32 research outputs found
Anomalous Bias Dependence of Spin Torque in Magnetic Tunnel Junctions
We predict an anomalous bias dependence of the spin transfer torque parallel
to interface, , in magnetic tunnel junctions (MTJ), which can be
selectively tuned by the exchange splitting. It may exhibit a sign reversal
{\it without} a corresponding sign reversal of the bias or even a quadratic
bias dependence. We demonstrate that the underlying mechanism is the interplay
of spin currents for the ferromagnetic (antiferromagnetic) configurations,
which vary linearly (quadratically) with bias, respectively, due to the
symmetric (asymmetric) nature of the barrier. The spin transfer torque
perpendicular to interface exhibits a quadratic bias dependence.Comment: 4 pages, 5 figure
Voltage Dependence of Spin Transfer Torque in Magnetic Tunnel Junctions
Theoretical investigations of spin transfer torque in magnetic tunnel
junctions using the tight-binding model in the framework of non-equilibrium
Green functions formalism are presented. We show that the behavior of the spin
transfer torque as a function of applied voltage can vary over a wide range
depending on the band parameters of the ferromagnetic electrodes and the
insulator that comprise the magnetic tunnel junction. The behavior of both the
parallel and perpendicular components of the spin torque is addressed. This
behavior is explained in terms of the spin and charge current dependence and on
the interplay between evanescent states in the insulator and the Fermi surfaces
of ferromagnetic electrodes comprising the junction. The origin of the
perpendicular (field-like) component of spin transfer torque at zero bias, i.e.
exchange coupling through the barrier between ferromagnetic electrodes is
discussed.Comment: 5 pages,4 figure
Financial Fraud Detection using Quantum Graph Neural Networks
Financial fraud detection is essential for preventing significant financial
losses and maintaining the reputation of financial institutions. However,
conventional methods of detecting financial fraud have limited effectiveness,
necessitating the need for new approaches to improve detection rates. In this
paper, we propose a novel approach for detecting financial fraud using Quantum
Graph Neural Networks (QGNNs). QGNNs are a type of neural network that can
process graph-structured data and leverage the power of Quantum Computing (QC)
to perform computations more efficiently than classical neural networks. Our
approach uses Variational Quantum Circuits (VQC) to enhance the performance of
the QGNN. In order to evaluate the efficiency of our proposed method, we
compared the performance of QGNNs to Classical Graph Neural Networks using a
real-world financial fraud detection dataset. The results of our experiments
showed that QGNNs achieved an AUC of , which outperformed classical GNNs.
Our research highlights the potential of QGNNs and suggests that QGNNs are a
promising new approach for improving financial fraud detection.Comment: 15 pages, 18 figures, 4 table
Vertical current induced domain wall motion in MgO-based magnetic tunnel junction with low current densities
Shifting electrically a magnetic domain wall (DW) by the spin transfer
mechanism is one of the future ways foreseen for the switching of spintronic
memories or registers. The classical geometries where the current is injected
in the plane of the magnetic layers suffer from a poor efficiency of the
intrinsic torques acting on the DWs. A way to circumvent this problem is to use
vertical current injection. In that case, theoretical calculations attribute
the microscopic origin of DW displacements to the out-of-plane (field-like)
spin transfer torque. Here we report experiments in which we controllably
displace a DW in the planar electrode of a magnetic tunnel junction by vertical
current injection. Our measurements confirm the major role of the out-of-plane
spin torque for DW motion, and allow to quantify this term precisely. The
involved current densities are about 100 times smaller than the one commonly
observed with in-plane currents. Step by step resistance switching of the
magnetic tunnel junction opens a new way for the realization of spintronic
memristive devices
Effect of resistance feedback on spin torque-induced switching of nanomagnets
In large magnetoresistance devices spin torque-induced changes in resistance
can produce GHz current and voltage oscillations which can affect magnetization
reversal. In addition, capacitive shunting in large resistance devices can
