Thin Sandwiches Research Articles

Characterization by Auger spectroscopy of niobium–aluminum oxide–niobium sandwiches for Josephson junctions

Auger electron spectroscopy (AES) combined with ion sputtering was used to study the chemical composition of Nb–Al2O3–Nb thin film sandwiches. Attention was focused on the interfaces between the aluminum oxide and the two niobium films, because they are responsible for the electrical quality of the Josephson junctions based on these sandwiches. The real composition of the analyzed interfaces as a function of thickness was reconstructed from the AES signal intensities versus time data through an approximate conversion method. The obtained results show that the niobium layers adjacent to these interfaces are less pure than the rest of the films, due to aluminum and oxygen diffusion: a gradual aluminum diffusion up to 10–20 nm in the base niobium film was found.

Quantum singularities in the angular dependence of giant magnetoresistance in ultrathin ferromagnetic sandwiches

We present an analytical quantum statistical theory of the angular variation of the giant magnetoresistance (GMR) in thin ferromagnetic sandwiches (current flowing in the plane of the layers). Both spin-dependent scattering of conduction electrons and the existence of spin-dependent potential barriers between the successive layers are taken into account. We show that quantum singularities can arise in the variation of the electrical resistivity of the structure with the angle between the magnetizations in the ferromagnetic layers. These singularities result from the quantization of the momentum of electrons in the direction perpendicular to the plane of the layers.

Biquadratic exchange coupling in Fe/Cr/Fe(100) and Fe/Ag/Fe(100) thin film sandwiches (abstract)

We have investigated the magnetic exchange coupling between an Fe film and an Fe(100) single crystal whisker substrate separated by varying thickness Cr or Ag spacer layers. The magnetization of the Fe was measured using scanning electron microscopy with polarization analysis. Reflection high-energy electron diffraction and scanning tunneling microscopy (STM) were used to analyze the film structure. In addition to the oscillatory thickness-dependent bilinear exchange coupling, biquadratic exchange coupling was observed in both films. At spacer thicknesses that correspond to the transitions between ferromagnetic and antiferromagnetic coupling the Fe film magnetization was in-plane and orthogonal to the ferromagnetic alignment. Our observations generally agree with the model proposed by Slonczewski1 in which the biquadratic coupling originates from the bilinear coupling through spatial fluctuations of the spacer thickness. If these fluctuations occur over small enough length scales and the bilinear coupling varies between ferromagnetic and antiferromagnetic with monolayer thickness changes, then the Fe film intralayer exchange stiffness forces biquadratic coupling of the film to the substrate. We have observed the necessary short-period coupling in both Fe/Cr/Fe and Fe/Ag/Fe and we have measured the length scale of the thickness variations on a similar sample using a STM. One unexpected result in Fe/Ag/Fe was that the amount of biquadratic coupling increased with increasing top Fe layer thickness. No similar dependence was observed for Fe/Cr/Fe.

Switching behaviour of lead phthalocyanine thin films

Abstract Switching phenomena were found in thin film sandwiches of lead phthalocyanine. The structure and properties of these films depend on the deposition technology. We obtained a voltage controlled electrical switching and memory effect. Starting with insulating off-states we obtained at electric fields of 3 to 5 × lO4Vcm−1 a conductivity jump to higher values by up to ten orders of magnitude. Switching times shorter than 0·5 μs is were observed.

On direction of the magnetization of thin films and sandwiches as a function of temperature-II

The perpendicular to in-plane transition of thin ferromagnetic films is discussed. A simple theory by P.J. Jensen & K.H. Benemann, Phys. Rev. B42, 849 (1990) is compared with a renormalization treatment of the problem by D. Pescia & V.L. Pokrovsky, Phys. Rev. Lett. 65, 2599 (1990). Furthermore, extensions of these theories are studied.

Direction of the magnetization of thin films and sandwiches as a function of temperature

We present a simple model for the temperature and thickness dependence of the direction of magnetization of ferromagnetic thin films or sandwiches. Above a temperature that is much smaller than the Curie temperature, the magnetization in the presence of a uniaxial anisotropy is aligned completely parallel to the surface plane. The main reason for this is the entropy of the magnetization vector. Also, the direction of magnetization is determined by the competition between uniaxial and shape anisotropy. The decrease of the magnetic ordering for increasing temperature turns out to be not so important.

