arXiv subjects

M. Chshiev

Publications and source records attributed to M. Chshiev.

36 records · Page 2Linked to original sources

arXiv2014

Impurity induced enhancement of perpendicular magnetic anisotropy in Fe/MgO tunnel junctions

Using first-principles calculations, we investigated the impact of chromium (Cr) and vanadium (V) impurities on the magnetic anisotropy and spin polarization in Fe/MgO magnetic tunnel junctions. It is demonstrated using layer resolved anisotropy calculation technique, that while the impurity near the interface has a drastic effect in decreasing the perpendicular magnetic anisotropy (PMA), its position within the bulk allows maintaining high surface PMA. Moreover, the effective magnetic anisotropy has a strong tendency to go from in-plane to out-of-plane character as a function of Cr and V concentration favoring out-of-plane magnetization direction for ~1.5 nm thick Fe layers at impurity concentrations above 20 %. At the same time, spin polarization is not affected and even enhanced in most situations favoring an increase of tunnel magnetoresistance (TMR) values.

cond-mat.mtrl-sci↗

arXiv2013

Anatomy of perpendicular magnetic anisotropy in Fe/MgO magnetic tunnel junctions: First principles insight

Using first-principles calculations, we elucidate microscopic mechanisms of perpendicular magnetic anisotropy (PMA)in Fe/MgO magnetic tunnel junctions through evaluation of orbital and layer resolved contributions into the total anisotropy value. It is demonstrated that the origin of the large PMA values is far beyond simply considering the hybridization between Fe-3d$ and O-2p orbitals at the interface between the metal and the insulator. On-site projected analysis show that the anisotropy energy is not localized at the interface but it rather propagates into the bulk showing an attenuating oscillatory behavior which depends on orbital character of contributing states and interfacial conditions. Furthermore, it is found in most situations that states with $d_{yz(xz)}$ and $d_{z^2}$ character tend always to maintain the PMA while those with $d_{xy}$ and $d_{x^2-y^2}$ character tend to favor the in-plane anisotropy. It is also found that while MgO thickness has no influence on PMA, the calculated perpendicular magnetic anisotropy oscillates as a function of Fe thickness with a period of 2ML and reaches a maximum value of 3.6 mJ/m$^2$.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic Insulator-Induced Proximity Effects in Graphene: Spin Filtering and Exchange Splitting Gaps

We report on first-principles calculations of spin-dependent properties in graphene induced by its interaction with a nearby magnetic insulator (Europium oxide, EuO). The magnetic proximity effect results in spin polarization of graphene $π$ orbitals by up to 24 %, together with large exchange splitting bandgap of about 36 meV. The position of the Dirac cone is further shown to depend strongly on the graphene-EuO interlayer. These findings point towards the possible engineering of spin gating by proximity effect at relatively high temperature, which stands as a hallmark for future all-spin information processing technologies.

cond-mat.mtrl-sci↗

arXiv2012

Giant Spin Hall Effect Induced by Skew Scattering from Bismuth Impurities inside Thin Film CuBi Alloys

We demonstrate that a giant spin Hall effect (SHE) can be induced by introducing a small amount of Bi impurities in Cu. Our analysis based on a new 3-dimensional finite element treatment of spin transport shows that the sign of the SHE induced by the Bi impurities is negative and its spin Hall (SH) angle amounts to -0.24. Such a negative large SH angle in CuBi alloys can be explained by applying the resonant scattering model proposed by Fert and Levy [Phys. Rev. Lett. 106, 157208 (2011)] to 6p impurities.

cond-mat.mes-hall↗

arXiv2011

Symmetry Dependent Scattering by Minority Interface Resonance States in Single-crystal Magnetic Tunnel Junctions

Symmetry dependent scattering effect by minority interface resonance states (IRS) has been evidenced in full-epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs). Two types of samples with and without carbon doped bottom Fe/MgO interface were fabricated to represent two different types of IRS in the minority channel in the vicinity of the Fermi level. By analysis of the first- principles calculated local density of states (LDOS) and the temperature dependence of conductance in parallel configuration at low bias, we show that the IRS in the carbon free sample is dominated by the delta5 symmetry. This has a major contribution on the majority deltai to delta5 channel scattering and explains the enhancement of the delta5 conductance in the parallel configuration at low temperature. Furthermore, the spectral composition of the IRS in the carbon doped interface is found to be dominated by the delta1 symmetry, which is responsible for the suppression of delta5 channel in the parallel conductance.