further reduce the current, adversely affecting spin torque switching. Here, we
simultaneously solve the Landau-Lifshitz-Gilbert equation with spin torque and
the transmission line telegrapher's equations to study the effects of
resistance feedback and capacitance on magnetization reversal of both spin
valves and magnetic tunnel junctions. While for spin valves parallel (P) to
anti-parallel (AP) switching is adversely affected by the resistance feedback
due to saturation of the spin torque, in low resistance magnetic tunnel
junctions P-AP switching is enhanced. We study the effect of resistance
feedback on the switching time of MTJ's, and show that magnetization switching
is only affected by capacitive shunting in the pF range.Comment: 8 page
Magnetoresistance and spin-transfer torque in magnetic tunnel junctions
We comment on both recent progress and lingering puzzles related to research
on magnetic tunnel junctions (MTJs). MTJs are already being used in
applications such as magnetic-field sensors in the read heads of disk drives,
and they may also be the first device geometry in which spin-torque effects are
applied to manipulate magnetic dynamics, in order to make nonvolatile magnetic
random access memory. However, there remain many unanswered questions about
such basic properties as the magnetoresistance of MTJs, how their properties
change as a function of tunnel-barrier thickness and applied bias, and what are
the magnitude and direction of the spin-transfer-torque vector induced by a
tunnel current.Comment: 37 pages, 2 figures. Contribution to a collection of "Current
Perspectives" articles on spin transfer torque now available in the Journal
of Magnetism and Magnetic Material
Spin Transfer Torque and Tunneling Magnetoresistance Dependences on the Finite Bias Voltages and Insulator Barrier Energy
We investigate the dependence of perpendicular and parallel spin transfer
torque (STT) and tunneling magnetoresistance (TMR) on the insulator barrier
energy in the magnetic tunnel junction (MTJ). We employed single orbit tight
binding model combined with the Keldysh non-equilibrium Green's function method
in order to calculate the perpendicular and parallel STT, and TMR in MTJ with
the finite bias voltages. The dependences of STT and TMR on the insulator
barrier energy are calculated for the semi-infinite half metallic ferromagnetic
electrodes. We find that perfect linear relation between the parallel STT and
the tunneling current for the wide range of the insulator barrier energy.
Furthermore, the TMR also depends on the insulator barrier energy, which
contradicts to the Julliere's simple model
Magnetic domain wall motion by spin transfer
The discovery that a spin polarized current can exert a large torque on a
ferromagnet through a transfusion of spin angular momentum, offers a new way to
control a magnetization by simple current injection, without the help of an
applied external field. Spin transfer can be used to induce magnetization
reversals and oscillations, or to control the position of a magnetic domain
wall. In this review, we focus on this last mechanism, which is today the
subject of an extensive research, both because the microscopic details for its
origin are still debated, but also because promising applications are at stake
for non-volatile magnetic memories
Time-resolved detection of spin-transfer-driven ferromagnetic resonance and spin torque measurement in magnetic tunnel junctions
Several experimental techniques have been introduced in recent years in
attempts to measure spin transfer torque in magnetic tunnel junctions (MTJs).
The dependence of spin torque on bias is important for understanding
fundamental spin physics in magnetic devices and for applications. However,
previous techniques have provided only indirect measures of the torque and
their results to date for the bias dependence are qualitatively and
quantitatively inconsistent. Here we demonstrate that spin torque in MTJs can
be measured directly by using time-domain techniques to detect resonant
magnetic precession in response to an oscillating spin torque. The technique is
accurate in the high-bias regime relevant for applications, and because it
detects directly small-angle linear-response magnetic dynamics caused by spin
torque it is relatively immune to artifacts affecting competing techniques. At
high bias we find that the spin torque vector differs markedly from the simple
lowest-order Taylor series approximations commonly assumed.Comment: 29 pages, 5 figures including supplementary materia