M. Chshiev - Academia.edu (original) (raw)
Papers by M. Chshiev
Applied Physics Letters, 2014
Physical Review B, 2011
Using first-principles calculations, we explore the electronic and magnetic properties of graphen... more Using first-principles calculations, we explore the electronic and magnetic properties of graphene nanomesh (GNM), a regular network of large vacancies, produced either by lithography or nanoimprint. When removing an equal number of A and B sites of the graphene bipartite lattice, the nanomesh made mostly of zigzag (armchair) -type edges exhibit antiferromagnetic (spin unpolarized) states. In contrast, in situations of sublattice symmetry breaking, stable ferri(o)magnetic states are obtained. For hydrogen-passivated nanomesh, the formation energy is dramatically decreased, and ground state is found to strongly depend on the vacancies shape and size. For triangular-shaped holes, the obtained net magnetic moments increase with the number difference of removed A and B sites in agreement with Lieb's theorem for even A + B. For odd A + B triangular meshes and all cases of nontriangular nanomeshes, including the one with even A + B, Lieb's theorem does not hold anymore, which can be partially attributed to the introduction of armchair edges. In addition, large triangular-shaped GNMs could be as robust as nontriangular GNMs, providing a possible solution to overcome one of the crucial challenges for the sp magnetism. Finally, significant exchange-splitting values as large as ∼ 0.5 eV can be obtained for highly asymmetric structures evidencing the potential of GNM for room-temperature carbon-based spintronics. These results demonstrate that a turn from zero-dimensional graphene nanoflakes throughout one-dimensional graphene nanoribbons with zigzag edges to GNM breaks localization of unpaired electrons and provides deviation from the rules based on Lieb's theorem. Such delocalization of the electrons leads the switch of the ground state of a system from an antiferromagnetic narrow gap insulator discussed for graphene nanoribons to a ferromagnetic or nonmagnetic metal.
Physical Review B, 2011
The perpendicular magnetic anisotropy (PMA) arising at the interface between ferromagnetic transi... more The perpendicular magnetic anisotropy (PMA) arising at the interface between ferromagnetic transition metals and metallic oxides was 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. Furthermore, it is shown that the PMA value weakens in the case of over-or underoxidation due to the fact that oxygen p z and transition metal d z 2 orbital overlap is strongly affected by disorder, in agreement with experimental observations in magnetic tunnel junctions.
physica status solidi (b), 2012
The zigzag-type atomic structure at edges of graphenes theoretically produces flat energy band. B... more The zigzag-type atomic structure at edges of graphenes theoretically produces flat energy band. Because electrons have infinite effective mass at the flat band, they localize at zigzag edges with high densities. The localized electron spins are spontaneously polarized due to mutual Coulomb interaction in spite of a material consisting of only carbon atom with sp 2 bonds. However, in most experimental studies, spin polarization (such as ferromagnetism) has been observed in defect-related carbon systems. Here, we fabricate honeycomblike arrays of low-defect hexagonal antidots (nanopores) terminated by hydrogen atoms on graphenes. They are prepared by a non-lithographic method using nanoporous alumina templates. We find large-magnitude ferromagnetism arising from polarized electron spins localizing at the zigzag antidot edges. Moreover, weak hysteresis loop in magnetoresistance and also spin pumping effect are found for perpendicular and parallel magnetic fields applied to the few-layer antidot lattices with larger inter-antidot space. These promise to be a realization of rare-element free magnets and also novel spintronic devices such as all-carbon spin transistors.
Journal of Magnetism and Magnetic Materials, 1998
An analytical quantum-statistical theory of giant magnetoresistance (GMR) in magnetic sandwiches ... more An analytical quantum-statistical theory of giant magnetoresistance (GMR) in magnetic sandwiches for current in-plane geometry (CIP) is developed taking into account quantization of electron motion perpendicular to plane direction and spin-dependent reflection and scattering of conduction electrons at interfaces. We adopted free electron model described by four parameters: mean free paths in the bulk and scattering amplitudes (coherent potentials) at interfaces for spin-up and spin-down electrons. The conductivities and GMR were calculated using Kubo formalism and Green functions technique in mixed real-space-mometum representation. Well-defined oscillations of CIP GMR with respect to thicknesses of the various layers due to the electron reflection on spin-dependent potential barriers at interfaces are predicted. The relative role of spin-dependent bulk and interfacial scattering and the influence of potential barriers at the interfaces on CIP GMR are investigated.