Fabrication of S-N-S Josephson junctions of Y-Ba-Cu-O/Au/Nb thin film sandwiches

The authors fabricated S-N-S (superconductor-normal-superconductor) Josephson junctions which consist of Y-Ba-Cu-O/Au/Nb thin-film sandwiches with various thicknesses of the Au barrier. Due to the AC-Josephson effect, Shapiro steps are observed in the S-N-S junctions under microwave radiation. The magnetic-field dependence of the critical current shows that the junctions behave as self-field-limited Josephson junctions. From the temperature dependence of the critical current near the transition temperature of the junctions, it is found that the thickness of the Au barrier is shorter than the coherence length of Au.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetic properties of ferromagnetic thin sandwiches correlated to their crystallographic structure and interface morphology: HREM and NMR study

High resolution electron microscopy (HREM) studies were made on Au/Co/Au sandwiches having ultrathin Co layers. These films were shown previously to possess an important interface anisotropy. We can conclude from the present experiments that there are some fluctuations of the c axis around the film normal, and change in lattice parameter for certain crystallites. (The Co layer is composed of small crystallites of about 7 nm.) All these crystallographic characteristics explain why the Co NMR spectrum is asymmetric with a low frequency tail.

Generalized de Gennes-Takahashi-Tachiki proximity effect theory

We generalize the de Gennes-Takahashi-Tachiki theory of coupled dirty superconductors to include the effects of orbital diamagnetism, spin (Pauli) paramagnetism, spin-orbit scattering, and magnetic impurity scattering. Some new expressions for thin film sandwiches and superlattices are obtained.

Magnetic properties of thin fcc iron films on Cu(001) (invited)

Thin epitaxial iron overlayers and sandwiches on Cu(001) have been investigated by spin-polarized photoemission. The magnetization was measured as a function of perpendicularly applied field and temperature. Up to 14 monolayers (ML) of iron grow epitaxially on Cu(001) in the fcc phase and are ferromagnetic. At 30 K a remanence magnetization perpendicular to the plane of the film is observed for both the Fe/Cu(001) and the Cu/Fe/Cu(001) systems with Fe layers thicker than 2 ML. The coercive field, measured as function of temperature for a 10-ML Fe film, decreases rapidly from 2.7 kOe at 30 K to about 50 Oe at 200 K. The Curie temperature is 300 K for films thicker than 8 ML and increases for thinner films up to 500 K. The transition from fcc to bcc iron occurs at 15 ML. It is recognized by a jump of the Curie temperature to 1000 K and by the disappearance of the remanence magnetization at 30 K. Simultaneous evaporation of Fe and Cu at various rates also results in epitaxial overlayers with characteristic magnetic features. The fcc Fe films are suitable for thermomagnetic recording. Writing and reading has been carried out by a UV excimer laser. Reading was performed by analyzing the sign of the spin polarization of the photoelectrons.

Measurements of hydrogen profiles in some thin film metal sandwiches using the 15N reaction method

Abstract Hydrogen depth profiles were measured using the nuclear resonance reaction 1 H( 15 N, αγ) 12 C in some thin film metal sandwiches deposited on stainless steel substrates. The samples examined were TaNb, NbTa and TaV, which were prepared by evaporation in an ultrahigh vacuum chamber. The hydrogen to metal ratios in the components of each sandwich were determined quantitatively. The samples were annealed at several temperatures in situ and after cooling to room temperature the profiles of the remaining hydrogen were measured. The observed changes in the hydrogen profiles are discussed qualitatively.

Pulsed laser reactive quenching at liquid-solid interface

Recent results on high-power pulsed-laser-induced transformations at liquid-solid interface are described in the context of synthesis of new metastable phases of materials. Specifically two types of problems are reported: (i) laser-induced synthesis of compound film at liquid solid interface, this process being termed “reactive quenching”, and (ii) laser-induced alloying of layered structure under liquid medium wherein the reactive aspect plays the minimal role. So far the reactive quenching process has been studied for different metals, compounds and thin film sandwiches in various liquid ambients such as H2O, liquid ammonia (NH3), liquid N2 and benzene (C6H6). The identification of the metastable phases and the microstructural transformations therein subsequent to laser processing and thermal annealing have been brought out by employing a range of techniques such as conversion electron Mossbauer spectroscopy (CEMS), X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), etc. Laser-induced alloying of layered structure in liquid ambient has been investigated in case of Fe/Al and Fe/B systems. The possible mechanism which could be responsible for the observed effects is discussed on the basis of time-resolved reflectivity measurements.

