi2 crystals, which follows a VRH conduction mechanism, can be expressed by a scaling expression of the form ln(ρ/ρ0)=Af(T/Tx). The characteristic and transition temperatures, as well as the complete set of parameters describing the properties of the localized holes (the localization radius, the dielectric permittivity, the width of the Coulomb gap Δ, and the values of the density of states at the Fermi level) are determined. The data above indicate existence of a rigid gap δ in the spectrum of the DOS in addition to Δ and point out to the polaronic nature of the charge carriers in the investigated compound.

Changes in the magnetization structure of an antiferromagnetically afm -coupled metallic multilayer as a function of the applied field H along the easy axis may involve both spin-flip and spin-flop events. The latter are widely discussed as the origin of the characteristic shapes of the magnetization M H and giant magnetoresistance GMR curves. In this work, we demonstrate the influence of spin-flip processes, which result in very different magnetization reversal and resistivity characteristics as compared to the spin-flop case: sharp, steplike GMR and magnetization changes for both the surface and internal layers—including magnetic viscosity effects—are observed. By means of Kerr microscopy, Kerr magnetometry, GMR, and magnetization measurements we show that spin-flip transitions via domain wall displacement constitute the relevant mechanism of magnetization reversal, provided that the anisotropy field HK in the multilayer surpasses the antiferromagnetic coupling field Hafm. In this case, a linear dependence of the GMR on the magnetization is observed, whereas for fields applied along the hard axis magnetization rotation results in a quadratic dependence. The strong change of the ratio HK /Hafm could be realized for measuring temperatures between 4.2 and 470 K in a series of wedge-type NiFe /Cu multilayers prepared by dc magnetron sputtering and showing GMR amplitudes of up to 12% 300 K and 28% 4.2 K in the second afm coupling maximum with extremely low values of the afm coupling strength.

This paper concerns with giant magnetoresistance (MR) effects in organic spin valves, which are realized as layered (La,Sr)MnO3 (LSMO)-based junctions with tris-(8, hydroxyquinoline) aluminum (Alq3)-spacer and ferromagnetic top layers. The experimental work was focused on the understanding of the transport behavior in this type of magnetic switching elements. The device preparation was carried out in an ultrahigh vacuum chamber equipped with a mask changer by evaporation and sputtering on SrTiO3 substrates with LSMO stripes deposited by pulsed laser technique. The field and temperature dependences of the MR of the prepared elements are studied. Spin-valve effects at 4.2K have been observed in a broad resistance interval from 50Ω to MΩ range, however, without systematic dependence on spacer layer thickness and device area. In some samples, the MR changes sign as a function of the bias voltage. The observed similarity in the bias voltages dependences of the MR in comparison with conventional magnetic tunnel junctions with oxide barriers suggests a description of the found effects within the classical tunneling concept. This assumption is also confirmed by a similar switching behavior observed on ferromagnetically contacted carbon nanotube devices. The proposed model implies the realization of the transport via local Co chains embedded in the Alq3 layer and spin dependent tunneling over barriers at the interface Co grains∕Alq3∕LSMO. The existence of conducting Co chains within the organics is supported by transmission electron microscopic∕electron energy loss spectroscopic studies on cross-sectional samples from analogous layer stacks.

Resistivity ρ(T) of Co-doped n-type FeSi2 single crystals is investigated in the temperature range of 4.2K⩽T⩽300K, and analyzed within the framework of different hopping conductivity models according to the relation ρ(T)=ApT1∕pexp[(T0∕T)1∕p]. Both the Mott regime with p=4 [Metal-Insulator Transitions (Taylor & Francis, London, 1990)] and the Shklovskii-Efros regime with p=2 [Electronic Properties of Doped Semiconductors (Springer, Berlin, 1984)] within the variable-range hopping conductivity are observed, as well as a crossover of them, where the temperature dependence of the resistivity follows a scaling law ln(ρ∕ρ0)=Af(T∕Tx). The values of the characteristic (T0) and transition temperatures and the width of the soft Coulomb gap in the spectrum of the density of the localized states (DOS) Δ≈0.5–1.6meV are determined. In addition to Δ, our results suggest existence of the rigid gap in the spectrum of the DOS with the value up to δ≈0.35meV. The rigid gap is consistent with a polaronic nature of the charge carriers in the investigated compound.

The magnetization dynamics of a constrained cross-tie wall in a 50-nm-thick Ni81Fe19 element is investigated using static and time-resolved wide-field Kerr microscopy. By applying a series of short magnetic-field pulses to the sample the creation of additional vortex antivortex pairs within the cross-tie wall is observed. The cross-tie spacing decreases by a factor of 2–4 relative to the initial equilibrium state. By using sinusoidal and pulsed magnetic-field excitations, a discrimination between the influence of the repetition rate and the rise time of the applied fields is possible. We found a logarithmic increase of the Bloch-line number with decreasing rise time. A resonant excitation with increased repetition rate decreases the cross-tie spacing even further. From direct stroboscopic observation of the precessional remagnetization process, a mechanism for vortex multiplication is proposed.