A theory of giant magnetoresistance (GMR) in magnetic sandwiches F/P/F for current in plane (CIP)... more A theory of giant magnetoresistance (GMR) in magnetic sandwiches F/P/F for current in plane (CIP) and current perpendicular to plane (CPP) geometries is developed. We adopted the free electron model described by four parameters: mean free paths and scattering amplitudes (coherent potentials) at the interfaces for spin-up and spin-down electrons. For both CIP and CPP geometries, we calculated the conductivities and GMR using Kubo formalism and the Green function technique in mixed real space-momentum representation. The final expressions for GMR in both geometries were obtained using the same microscopic parameters. Main attention was paid to the relative role of spin-dependent bulk and interfacial scattering. It was shown that increasing of surface scattering for fixed spin asymmetry leads to non-monotonic behaviour of CIP GMR due to renormalization of the scattering amplitude. In the case of CPP geometry the dependence of GMR on interfacial scattering amplitude is monotonic.
![Research paper thumbnail of Half-metallic L2[sub 1] structures with (001) planar insertions](https://attachments.academia-assets.com/42454738/thumbnails/1.jpg)
](Journal of Applied Physics, 2008
A number of L2 1 phase alloys (composition X 2 YZ) are half-metallic. Although this structure is ... more 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".
Journal of Applied Physics, 2012
IEEE Transactions on Magnetics, 2000
Theoretical investigations of spin transfer torque in magnetic tunnel junctions using the tight-b... more 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.
Applied Physics Letters, 2014
We report on first-principles calculations of spin-dependent properties in graphene induced by it... more 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.
... This makes half-metals interesting potential candidates for information storage technologies ... more ... This makes half-metals interesting potential candidates for information storage technologies especially for use in CPP/GMR read head devices and spin ... within the spin torque correlation model for different half-metallic structures, including the Heusler alloys Co$_{2}$MnSi, Co$_ ...
The charge and spin diffusion equations taking into account spin-flip and spin-transfer torque we... more 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. 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
Applied Physics Letters, 2014
Physical Review B, 2011
Using first-principles calculations, we explore the electronic and magnetic properties of graphen... more Using first-principles calculations, we explore the electronic and magnetic properties of graphene nanomesh (GNM), a regular network of large vacancies, produced either by lithography or nanoimprint. When removing an equal number of A and B sites of the graphene bipartite lattice, the nanomesh made mostly of zigzag (armchair) -type edges exhibit antiferromagnetic (spin unpolarized) states. In contrast, in situations of sublattice symmetry breaking, stable ferri(o)magnetic states are obtained. For hydrogen-passivated nanomesh, the formation energy is dramatically decreased, and ground state is found to strongly depend on the vacancies shape and size. For triangular-shaped holes, the obtained net magnetic moments increase with the number difference of removed A and B sites in agreement with Lieb's theorem for even A + B. For odd A + B triangular meshes and all cases of nontriangular nanomeshes, including the one with even A + B, Lieb's theorem does not hold anymore, which can be partially attributed to the introduction of armchair edges. In addition, large triangular-shaped GNMs could be as robust as nontriangular GNMs, providing a possible solution to overcome one of the crucial challenges for the sp magnetism. Finally, significant exchange-splitting values as large as ∼ 0.5 eV can be obtained for highly asymmetric structures evidencing the potential of GNM for room-temperature carbon-based spintronics. These results demonstrate that a turn from zero-dimensional graphene nanoflakes throughout one-dimensional graphene nanoribbons with zigzag edges to GNM breaks localization of unpaired electrons and provides deviation from the rules based on Lieb's theorem. Such delocalization of the electrons leads the switch of the ground state of a system from an antiferromagnetic narrow gap insulator discussed for graphene nanoribons to a ferromagnetic or nonmagnetic metal.