cond-mat.mtrl-sci↗

arXiv2011

Spin Transfer Torques induced by Spin Hall Effect

Spin accumulation and spin transfer torques induced by Spin Hall Effect in bi-layer structures comprising ferromagnetic and paramagnetic materials are theoretically investigated. The charge and spin diffusion equations taking into account spin-flip and spin Hall effect are formulated and solved analytically and numerically for in structures with simplified and complex geometry, respectively. It is demonstrated that spin torques could be efficiently produced by means of Spin Hall effect which may be further enhanced by modifying structure geometry.

cond-mat.mes-hall↗

arXiv2011

Spin-current vortices in current-perpendicular-to-plane nanoconstricted spin-valves

The charge and spin diffusion equations taking into account spin-flip and spin-transfer torque were numerically solved using a finite element method in complex non-collinear geometry with strongly inhomogeneous current flow. As an illustration, spin-dependent transport through a non-magnetic nanoconstriction separating two magnetic layers was investigated. Unexpected results such as vortices of spin-currents in the vicinity of the nanoconstriction were obtained. The angular variations of magnetoresistance and spin-transfer torque are strongly influenced by the structure geometry.

cond-mat.mes-hall↗

arXiv2010

First-principles investigation of the very large Perpendicular Magnetic Anisotropy at Fe|MgO Interfaces

The perpendicular magnetic anisotropy (PMA) arising at the interface between ferromagnetic transition metals and metallic oxides are investigated via first-principles calculations. In this work very large values of PMA up to 3 erg/cm$^2$ at Fe$|$MgO interfaces are reported in agreement with recent experiments. The origin of PMA is attributed to overlap between O-$p_z$ and transition metal $d_{z^2}$ orbitals hybridized with $d_{xz(yz)}$ orbitals with stronger spin-orbit coupling induced splitting around the Fermi level for perpendicular magnetization orientation. Furthemore, it is shown that the PMA value weakens in case of over- or underoxidation when oxygen $p_z$ and transition metal $d_{z^2}$ orbitals overlap is strongly affected by disorder, in agreement with experimental observations in magnetic tunnel junctions.

cond-mat.mtrl-sci↗

arXiv2010

Finite Element Modeling of Charge and Spin-currents in Magnetoresistive Pillars with Current Crowding Effects

The charge and spin diffusion equations taking into account spin-flip and spin-transfer torque were numerically solved using a finite element method in complex non-collinear geometry. This approach was used to study the spin-dependent transport in giant magnetoresistance metallic pillars sandwiched between extended electrodes as in magnetoresistive heads for hard disk drives. The charge current crowding around the boundaries between the electrodes and the pillar has a quite significant influence on the spin current.

cond-mat.mes-hall↗

arXiv2009

Voltage induced control and magnetoresistance of noncollinear frustrated magnets

Noncollinear frustrated magnets are proposed as a new class of spintronic materials with high magnetoresistance which can be controlled with relatively small applied voltages. It is demonstrated that their magnetic configuration strongly depends on position of the Fermi energy and applied voltage. The voltage induced control of noncollinear frustrated materials (VCFM) can be seen as a way to intrinsic control of colossal magnetoresistance (CMR) and is the bulk material counterpart of spin transfer torque concept used to control giant magnetoresistance in layered spin-valve structures.

cond-mat.mtrl-sci↗

arXiv2009

The two-bands model of the magnetic tunnel junctions in the Fe/Cr/MgO/Fe structure

In this paper we present theoretical studies of spin dependent transport in Fe/Cr/MgO/Fe tunnel junctions with non-collinear alignment of magnetizations of metallic layers comprising these MTJs. Calculations are performed with use of non-equilibrium Green function technique in the framework of Keldysh formalism. WKB approximation is used for wave and Green functions in the trapezoidal barrier region under applied voltage. Electronic structure of ferromagnetic electrodes is modeled with two bands model, i.e. with majority s-electrons and minority d-holes. Furthermore, we introduce s-d hybridization by calculating the corresponding perturbation corrections for the wave and Green functions.

cond-mat.mes-hall↗

arXiv2008

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.

