33 research outputs found
Diffusive model of current-in-plane-tunneling in double magnetic tunnel junctions
We propose a model that describes current-in-plane tunneling transport in
double barrier magnetic tunnel junctions in diffusive regime. Our study shows
that specific features appear in double junctions that are described by
introducing two typical length scales. The model may be used to measure the
magnetoresistance and the resistance area product of both barriers in
unpatterned stacks of double barrier magnetic tunnel junctions.Comment: 4 pages, 3 figure
Temperature induced crossover between 0 and pi states in S/F/S junctions
Ferromagnetic Josephson junctions can show at equilibrium a pi phase difference between the superconducting electrodes. We explain this pi state in an original way by a modified spectrum of Andreev bound states shifted by the exchange energy. A simplified expression for the spectral supercurrent density is calculated and the non-monotonic temperature dependence of the critical current is discussed. This model accounts for the cancellation of the critical current with temperature observed in a small range of barrier thickness in our Nb/Cu52Ni48/Nb junctions. This cancellation corresponds to an inversion of the supercurrent and to a ground state crossover from a 0 state to a pi state. This transition is caused both by the thermal distribution of quasi-particles and by the temperature dependence of the exchange energy. The experimental curves are well reproduced by our theoretical expression except for the very small amplitude of the supercurrent attributed to a large spin-flip scattering
Half-integer Shapiro steps at the 0-pi crossover of a ferromagnetic Josephson junction
We investigate the current-phase relation of S/F/S junctions near the
crossover between the 0 and the pi ground states. We use Nb/CuNi/Nb junctions
where this crossover is driven both by thickness and temperature. For a certain
thickness a non-zero minimum of critical current is observed at the crossover
temperature. We analyze this residual supercurrent by applying a high frequency
excitation and observe the formation of half-integer Shapiro steps. We
attribute these fractional steps to a doubling of the Josephson frequency due
to a sin(2*phi) current-phase relation. This phase dependence is explained by
the splitting of the energy levels in the ferromagnetic exchange field.Comment: 4 pages, 5 figures, accepted for publication in Phys. Rev. Let
The skyrmion switch: turning magnetic skyrmion bubbles on and off with an electric field
Nanoscale magnetic skyrmions are considered as potential information carriers
for future spintronics memory and logic devices. Such applications will require
the control of their local creation and annihilation, which involves so far
solutions that are either energy consuming or difficult to integrate. Here we
demonstrate the control of skyrmion bubbles nucleation and annihilation using
electric field gating, an easily integrable and potentially energetically
efficient solution. We present a detailed stability diagram of the skyrmion
bubbles in a Pt/Co/oxide trilayer and show that their stability can be
controlled via an applied electric field. An analytical bubble model, with the
Dzyaloshinskii-Moriya interaction imbedded in the domain wall energy, account
for the observed electrical skyrmion switching effect. This allows us to unveil
the origin of the electrical control of skyrmions stability and to show that
both magnetic dipolar interaction and the Dzyaloshinskii-Moriya interaction
play an important role in the skyrmion bubble stabilization
Gate-Controlled Skyrmion Chirality
Magnetic skyrmions are localized chiral spin textures, which offer great
promise to store and process information at the nanoscale. In the presence of
asymmetric exchange interactions, their chirality, which governs their
dynamics, is generally considered as an intrinsic parameter set during the
sample deposition. In this work, we experimentally demonstrate that this key
parameter can be controlled by a gate voltage. We observed that the
current-induced skyrmion motion can be reversed by the application of a gate
voltage. This local and dynamical reversal of the skyrmion chirality is due to
a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we
attribute to ionic migration of oxygen under gate voltage. Micromagnetic
simulations show that the chirality reversal is a continuous transformation, in
which the skyrmion is conserved. This gate-controlled chirality provides a
local and dynamical degree of freedom, yielding new functionalities to
skyrmion-based logic devices.Comment: 4 figure
Transport and vortex pinning in micron-size superconducting Nb films
We have carried out Hall measurements on thin films of Nb in the flux-flow regime. The Hall bars were several microns in scale. Oscillations with magnetic field in the transverse and longitudinal resistances between the depinning field Bd and the upper critical field Bc2 are observed below Tc. The Hall effect may even change sign. The transverse and longitudinal resistances are interpreted in terms of current-driven motion of vortices in the presence of a few impurities. Simulations from time-dependent Ginzburg-Landau equations confirm this argument
Tunability versus deviation sensitivity in a nonlinear vortex oscillator
International audienc
Barrier Breakdown Mechanisms in MgO-Based Magnetic Tunnel Junctions and Correlation With Low-Frequency Noise
International audienc