Time domain response of Al/Dy2O3/Al thin film capacitors and their a.c. characteristics

Abstract The charging and discharging current responses of dysprosium oxide thin film sandwiches after step voltage excitation are reported. Experimental measurements were carried out for times ranging from 100 μs to 1000 s and at temperature varying between 298 and 476 K.The transient currents follow the laws I ( t )∝ t − n ( T ) ( n = 0.22 − 0.60) and I ( t ) ∝ t −1.02 for short and long periods respectively. On the basis of the above results a.c. characteristics of the complex capacitance were evaluated and the dielectric response is discussed.

The voltage-current characteristics of thin MIM sandwiches with SiO x /Bi2O3 as the insulator

Regions of current-controlled and voltage-controlled negative resistance (CCNR and VCNR) have been observed in thin films of Bi2O3/SiOx with aluminium electrodes. ESCA measurements revealed the formation of metallic bismuth impurities in the dielectric, associated with a small activation energy for conductance, prior to any electroforming process. With copper electrodes only VCNR was observed. These thin films have been found to possess relatively high concentrations of dangling bonds.

Non-linear and hysteresis effects in the current-voltage characteristics of Ti/WO3/Ir thin film sandwiches

The current-voltage ( I–V ) characteristics of Ti/WO 3 /Ir thin film sandwiches were studied. Clear evidence of rectification was observed in the device. Marked I–V hysteresis was observed when the device was aged using a triangular wave of frequency 0.1 Hz (± 2.7 V). The hysteresis followed a triangular wave of frequency 1 kHz, indicating that the device can be used for high speed switching of electrochromic displays. The hysteresis is attributed to stretching and shrinkage of the depletion layer which is induced by protons injected into the WO 3 film.

Influence of paramagnons on proximity effect tunneling

The theory of superconductor-normal metal thin proximity sandwiches within McMillan's tunneling model is written down for the case when paramagnons are present in the normal side. It is shown that results of proximity effect tunneling experiments in Pd-Pb sandwiches are not inconsistent with a large paramagnon effect in Pd and with the calculated value λ ep = 0.41 for the electron-phonon mass enhancement parameter.

Polarized memory switching effects in Ag 2Se/Se/M thin film sandwiches

The current-voltage characteristics of Ag 2Se/Se/M thin film sandwiches were studied as functions of the shapes of the electrodes, which were either symmetrical or asymmetrical, as well as their composition (M is a metal (gold, chromium or silver) or Ag 2Se). A polarized memory switching effect independent of air exposure was observed. The first commutation occurs at a formation voltage V F which is dependent on the selenium thickness. After some cycles this voltage stabilizes to a value V S (where V S < V F ). V S is a function of temperature, frequency and the maximum reverse voltage but is independent of the selenium thickness. All these results are explained in terms of Ag + ionic diffusion: the formation is related to the growth of Ag 2Se dendrites through selenium as shown by temperature-dependent experiments. Furthermore the variations in V S are accounted for by ionic motion localized at the M-Se interface.

Polarized memory switching effects in Ag2Se/Se/M thin film sandwiches

Polarized memory switching effects in Ag2Se/Se/M thin film sandwiches

Measurement of the proximity effect in ferromagnetic-normal metal thin film sandwiches

A spin polarized tunneling measurement shows that ferromagnetism is suppressed in films of Ni thinner than three atomic layers when in contact with Al. In contrast, ferromagnetism was found in Fe, Co, Gd, and Tm films only one atomic layer thick when backed with Al. The present results agree with measurements of Ni, Fe, and Co using the anomalous Hall effect, but do not agree with photoemission results on Ni.

Theory of thin proximity-effect sandwiches. II. Effects ofs-wave elastic scattering

We show that Anderson's theorem concerning s-wave elastic scattering is inapplicable to certain types of inhomogeneous superconductors. The example of a proximity-effect sandwich consisting of a thin normal N metal in perfect contact with a thick superconducting (S)metal is considered in detail. The proper treatment of s-wave elastic scattering in the N metal is developed. Previously inexplicable experimental results on tunneling into NS sandwiches are found to be understandable in terms of the theory presented.