Interlayer Coupling Strength Research Articles

Static and Dynamic Coupling Transitions of Vortex Lattices in Disordered Anisotropic Superconductors

We use three-dimensional molecular dynamics simulations of magnetically interacting pancake vortices to study vortex matter in disordered, highly anisotropic materials such as BSCCO. We observe a sharp 3D-2D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current reminiscent of a second peak effect. We find that decoupled pancakes, when driven, simultaneously recouple and order into a crystalline-like state at high drives. We construct a dynamic phase diagram and show that the dynamic recoupling transition is associated with a double peak in dV/dI.

Temperature dependence of the strength of the interlayer coupling of anisotropic HTS in normal state

It is shown that strong temperature dependence of the strength of interlayer coupling t c ( T) for anisotropic HTS in normal state is due by 2D superconducting fluctuations in CuO 2-plane, which lead to the different temperature dependencies of in-plane and out-of-plane coherent lengths.

Influence of surface roughness andH2adsorption on the interlayer coupling in Ni/Cu/Ni trilayers on Cu(001)

The interlayer coupling strength in Ni/Cu/Ni trilayers on Cu(001) depends strongly on the roughness of the surface of the topmost Ni layer and on the hydrogen coverage. Smoothing of the Ni surface increases mostly the coupling strength of the short period oscillation contribution. Hydrogen adsorption causes an enhancement and a phase change of this short period.

JOSEPHSON COUPLING STRENGTHS AND OVERLAPPING OF DIMENSIONS IN THE CROSSOVER REGION IN IRRADIATED MULTIPERIODIC HIGH-Tc SUPERCONDUCTORS

We have bombarded (Bi,Pb)-2223 superconductors with 40 MeV α-particles to study the effect of irradiation on the dimensional crossover in fluctuation-induced conductivity. The pair-breaking parameter decreases with the irradiation and suppression of the critical temperature is obtained. The structural modification of the layer in the high-T c superconductors induced by irradiation affects the extent of the overlapping region in the process of dimensional crossover. The width of the overlapping regime is evaluated for the various doses of irradiation using the different possible relations for the normal state resistivity. The variation of the transition width (Δ T0) with the increase in dose of irradiation is discussed in terms of modifications of interlayer and intralayer Josephson coupling strengths.

Correlation of magnetic and microstructural properties of obliquely deposited Co/Cr thin films

Single layers of Si(111)/Co(200 Å)/Ag(30 Å) and trilayers of Si(111)/Co(200 Å)/Cr(15 Å)/Co(200 Å)/Ag(30 Å) were deposited, using e-beam evaporation, at oblique angles of incidence to the sample normal (in particular 0°, 25°, 40°, 70°). The magnetic properties of the samples were studied using the technique of ferromagnetic resonance, while the microstructure was imaged using high resolution electron microscopy. We find that the angle of deposition of Co has a critical effect on the magnetic anisotropy of the samples, and on the interlayer coupling strength between the ferromagnetic layers of the trilayer samples. It is shown that these effects are due to the microstructure of the samples which is controlled by the angle of Co deposition with respect to the sample normal.

Numerical simulations of elastic properties of flux-line lattices in high-T/sub c/ superconductors

We propose a quantitative model of the effective interaction between the magnetic flux-lines in a layered superconductor. It is derived from the Lawrence-Doniach model with all of its parameters derivable from experiments. Numerical simulations are performed using the Langevin dynamics technique for parameters suitable for Bi-2212. We studied the elastic properties of flux line lattices with various values of interlayer coupling strength and pinning strengths. We show that the softening of the flux-line lattices with increasing temperature is mainly due to the increase of configurational entropy of the lattice. The calculated results show good agreement with experimental observations and provide a unique way to study the properties of flux-line lattices with random pinning.

Symmetry influence on interlayer coupling in epitaxial Co/Cr trilayers grown on MgO (1 0 0) and (1 1 0) substrates

Trilayers of Co(15 Å)/Cr(x)/Co(17 Å) have been epitaxially sputtered onto MgO (1 0 0) and (1 1 0) substrates coated with Cr(1 0 0) and (2 1 1) buffer layers, respectively. The Cr thickness x is varied from 6 to 50 Å. Both sample sets have the Co c-axis lying in the plane of the film, however, due to the four-fold symmetry of MgO (1 0 0), the Co layers form bicrystals with perpendicularly oriented c-axis. Ferromagnetic resonance measurements clearly show large two-fold and four-fold magnetic in-plane anisotropy for MgO (1 1 0) and MgO (1 0 0) samples, respectively. Magnetization measurements reveal interlayer coupling strengths peaked at a Cr thickness of approximately 10 Å for both symmetries, but the strength decreases more slowly with increasing t Cr in the MgO(1 0 0) system.