Physical Review B, 2011
The perpendicular magnetic anisotropy (PMA) arising at the interface between ferromagnetic transi... more The perpendicular magnetic anisotropy (PMA) arising at the interface between ferromagnetic transition metals and metallic oxides was 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. Furthermore, it is shown that the PMA value weakens in the case of over-or underoxidation due to the fact that oxygen p z and transition metal d z 2 orbital overlap is strongly affected by disorder, in agreement with experimental observations in magnetic tunnel junctions.
physica status solidi (b), 2012
The zigzag-type atomic structure at edges of graphenes theoretically produces flat energy band. B... more The zigzag-type atomic structure at edges of graphenes theoretically produces flat energy band. Because electrons have infinite effective mass at the flat band, they localize at zigzag edges with high densities. The localized electron spins are spontaneously polarized due to mutual Coulomb interaction in spite of a material consisting of only carbon atom with sp 2 bonds. However, in most experimental studies, spin polarization (such as ferromagnetism) has been observed in defect-related carbon systems. Here, we fabricate honeycomblike arrays of low-defect hexagonal antidots (nanopores) terminated by hydrogen atoms on graphenes. They are prepared by a non-lithographic method using nanoporous alumina templates. We find large-magnitude ferromagnetism arising from polarized electron spins localizing at the zigzag antidot edges. Moreover, weak hysteresis loop in magnetoresistance and also spin pumping effect are found for perpendicular and parallel magnetic fields applied to the few-layer antidot lattices with larger inter-antidot space. These promise to be a realization of rare-element free magnets and also novel spintronic devices such as all-carbon spin transistors.
Journal of Magnetism and Magnetic Materials, 1998
An analytical quantum-statistical theory of giant magnetoresistance (GMR) in magnetic sandwiches ... more An analytical quantum-statistical theory of giant magnetoresistance (GMR) in magnetic sandwiches for current in-plane geometry (CIP) is developed taking into account quantization of electron motion perpendicular to plane direction and spin-dependent reflection and scattering of conduction electrons at interfaces. We adopted free electron model described by four parameters: mean free paths in the bulk and scattering amplitudes (coherent potentials) at interfaces for spin-up and spin-down electrons. The conductivities and GMR were calculated using Kubo formalism and Green functions technique in mixed real-space-mometum representation. Well-defined oscillations of CIP GMR with respect to thicknesses of the various layers due to the electron reflection on spin-dependent potential barriers at interfaces are predicted. The relative role of spin-dependent bulk and interfacial scattering and the influence of potential barriers at the interfaces on CIP GMR are investigated.
A theory of giant magnetoresistance (GMR) in magnetic sandwiches F/P/F for current in plane (CIP)... more A theory of giant magnetoresistance (GMR) in magnetic sandwiches F/P/F for current in plane (CIP) and current perpendicular to plane (CPP) geometries is developed. We adopted the free electron model described by four parameters: mean free paths and scattering amplitudes (coherent potentials) at the interfaces for spin-up and spin-down electrons. For both CIP and CPP geometries, we calculated the conductivities and GMR using Kubo formalism and the Green function technique in mixed real space-momentum representation. The final expressions for GMR in both geometries were obtained using the same microscopic parameters. Main attention was paid to the relative role of spin-dependent bulk and interfacial scattering. It was shown that increasing of surface scattering for fixed spin asymmetry leads to non-monotonic behaviour of CIP GMR due to renormalization of the scattering amplitude. In the case of CPP geometry the dependence of GMR on interfacial scattering amplitude is monotonic.
Journal of Applied Physics, 2008
A number of L2 1 phase alloys (composition X 2 YZ) are half-metallic. Although this structure is ... more 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".
Journal of Applied Physics, 2012
IEEE Transactions on Magnetics, 2000
Theoretical investigations of spin transfer torque in magnetic tunnel junctions using the tight-b... more 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.
Applied Physics Letters, 2014
We report on first-principles calculations of spin-dependent properties in graphene induced by it... more 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.
... This makes half-metals interesting potential candidates for information storage technologies ... more ... This makes half-metals interesting potential candidates for information storage technologies especially for use in CPP/GMR read head devices and spin ... within the spin torque correlation model for different half-metallic structures, including the Heusler alloys Co$_{2}$MnSi, Co$_ ...
The charge and spin diffusion equations taking into account spin-flip and spin-transfer torque we... more 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. 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