cond-mat.mtrl-sci↗

arXiv2008

Half-Metallic L2$_1$ Structures with (001) Planar Insertions

A number of L2$_1$ phase alloys (composition X$_2$YZ) are half-metallic. Although this structure is typically described in terms of an fcc Bravais lattice with a 4 atom basis, it can be viewed more simply as a variant of bcc or B2 in which planes of X$_2$ alternate with planes of YZ along the 001 direction. Using ab-initio electronic structure calculations, we have investigated planar insertions along 001 into the L2$_1$ structure. For most scenarios, insertion of single or double atomic layers of Cr into Co$_2$MnGe or Co$_2$MnSi did not destroy the half-metallic property. One insertion of a Cr layer into Co$_2$MnGe was observed to increase the gap. In fact, we observed that for a large number of insertions using various transition metals or combinations of transition metals and non-transition metals, the band gap in the minority channel at the Fermi energy remains. An ad hoc rule that seems to partially capture the tendency to form half-metals can be formulated as: "001 planar insertions that can plausibly yield 8 down spin electrons on the X$_2$ layer and 4 down spin electrons on the YZ layer yield half-metals".

cond-mat.mtrl-sci↗

arXiv2005

Spin Dependent Tunneling in FM|semiconductor|FM structures

Here we show that ordinary band structure codes can be used to understand the mechanisms of coherent spin-injection at interfaces between ferromagnets and semiconductors. This approach allows the screening of different material combinations for properties useful for obtaining high tunneling magnetoresistance (TMR). We used the Vienna Ab-initio Simulation Code (VASP) to calculate the wave function character of each band in periodic epitaxial Fe(100)|GaAs(100) and Fe(100)|ZnSe(100) structures. It is shown that Fe wave functions of different symmetry near Fermi energy decay differently in the GaAs and ZnSe.

cond-mat.mtrl-sci↗

arXiv2005

Spin-Polarized Current Induced Torque in Magnetic Tunnel Junctions

We present tight-binding calculations of the spin torque in non-collinear magnetic tunnel junctions based on the non-equilibrium Green functions approach. We have calculated the spin torque via the effective local magnetic moment approach and the divergence of the spin current. We show that both methods are equivalent, i.e. the absorption of the spin current at the interface is equivalent to the exchange interaction between the electron spins and the local magnetization. The transverse components of the spin torque parallel and perpendicular to the interface oscillate with different phase and decay in the ferromagnetic layer (FM) as a function of the distance from the interface. The period of oscillations is inversely proportional to the difference between the Fermi-momentum of the majority and minority electrons. The phase difference between the two transverse components of the spin torque is due to the precession of the electron spins around the exchange field in the FM layer. In absence of applied bias and for a relatively thin barrier the perpendicular component of the spin torque to the interface is non-zero due to the exchange coupling between the FM layers across the barrier.

cond-mat.other↗

arXiv2004

Role of heating and current-induced forces in the stability of atomic wires

We investigate the role of local heating and forces on ions in the stability of current-carrying aluminum wires. We find that heating increases with wire length due to a red shift of the frequency spectrum. Nevertheless, the local temperature of the wire is relatively low for a wide range of biases provided good thermal contact exists between the wire and the bulk electrodes. On the contrary, current-induced forces increase substantially as a function of bias and reach bond-breaking values at about 1 V. These results suggest that local heating promotes low-bias instabilities if dissipation into the bulk electrodes is not efficient, while current-induced forces are mainly responsible for the wire break-up at large biases. We compare these results to experimental observations.

cond-mat.mes-hall↗

arXiv2004

Impurity-induced tuning of quantum well states in spin-dependent resonant tunneling

We report exact model calculations of the spin-dependent tunneling in double magnetic tunnel junctions in the presence of impurities in the well. We show that the impurity can tune selectively the spin channels giving rise to a wide variety of interesting and novel transport phenomena. The tunneling magnetoresistance, the spin polarization and the local current can be dramatically enhanced or suppressed by impurities. The underlying mechanism is the impurity-induced shift of the quantum well states (QWS) which depends on the impurity potential, impurity position and the symmetry of the QWS.

cond-mat.mtrl-sci↗

arXiv2001

Magnetic Diode Effect in Double Barrier Tunnel Junctions

A quantum statistical theory of spin-dependent tunneling through asymmetric magnetic double barrier junctions is presented which describes $both$ ballistic and diffuse tunneling by a single analytical expression. It is evidenced that the key parameter for the transition between these two tunneling regimes is the electron scattering. For these junctions a strong asymmetric behaviour in the I-V characteristics and the tunnel magnetoresistance (TMR) is predicted which can be controlled by an applied magnetic field. This phenomenon relates to the quantum well states in the middle metallic layer. The corresponding resonances in the current and the TMR are drastically phase shifted under positive and negative voltage.

cond-mat.mtrl-sci↗