Search results for "Tunnelling"
showing 10 items of 218 documents
Impact of Annealing Temperature on Tunneling Magnetoresistance Multilayer Stacks
2020
The effect of annealing temperatures on the tunnel magnetoresistance (TMR) of MgO-based magnetic tunnel junctions (MTJs) has been investigated for annealing between 190 and 370°C. The TMR shows a maximum value of 215% at an annealing temperature of 330°C. A strong sensitivity of the TMR and the exchange bias of the pinned ferromagnetic layers on the annealing temperature are observed. Depending on sensor application requirements, the MTJ can be optimized either for stability and pinning strength or for a high TMR signal by choosing the appropriate annealing temperature. The switching mechanism of the ferromagnetic layers in the MTJ and the influence of the annealing on the layer properties,…
A Local Study of the Transport Mechanisms in MoS2 Layers for Magnetic Tunnel Junctions
2018
MoS2-based vertical spintronic devices have attracted an increasing interest thanks to theoretical predictions of large magnetoresistance signals. However, experimental performances are still far from expectations. Here, we carry out the local electrical characterization of thin MoS2 flakes in a Co/Al2O3/MoS2 structure through conductive tip AFM measurements. We show that thin MoS2 presents a metallic behavior with a strong lateral transport contribution that hinders the direct tunnelling through thin layers. Indeed, no resistance dependence is observed with the flake thickness. These findings reveal a spin depolarization source in the MoS2-based spin valves, thus pointing to possible solut…
Influence of the MgO barrier thickness on the lifetime characteristics of magnetic tunnelling junctions for sensors
2016
Magnetic tunnelling junctions increasingly enter the market for magnetic sensor applications. Thus, technological parameters such as the lifetime characteristics become more and more important. Here, an analysis of the lifetime characteristics of magnetic tunnelling junctions using the Weibull statistical distribution for CoFeB/MgO/CoFeB junctions is presented. The Weibull distribution is governed by two parameters, the characteristic lifetime η of the population and the shape parameter β, which gives information about the presence of an infant mortality. The suitability of the Weibull distribution is demonstrated for the description of dielectric breakdown processes in MgO-based tunnelling…
Laser-induced enhancement of tunneling in NHD2
2012
We apply and explore techniques aiming at enhancing the tunneling by laser fields, originally developed for a one-dimensional model, to a complete six-dimensional vibrational model of the inversion motion in NHD(2). The computational study is performed with the multi-configuration time-dependent Hartree method. Assuming an ideal three-dimensional alignment we obtain a driven tunneling time twenty times smaller than the natural one, in rather good agreement with an oversimplified three-state model. In the case of one-dimensional alignment, a linearly polarized field leads to a poor enhancement of the tunneling probability, after averaging over the rotation about the alignment axis, whereas a…
Supersymmetric structures for second order differential operators
2012
Necessary and sufficient conditions are obtained for a real semiclassical partial differential operator of order two to possess a supersymmetric structure. For the operator coming from a chain of oscillators, coupled to two heat baths, we show the non-existence of a smooth supersymmetric structure, for a suitable interaction potential, provided that the temperatures of the baths are different.
Properties of native ultrathin aluminium oxide tunnel barriers
2003
We have investigated planar metal–insulator–metal tunnel junctions with aluminium oxide as the dielectricum. These oxide barriers were grown on an aluminium electrode in pure oxygen at room temperature till saturation. By applying the Simmons model we derived discrete widths of the tunnelling barrier, separated by Δs ≈ 0.38 nm. This corresponds to the addition of single layers of oxygen atoms. The minimum thickness of s0 ≈ 0.54 nm is then due to a double layer of oxygen. We found a strong and systematic dependence of the barrier height on the barrier thickness. Breakdown fields up to 5 GV m−1 were reached. They decreased strongly with increasing barrier thickness. Electrical breakdown could…
Dynamics of spin state conversion processes in the solid state
1989
High spin (HS) ⇌ low spin (LS) conversions in transition metal complexes are nonradiative transitions between spin states. In this contribution, we present a study of the temperature and pressure dependence of the HS ⇌ LS intersystem crossing dynamics. For some iron(II) spin-crossover complexes, the rate constants were determined by line shape analysis of57Fe Mossbauer spectra. Their temperature dependence is described by an Arrhenius equation, their pressure dependence is interpreted within absolute rate theory. HS → LS conversion rates at low temperatures were determined from the relaxation of light-induced formation of HS states, monitored by optical spectroscopy. Deviations from a simpl…
Intersystem crossing in Fe(II) coordination compounds
1994
Fe(II) spin-crossover systems can be quantitatively converted from the low-spin (LS) to the high-spin (HS) state well below the thermal transition temperature by irradiating either into the metal-ligand charge transfer or d-d absorption bands, and even in low-spin systems a transient population of the HS state can be achieved. This fact can be made use of to determine HS → LS relaxation rate constants for a wide variety of Fe(II) spin-crossover and low-spin systems. The HS → LS relaxation shows strong deviations from an Arrhenius behaviour, with nearly temperature-independent tunnelling below ∼70 K and a thermally activated process above ∼100 K. The range of more than 12 orders of magnitude…
A Contradiction between Pulsed and Steady-State Studies in the Recombination Kinetics of Close Frenkel Defects in KBr and KCl Crystals
1994
Theoretical study of the kinetics of the correlated annealing of pairs of close (geminate) F-H centers in KCl and KBr crystals controlled by their diffusion and elastic attraction shows that the multi-step (kink) decay in defect concentrations observed more than once in thermostimulated experiments takes place only for very close F-H center pairs which are no further than fourth nearest neighbors. On the other hand, it is demonstrated (both theoretically and experimentally) that such F-H center pairs should be destroyed by the tunneling recombination already at time ≤10 -4 s, i.e. much before beginning of the thermostimulated experiments. Possible explanations of this contradiction are sugg…
Magnetic tunneling junctions with the Heusler compound
2005
Abstract Certain Heusler phases belong to the materials which are discussed as potential half metals. Here, results of tunneling experiments with the full-Heusler alloy Co 2 Cr 0.6 Fe 0.4 Al are presented. The Heusler alloy is used as an electrode of magnetic tunneling junctions. The junctions are deposited by magnetron DC sputtering using shadow mask techniques with AlO x as a barrier and cobalt as counter electrode. Measurements of the magnetoresistive differential conductivity in a temperature range between 4 and 300 K are shown. An analysis of the barrier properties applying the Simmons model to the bias dependent junction conductivity is performed. VSM measurements were carried out to …