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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. [ABSTRACT FROM AUTHOR]

The effect of structural relaxation and interfacial oxidation is demonstrated from first principles to have a crucial impact on interlayer exchange coupling (IEC) in crystalline Fe|MgO|Fe magnetic tunnel junctions (MTJs). It is shown that the IEC becomes antiferromagnetic for the relaxed structure in contrast to ferromagnetic for unrelaxed Fe|MgO|Fe MTJs. Furthermore, the antiferromagnetic IEC is strongly enhanced in the presence of oxygen vacancies while it is decreased by overoxidation and may even become ferromagnetic for sufficiently high oxygen concentration at the Fe|MgO interface. These results were supported using a tight-binding model and provide an explanation for recent experimental studies. [ABSTRACT FROM AUTHOR]

Quantum coherent transport in double barrier tunnel junctions has been exploited for building micrometric-size semiconductor-free diodes. At room temperature, we observe strongly asymmetric current-voltage characteristics with an asymmetry ratio increasing with the bias voltage, reaching a maximum of 20 at 1 V. Our experimental data can be perfectly explained using a theoretical model involving resonance-assisted tunneling. The coherent/resonant tunneling regime is achieved using metallic 3 nm diameter monodisperse Cu clusters, sandwiched between two Al[sub 2]O[sub 3] barriers. © 2001 American Institute of Physics. [DOI: 10.1063/1.1426685]. [ABSTRACT FROM AUTHOR]

A number of L21 phase alloys (composition X2YZ) are half-metallic. Although this structure is typically described in terms of a fcc Bravais lattice with a four atom basis, it can be viewed more simply as a variant of bcc or B2 in which planes of X2 alternate with planes of YZ along the 001 direction. Using ab initio electronic structure calculations, we have investigated planar insertions along 001 into the L21 structure. For most scenarios, insertion of single or double atomic layers of Cr into Co2MnGe or Co2MnSi did not destroy the half-metallic property. One insertion of a Cr layer into Co2MnGe 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 nontransition 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 eight down spin electrons on the X2 layer and four down spin electrons on the YZ layer yield half-metals.” [ABSTRACT FROM AUTHOR]

Endurance of MgO-based magnetic tunnel junctions has been studied using a time-dependent dielectric breakdown method. Series of successive electrical pulses were applied until electrical breakdown of the tunnel barrier. We show that two electrical breakdown regimes exist depending on the time interval Δt between pulses compared to a characteristic escape time of trapped electrons τ0 ∼ 100 ns. For Δt < τ0, breakdown is caused by a high average charge trapped in the barrier. For Δt > τ0, breakdown is ascribed to large temporal modulation of trapped charges causing alternating stress in the barrier oxide. Between these two regimes, the tunnel junctions reach a very high endurance. [ABSTRACT FROM AUTHOR]

First-principles calculations of the electronic structures of chalcogenide spinels with anion vacancy and substitutions are presented. Our calculations predict that Cu(Cd)Cr2S(Se)4-x, CuCr2S(Se)4-xEx (E=F, Cl, Br), and CdCr2S(Se)4-xDx (D=N, P, As) can be half-metallic over a range of concentrations. The magnetic moment is found to scale approximately linearly on adding or withdrawing electrons and varies between 5.0 and 6.0 μB/f.u. [ABSTRACT FROM AUTHOR]

First-principles calculations indicate that the chalcogenide spinels CdxCu1-xCr2S4 and CdxCu1-xCr2Se4 can be ferromagnetic metals, half-metals, or semiconductors depending on the Cd concentration x. In particular, CdxCu1-xCr2S4 can be tuned to be effectively half-metallic for x values between 0.25 and 0.875 and CdxCu1-xCr2Se4 can be half-metallic for x between 0.625 and 0.875. For x<0.25 for CdxCu1-xCr2S4 and for x<0.625 for CdxCu1-xCr2Se4, these materials are predicted to be ferromagnetic metals. For x>0.875 both systems are predicted to be ferromagnetic semiconductors. [ABSTRACT FROM AUTHOR]

Theoretical studies of spin dependent transport in Fe/Cr/MgO/Fe tunnel junctions with noncollinear alignment of magnetizations of metallic layers comprising these magnetic tunnel junctions are presented. Calculations are performed with use of nonequilibrium Green function technique in the framework of the Keldysh formalism. Electronic band structure of ferromagnetic electrodes is modeled within a two-band model with majority and minority states being s-like and d-like electrons, respectively. Furthermore, interfacial s-d hybridization is taken into account and calculated using perturbation corrections for the wave and Green functions. It is shown that in the presence of Cr layer at the Fe/MgO interface, the contribution from s-d hybridization to the total current is much stronger in the antiparallel magnetizations configuration compared to the parallel one leading to decrease in tunnel magnetoresistance values in agreement with earlier reports. [ABSTRACT FROM AUTHOR]