PtMn single and dual spin valves with synthetic ferrimagnet pinned layers

Spin valve films with the PtMn/Co/Ru/Co synthetic ferrimagnet pinned layer, in which the magnetization of the two Co layers is strongly coupled in an antiparallel orientation and the magnetization direction of one of the Co layers is pinned by unidirectional exchange coupling with PtMn, were investigated. PtMn synthetic ferrimagnet pinned layers were demonstrated to exhibit strong unidirectional exchange field Hex exceeding 2500 Oe at an optimum PtMn/Co/Ru/Co thickness combination for either PtMn on top or at the bottom. The Hex of PtMn/Co/Ru/Co rapidly increased at the PtMn layer thickness around 10–12 nm and was nearly constant from 12 to 30 nm. It was also found that the saturation field of the Co(3 nm)/Ru(0.8 nm)/Co(2 nm) sandwich, which represents the antiparallel interlayer coupling strength through Ru, remained 2200 Oe even at 300 °C. Thus the combination of the high blocking temperature of 380 °C for PtMn and the large antiparallel exchange interaction of Co/Ru/Co up to 300 °C makes the pinned layer highly stable against various magnetic fields during Joule heating of spin valve sensor stripes. Further, the PtMn synthetic-ferri-pinned-type dual spin valve films showed a large unidirectional exchange field of 2000 Oe and a giant magnetoresistance ratio of 11.0%.

On the free layer reversal mechanism of FeMn-biasedspin-valves with parallel anisotropy

Transmission electron microscopy has been used to investigate the `free' layer reversal mechanism of a range of FeMn-biased spin-valves with parallel anisotropy. Fresnel imaging enabled three modes of reversal to be directly observed for different in-plane applied field orientations and interlayer coupling strengths. Low-angle diffraction has provided some quantitative information on the level of dispersion involved in the reversals. Comparison with a modified Stoner-Wohlfarth coherent rotation model is made and magnetic modelling of the free layer energy variation as a function of magnetization orientation is presented. This has helped provide a deeper understanding of the observed discrepancies between the model and experimental data. Finally, a qualitative description of the observed domain processes is given in relation to the free layer energy for the corresponding experimental conditions.

Inhomogeneous interlayer Josephson coupling in

In a layered superconductor, the interlayer coupling strength determines the spatial variation of the magnetic fields parallel to the layers. Scanning SQUID microscope images of an edge of a single crystal of the layered organic superconductor show regions of inhomogeneous coupling as well as trapped interlayer Josephson vortices. Quantitative modelling of individual isolated vortices indicates an interlayer penetration depth . Previous bulk measurements that indicated a weaker interlayer coupling may have been dominated by inhomogeneities.

Isothermal Elastic Constants of Flux-Line Lattice in Layered Superconductors

A model of the effective interaction between the magnetic flux-lines in a layered superconductor is derived from the Lawrence–Doniach model. We show analytically that the intralayer interaction energy can be evaluated using the Ewald summation technique. The melting of flux line lattices is studied using Langevin dynamics simulation of the model with various values of interlayer coupling strength and pinning intensities. The thermal fluctuation terms of the isothermal shear modulus are found to increase sharply at the melting transition temperature for systems with or without pinning, while the structural order parameters were close to zero at all temperatures for systems with strong pinning. The melting of the pinned flux-line lattice is discussed in the context of its elastic properties.

Spectral Properties of Three-Dimensional Layered Quantum Hall Systems

We investigate the spectral statistics of a network model for a three dimensional layered quantum Hall system numerically. The scaling of the quantity $J_0={1/2}< s^2>$ is used to determine the critical exponent $\nu$ for several interlayer coupling strengths. Furthermore, we determine the level spacing distribution $P(s)$ as well as the spectral compressibility $\chi$ at criticality. We show that the tail of $P(s)$ decays as $\exp(-\kappa s)$ with $\kappa=1/(2\chi)$ and also numerically verify the equation $\chi=(d-D_2)/(2d)$, where $D_2$ is the correlation dimension and $d=3$ the spatial dimension.

Numerical simulations of flux-line lattices in layered superconductors

A model of the effective interaction between the magnetic flux-lines in a layered superconductor is derived from the Lawrence–Doniach model. We show analytically that the intralayer interaction energy can be evaluated using the Ewald summation technique. To illustrate the usefulness of the simulation technique, the isothermal shear modulus c 66 of flux-line lattices with various values of the interlayer coupling strength was obtained with a Langevin dynamics simulation.

Magnetic Properties of Layered Heisenberg Ferrimagnets

The method of spin-wave theory is used to study the magnetic properties of layered Heisenberg ferrimagnets. The internal energy and specific heat per site are calculated for different interlayer coupling strengths and temperatures. In the low temperature region θ (= K B T/6J) « θ 1 (= (2 + δ)/3), a characteristic temperature θ o = δ is given (δ is the ratio of ferrimagnetic interplanar coupling to planar coupling), the asymptotic expressions of internal energy anti specific heat are obtained for θ » θ o and θ « θ o , respectively. It is shown that the interlayer coupling has an important effect on the magnetic properties of ferrimagnets.