The switching of extended Ni81Fe19 thin film elements with a thickness of 50 nm and various shapes (squared, rectangular, pointed) has been studied by time-resolved stroboscopic Kerr microscopy based on a conventional wide-field optical polarization microscope. The elements are deposited on coplanar strip-lines that generate field pulses driven by electronic pulse generators. Time resolution is obtained by imaging with a gated and intensified charge-coupled device camera. The opening can be varied from 250 ps to continuous exposure, allowing the comparison of fast magnetization processes and quasistatic switching in slowly varying fields. The latter is typically characterized by the formation of a concertina domain pattern that irreversibly decays in a multidomain ground state by the abrupt motion of vortices and domain walls. After excitation with fast field pulses similar blocked patterns are formed. They dissolve by spatially inhomogeneous rotational processes involving cross-tie-wall-like domain boundaries.

The magnetic and electrical properties of β-FeSi2 single crystals have been studied. It is shown that the magnetization behavior of Crand Ni-doped samples significantly depends on the cooling regime: cooling in zero external field (ZFC) and cooling with external field (FC). This resembles the properties of spin-glasses and indicates the presence of coupling between magnetic centers. The results of resistivity measurements are analyzed within the framework of different hopping conductivity models. Both the Mott and the Shklovskii-Efros regimes of the variable-range hopping are observed.

Conductive polypyrrole (Ppy) nanowires (see picture) can be grown from single adsorbed synthetic polyelectrolyte molecules by oxidative polycondensation of pyrrole. The location and length of the Ppy nanowires are defined by the location and length of the adsorbed single-molecule templates. The diameter of the Ppy nanowires varies from a few to hundreds of nanometers and can be adjusted by varying the reaction conditions. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2005/z501354_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

The resistivity of Cr-doped p-type β-FeSi2 single crystals has been investigated in the temperature interval of T=4.2–300K. The data are analyzed within the framework of the variable-range hopping (VRH) mechanism for both the Mott and the Shklovskii–Efros regimes. With decreasing temperature a crossover between the Mott and the Shklovskii–Efros VRH conductivity regimes is observed. It is shown that the temperature dependence of the resistivity, ρ (T), of Cr-doped β-FeSi2 crystals, that follows a VRH conductivity mechanism, obeys a scaling law ln(ρ∕ρ0)=Af(T∕Tx). The values of the characteristic (T0) and the transition temperatures (Tv), as well as the width of the soft Coulomb gap Δ in the spectrum of the density of the localized states (DOS) are determined. The observed values of the VRH transition and characteristic temperatures indicate the existence of a rigid gap δ in the spectrum of the DOS in addition to Δ. This points to the polaronic nature of the charge carriers in the investigated compound.

AlOx tunnel barriers prepared by oxidizing ultra-thin Al films of various thickness by means of a Rf wave resonance plasma beam source were investigated to understand the influence of the plasma oxidation conditions on the junction resistance Rj, the magnetoresistance ratio (MR) and the switching characteristics of exchange-biased magnetic tunnel junctions (MTJs) with NiMn pinning layer. The junction properties were characterized as a function of oxidation time, plasma power and distance between plasma source and sample. The MR of the as-deposited junction was about 15%. The highest exchange bias fields (17mT) and pinned layer coercivities (23mT) can be achieved with extended annealing at low temperatures T≅320∘C or with a rapid annealing at T≅400∘C, respectively. Short-time annealing (1min) at intermediate temperatures (350°C) and field cooling at 1T leads to the highest MR effect of 35% at room temperature and 55% at 4.2K.

Electron spin is not exploited in conventional semiconductor transistors. However, spin dependent devices have, significant potential for data storage technology and future electronic devices, Due to their large spin-flip scattering length, carbon nanotubes (CNTs) are a promising candidate for spin dependent devices. High magneto resistivity effects were recently discovered on ferromagnetically contacted CNTs [1-2]. Ferromagnetically filled multimall CNTs (MWCNTs) [3] represent an exciting compound material class, potentially providing excellent magnetic, electrical and mechanical properties. However, electric transport measurement results exist only on as grown two-dimensional arrays of aligned Fe-filled MWCNTs [4]. In order to realize future nanotube spintronic devices, it is necessary to understand the transport properties of both a nanotube (i.e. excluding contacts) and a transition zone between the tube and the electrode. Single tube devices were produced by using an AC-electrophoresis deposition on predefined Au- or Ti-finger structures. The measured magnetotransport data show a broad spectrum of behaviours: positive and negative magnetoresistance, oscillations and shoulders.