Recent experiments indicate that films of V1-xCrxO2 may be obtained which retain the tetragonal rutile phase to low temperature. In order to better understand this system we have calculated its electronic structure using density functional theory in the generalized gradient approximation and density functional theory with empirical on-site Coulomb correlations (LDA+U). Within these approximations we find that the ground state of rutile phase V1-xCrxO2 is quite simple. Both V and Cr are in the +4 state, implying that the V and Cr ions have moments of 1μB and 2μB, respectively. Similar to CrO2, V1-xCrxO2 is predicted to be ferromagnetic and half-metallic. Our results appear to be consistent with the experimental observations that VO2 is paramagnetic and metallic for temperatures above 340 K where it is stable. It is not clear, however, that these results are completely consistent with recent experimental observations of ferromagnetism at low temperature in V1-xCrxO2 for x=0.1 and x=0.2. [ABSTRACT FROM AUTHOR]

We present tight-binding calculations of the spin torque in noncollinear magnetic tunnel junctions based on the nonequilibrium 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 the absence of applied bias and for a relatively thin barrier, the component of the spin torque perpendicular to the interface is nonzero due to the exchange coupling between the FM layers across the barrier. [ABSTRACT FROM AUTHOR]

Magnetization switching induced by spin transfer torque is used to write magnetic memories (Magnetic Random Access Memory, MRAM) but can be detrimental to the reading process. It would be quite convenient therefore to modulate the efficiency of spin transfer torque. A solution is adding an extra degree of freedom by using double barrier magnetic tunnel junctions with two spin-polarizers, with controllable relative magnetic alignment. We demonstrate, for these structures, that the amplitude of in-plane spin transfer torque on the middle free layer can be efficiently tuned via the magnetic configuration of the electrodes. Using the proposed design could thus pave the way towards more reliable read/write schemes for MRAM. Moreover, our results suggest an intriguing effect associated with the out-of-plane (field-like) spin transfer torque, which has to be further investigated. [ABSTRACT FROM AUTHOR]

We investigate the behaviour of spin transfer torque (STT) and tunnelling magnetoresistance (TMR) in epitaxial antiferromagnetic-based tunnel junctions using tight binding calculations in the framework of the Keldysh formalism. We find that the STT out-of-plane component exhibits a staggered spatial distribution similar to its in-plane component. This behaviour is specific to the use of a tunnel barrier and significantly differs from the out-of-plane torques reported in previous works using a metallic spacer. Additionally, we show that unlike conventional ferromagnetic-based tunnel junctions, the TMR can increase with applied bias and reach values comparable to typical magnetoresistances found for usual spin valves. [ABSTRACT FROM AUTHOR]

Spintronics relies on the spin dependent transport properties of ferromagnets (Fs). Although antiferromagnets (AFs) are used for their magnetic properties only, some fundamental F-spintronics phenomena like spin transfer torque, domain wall motion, and tunnel anisotropic magnetoresistance also occur with AFs, thus making AF-spintronics attractive. Here, room temperature critical depths and absorption mechanisms of spin currents in Ir20Mn80 and Fe50Mn50 are determined by F-resonance and spin pumping. In particular, we find room temperature critical depths originating from different absorption mechanisms: dephasing for Ir20Mn80 and spin flipping for Fe50Mn50. [ABSTRACT FROM AUTHOR]

The so-called zigzag edge of graphenes theoretically has localized electrons due to the presence of flat energy bands near the Fermi level. The localized electron spins are strongly polarized, resulting in ferromagnetism. We fabricate graphenes with honeycomb-like arrays of hydrogen-terminated and low-defect hexagonal nanopores by a nonlithographic method using nanoporous alumina templates. We report large-magnitude room-temperature ferromagnetism caused by electron spins localizing at the zigzag nanopore edges. This promises to be a realization of rare-element free, controllable, transparent, flexible, and mono-atomic layer magnets and novel spintronic devices. [ABSTRACT FROM AUTHOR]

Room temperature transport properties are reported in polycrystalline SrTiO3-based magnetic tunnel junctions deposited by ion beam sputtering. The junctions comprise CoFeB electrodes and the SrTiO3 barrier with thickness varied between 0.9 and 1.9 nm. Resistance area product values between 3 Ω.μm2 and 22 kΩ.μm2 have been measured with a tunnel magnetoresistance ratio ranging from 3.1 to 13% at room temperature. At low barrier thickness (1.2 nm), ferromagnetic coupling between electrodes is observed, indicating the presence of defects in the structure. A post-oxidation step was found to improve transport properties at lower barrier thickness. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A correction to retract the article "Graphene magnet realized by hydrogenated graphene nanopore arrays" that was published online on March 1, 2012 is presented.