Spin Filter Tunnel Junctions Research Articles

Tunnel-magneto resistance of double-barrier tunnel junctions in the presence of spin-orbit coupling within spin-filter barriers

The tunnel magneto-resistance ratio is investigated for double-barrier tunnel junctions with spin-filter (magnetic insulators) barriers. The transmission probability is calculated in the tight-binding model by the non-equilibrium Green's function formalism considering spin-orbit coupling within barriers. The formation of resonant states inside a non-magnetic spacer gives rise to an enhancement in tunnel magneto-resistance ratio. Double spin-filter tunnel junctions have also shown a higher tunnel magneto-resistance ratio compared to that in double-barrier magnetic tunnel junctions. These findings will be valuable to improve the performance of spintronics devices.

Spin splitting in EuO(111)/Si(111) spin-filter tunnel junctions with atomically sharp interface

We demonstrate the tunneling in spin-split barriers made of ferromagnetic EuO grown on Si(111) substrates by molecular beam epitaxy. For 6 nm thick EuO films with high crystal quality and atomically sharp interfaces, we find a barrier height lowering driven by the spin splitting below the Curie temperature of 35 K. We determined the splitting energy to be 0.56 ± 0.03 eV at 20 K which results in a spin polarization above 90%.

Structural and magnetic properties of La0.7Sr0.3MnO3/LaCoO3 heterostructures

Perovskite manganites are well known for their complex magnetic and electronic properties. Their heterostructures are even more perplexing, specifically when interfaced with magnetic or nonmagnetic insulators as they offer particular promise for applications such as spin filter tunnel junctions. This study explores the structural and magnetic properties of one such heterostructure, namely, La0.7Sr0.3MnO3/LaCoO3 (LSMO/LCO) bilayers as the thickness of the LCO layer is varied while keeping that of the LSMO layer fixed. Exceeding a threshold value while gradually increasing the LCO layer thickness, prominent side lobes are observed in the reciprocal space map signifying the formation of periodic structural striped domains in LCO. The magnetic properties of the bilayers, specifically the interfacial couplings, are markedly influenced by the variation of the LCO thickness with noteworthy consequences that include a tunable negative exchange bias.

Spin-polarized quasiparticle control in a double spin-filter tunnel junction

Spin-polarized quasiparticles can be easily created during spin-filtering through a ferromagnetic insulator (FI) in contact with a superconductor due to pair breaking effects at the interface. A combination FI-N-FI sandwiched between two superconductors can be used to create and analyze such spin-polarized quasiparticles through their nonequilibrium accumulation in the middle metallic (N) layer. We report spin-polarized quasiparticle regulation in a double spin-filter tunnel junction in the configuration NbN-GdN1-Ti-GdN2-NbN. The middle Ti layer provides magnetic decoupling between two ferromagnetic GdN and a place for nonequilibrium quasiparticle accumulation. The two GdN(1,2) layers were deposited under different conditions to introduce coercive contrast. The quasiparticle tunneling spectra has been measured at different temperatures to understand the tunneling mechanism in these double spin-filter junctions. The conductance spectra were found to be comparable to an asymmetric SINI'S-type tunnel junction. A hysteretic R-H loop with higher resistance for the antiparallel configuration compared to parallel state was observed, asserting the spin-polarized nature of quasiparticles. The hysteresis in the R-H loop was found to disappear for subgap bias current. This difference can be understood by considering modification of the interlayer coupling due to nonequilibrium spin-polarized quasiparticle accumulation in the Ti layer.

