65 research outputs found
Electric Control of Spin Injection into a Ferroelectric Semiconductor
Electric-field control of spin-dependent properties has become one of the most attractive phenomena in modern materials research due to the promise of new device functionalities. One of the paradigms in this approach is to electrically toggle the spin polarization of carriers injected into a semiconductor using ferroelectric polarization as a control parameter. Using first-principles density-functional calculations, we explore the effect of ferroelectric polarization of electron-doped BaTiO3 (n-BaTiO3) on the spin-polarized transmission across the SrRuO3/n-BaTiO3(001) interface. Our study reveals that, in this system, the interface transmission is negatively spin polarized and that ferroelectric polarization reversal leads to a change in the transport spin polarization from −65% to −98%. Analytical model calculations demonstrate that this is a general effect for ferromagnetic-metal–ferroelectric-semiconductor systems and, furthermore, that ferroelectric modulation can even reverse the sign of spin polarization. The predicted effect provides a nonvolatile mechanism to electrically control spin injection in semiconductor-based spintronics devices
Giant Electroresistance in Ferroelectric Tunnel Junctions
The interplay between the electron transport in metal/ferroelectric/metal
junctions with ultrathin ferroelectric barriers and the polarization state of a
barrier is investigated. Using a model which takes into account screening of
polarization charges in metallic electrodes and direct quantum tunneling across
a ferroelectric barrier we calculate the change in the tunneling conductance
associated with the polarization switching. We find the conductance change of a
few orders of magnitude for metallic electrodes with significantly different
screening lengths. This giant electroresistance effect is the consequence of a
different potential profile seen by transport electrons for the two opposite
polarization orientations.Comment: 4 page
Ferroelectric Dead Layer Driven by a Polar Interface
Based on first-principles and model calculations we investigate the effect of
polar interfaces on the ferroelectric stability of thin-film ferroelectrics. As
a representative model, we consider a TiO2-terminated BaTiO3 film with LaO
monolayers at the two interfaces that serve as doping layers. We find that the
polar interfaces create an intrinsic electric field that is screened by the
electron charge leaking into the BaTiO3 layer. The amount of the leaking charge
is controlled by the boundary conditions which are different for three
heterostructures considered, namely Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and
SrRuO3/LaO/BaTiO3/LaO. The intrinsic electric field forces ionic displacements
in BaTiO3 to produce the electric polarization directed into the interior of
the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces
that is non-switchable and thus detrimental to ferroelectricity. Our
first-principles and model calculations demonstrate that the effect is stronger
for a larger effective ionic charge at the interface and longer screening
length due to a stronger intrinsic electric field that penetrates deeper into
the ferroelectric. The predicted mechanism for a ferroelectric dead layer at
the interface controls the critical thickness for ferroelectricity in systems
with polar interfaces.Comment: 33 Pages, 5 figure
Spin blockade in ferromagnetic nanocontacts
Using a free-electron model and a linear response theory we investigate spin-dependent electronic transport in magnetic nanocontacts in the ballistic regime of conduction. We emphasize the fact that in atomic-size ferromagnetic contacts it is possible to achieve the conductance value of e2/h, which implies a fully spin-polarized electric current. We explore some consequences of this phenomenon. In particular, we show that the presence of a nonmagnetic region in the nanocontact separating two ferromagnetic electrodes can lead to a spin blockade resulting in very large values of magnetoresistance
Green's Matrix for a Second Order Self-Adjoint Matrix Differential Operator
A systematic construction of the Green's matrix for a second order,
self-adjoint matrix differential operator from the linearly independent
solutions of the corresponding homogeneous differential equation set is carried
out. We follow the general approach of extracting the Green's matrix from the
Green's matrix of the corresponding first order system. This construction is
required in the cases where the differential equation set cannot be turned to
an algebraic equation set via transform techniques.Comment: 19 page
Anomalous and Spin Hall Effects in a Magnetic Tunnel Junction with Rashba Spin-Orbit Coupling
Anomalous and spin Hall effects are investigated theoretically for a magnetic
tunnel junction where the applied voltage produces a Rashba spin-or bit
coupling within the tunneling barrier layer. The ferromagnetic electrodes are
the source of the spin-polarized current. The tunneling electrons experience a
spin-orbit coupling inside the barrier due to the applied electrical field.
Charge and spin Hall currents are calculated as functions of the position
inside the barrier and the angle between the magnetizations of the electrodes.
We find that both charge and spin Hall currents are located inside the barrier
near the in terfaces. The dependence of the currents on magnetic configuration
of the magnetic tunnel junction makes possible the manipulation by the Hall
currents via rotation of the magnetization of the electrodes.Comment: 10 pages, 4 figure
Ferroelectric tunnel junctions with graphene electrodes
Polarization-driven resistive switching in ferroelectric tunnel junctions (FTJs)—structures composed of two electrodes separated by an ultrathin ferroelectric barrier—offers new physics and materials functionalities, as well as exciting opportunities for the next generation of non-volatile memories and logic devices. Performance of FTJs is highly sensitive to the electrical boundary conditions, which can be controlled by electrode material and/or interface engineering. Here, we demonstrate the use of graphene as electrodes in FTJs that allows control of interface properties for significant enhancement of device performance. Ferroelectric polarization stability and resistive switching are strongly affected by a molecular layer at the graphene/BaTiO3 interface. For the FTJ with the interfacial ammonia layer we find an enhanced tunnelling electroresistance (TER) effect of 6 x 105%. The obtained results demonstrate a new approach based on using graphene electrodes for interface-facilitated polarization stability and enhancement of the TER effect, which can be exploited in the FTJbased devices
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