Microwave spectroscopic studies of obliquely deposited Co/Cr trilayers

Polycrystalline Co/Cr trilayers were grown by e-beam evaporation in ultra-high vacuum, with Co deposited at various oblique angles to the substrate. Microwave spectroscopic experiments were performed with the applied field in the in-plane and out-of-plane configurations to explore the resultant changes in anisotropy and interlayer coupling. The in-plane anisotropy, effective magnetisation and interlayer coupling strength are found to be greatly affected by the angle of Co deposition. In addition, it is found that samples deposited at intermediate angles exhibit absorption spectra which are more complicated than the two lines expected in a trilayer structure. This phenomenon is not fully understood at present, but may be due to the presence of Co atoms in the Cr interlayer. It can be concluded that the magnetic properties of these structures depend critically on the angle of deposition, and that the magnetic anisotropy and coupling can be modified by an appropriate choice of angle.

Magnetisation reversal mechanisms of FeMn-biased spin-valves as a function of interlayer coupling strength and field orientation

The Fresnel mode of Lorentz microscopy has been used to study the magnetisation reversal mechanisms of ‘free’ layers in a range of spin-valves in which the anisotropies of both magnetic layers are parallel. Fields were applied in situ in a modified TEM at various angles to the biasing direction on samples with different coupling strengths between ‘free’ and ‘pinned’ layers. Three distinct reversal modes were identified, as predicted using a modified Stoner-Wohlfarth theory. However, whilst the model only permits reversal by coherent rotation of magnetisation, domain processes were observed to assist two of the modes. A magnetic phase diagram has been constructed to show how reversal varies according to the orientation of the applied field and the coupling strength.

α-Irradiation induced changes in fluctuation-conductivity in (Bi, Pb)-2223 superconductors

We have irradiated (Bi, Pb)-2223 superconductors by 40 meV α-particles. The irradiation induces changes in resistivity as we increase the dose of irradiation. The various doses change the resistivity as a result of which the fluctuation induced conductivity also changes. We have analysed the experimental observations in the framework of Aslamazov-Larkin, Lawrence-Doniach and Maki-Thompson theory. We have extracted coherence lengths, dimensional crossover temperature and Josephson interlayer coupling strength. The role of disorders induced by irradiation has been discussed in this respect. The change in the fluctuation-conductivity is discussed in the light of the coherence length and the critical magnetic field H c .

In-Plane Paraconductivity of Bi2Sr2CaCu2O8 Single Crystals with Different Resistivity Anisotropies

Superconducting fluctuations have been studied by systematic measurements of the in-plane paraconductivity in Bi2Sr2CaCu2O8 single crystals with different resistivity anisotropies. It has been observed that the interlayer coupling strength decreases dramatically with the anisotropy of the sample, which significantly influences the dimensional behaviour of the in-plane paraconductivity. For all the samples studied, the in-plane paraconductivity is in good agreement with the Lawrence and Doniach model. For the sample of highest resistivity anisotropy, the behaviour of the in-plane paraconductivity is basically two-dimensional, but deviates significantly from two-dimensional behaviour as the resistivity anisotropy decreases.

Oxygen Knock-Out and Other Studies in α-Irradiated Polycrystalline Bi-2212 Superconductor

Bulk polycrystalline samples of Bi 2 Sr 2 CaCu 2 O 8+x (Bi-2212) have been irradiated with 40 MeV α-particles. T c increases up to a certain dose. The increase in T c is correlated with the knock-out of oxygen, which has been verified by the determinations of the oxygen contents of the irradiated samples by iodometry. A model of the knock-out of oxygen is proposed on the basis of Monte-Carlo TRIM calculations. Resistivity versus temperature of the irradiated samples shows fairly metallic behaviour up to a certain dose. Excess conductivity analysis shows a cross-over from 2D to 3D behaviour in conductivity for the unirradiated sample. However, for irradiated samples, the critical fluctuation regime sets in. The interlayer coupling strengths decrease with the increase, in the irradiation dose. The sample with the highest dose shows a nonmetallic behaviour in resistivity. A detailed analysis shows a conductivity behaviour in the nonmetallic region characteristic of three-dimensional variable range hopping of charge carriers.

Ion beam deposition and structural characterization of GMR spin valves

NiO exchange-biased "bottom" spin valves of the type NiO/NiFe/Co/Cu/Co/NiFe and FeMn exchange-biased "top" spin valves of the type NiFe/Co/Cu/Co/NiFe/FeMn were deposited by ion-beam deposition (except the NiO layer). Their magnetic properties, magneto-transport, and microstructures are characterized and compared with corresponding GMR spin valves deposited by dc magnetron sputtering. High-resolution cross-sectional transmission electron microscopy and X-ray diffraction reveal microstructural differences between ion-beam-deposited and dc magnetron sputtered spin valves. In particular, film texture, surface morphology, GMR ratio, exchange bias, interlayer coupling strength, and coercivity vary widely, but property-structure-processing correlation can be identified. A GMR ratio of /spl sim/9.7% was obtained on random textured NiO exchange-biased bottom spin valves by ion-beam deposition.