Room temperature insulating ferromagnetism induced by charge transfer in ultrathin (110) La0.7Sr0.3MnO3 films

The achievement of high temperature ferromagnetism in perovskite manganites has proved both fundamentally and technologically important for spintronics devices. However, high operating temperatures have not been achieved due to the depression of the Curie temperature and the rapid spin filtering efficiency loss, which are the main obstacles for practical applications. Here, we report unexpected room temperature insulating ferromagnetism in ultrathin (110) oriented La0.7Sr0.3MnO3 (LSMO) films. The relationships between room temperature ferromagnetism, charge transfer, and orbital occupancy are investigated, with X-ray absorption spectroscopy (XAS) and X-ray linear dichroism (XLD) measurements. Our results suggest that the room temperature insulating ferromagnetism is originated from super-exchange interaction between Mn2+ and Mn3+. The formation of Mn2+ ions is related to the charge transfer induced by oxygen vacancies. Moreover, a preferential orbital occupancy of eg(3z2-r2) in Mn3+ ions is crucial to the in-plane super-exchange coupling in ultrathin (110) LSMO films, resulting in insulating ferromagnetic behavior. This work may lead to the development of barrier materials in spin filter tunnel junctions and understanding of ferromagnetic coupling in insulating perovskite films.

Four-state non-volatile memory in a multiferroic spin filter tunnel junction

We report a spin filter type multiferroic tunnel junction with a ferromagnetic/ferroelectric bilayer barrier. Memory functions of a spin filter magnetic tunnel junction and a ferroelectric tunnel junction are combined in this single device, producing four non-volatile resistive states that can be read out in a non-destructive manner. This concept is demonstrated in a LaNiO3/Pr0.8Ca0.2MnO3/BaTiO3/La0.7Sr0.3MnO3 all-oxide tunnel junction. The ferromagnetic insulator Pr0.8Ca0.2MnO3 serves as the spin filter and the ferromagnetic metal La0.7Sr0.3MnO3 is the spin analyzer. The ferroelectric polarization reversal in the BaTiO3 barrier switches the tunneling barrier height to produce a tunneling electroresistance. The ferroelectric switching also modulates the spin polarization and the spin filtering efficiency in Pr0.8Ca0.2MnO3.

Fully magnetic manganite spin filter tunnel junctions

In this paper we demonstrate spintronic devices which combine magnetic tunnel junctions with a spin-filtering tunnel barrier. These consist of an ultrathin ferromagnetic insulating barrier, Sm0.75Sr0.25MnO3, sandwiched between two ferromagnetic half-metallic manganite electrodes, La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3, in a nanopillar structure. Depending on the relative magnetic configurations of barrier and electrode layers, three resistance states are well defined, which therefore represent a potential three-state memory concept. These results open the way for the development of spintronic devices by exploiting the many degrees of freedom of perovskite manganite heterostructure systems.

The tunneling time in FM/I/SF/NM spin filter tunnel junctions

Based on the free electronic model and the Winful’s tunneling time theory, the spin dependent dwell time and the phase time in ferromagnetic metal/nonferromagnetic metal/spin filter layer/normal metal single spin filter tunneling junctions (FM/I/SF/NM) are investigated. The results show that, different from the traditional FM/I/FM structures, the transmission coefficients for up and down spin electrons are not equal due to the effects of the SF layer when the molecular fields in FM and SF layers are at antiparallel arrangement. With the higher incident electronic energies (the incident energies are higher than the barrier height), as the decreasing influences of the self-interference delay, the phase times and the dwell times tend to same for different spin orientations electrons. But with the lower incident electronic energies (the incident energies are smaller than the barrier height), as the increasing influences of the self-interference delay, the differences between the phase times and the dwell times appear for different spin orientations electrons. And, the barrier heights of the I and SF layers, and the widths and the molecular fields of spin filter barrier can induce large differences between the phase times and the dwell times for spin-up electrons. But for the spin-down electrons, the differences between the phase times and the dwell times are induced mainly by the changes of parameters of the SF layer and tinily by the varies of barrier heights of the I layer.

Resonant TMR inversion in LiF/EuS based spin-filter tunnel junctions

Resonant tunneling can lead to inverse tunnel magnetoresistance when impurity levels rather than direct tunneling dominate the transport process. We fabricated hybrid magnetic tunnel junctions of CoFe/LiF/EuS/Ti, with an epitaxial LiF energy barrier joined with a polycrystalline EuS spin-filter barrier. Due to the water solubility of LiF, the devices were fully packaged in situ. The devices showed sizeable positive TMR up to 16% at low bias voltages but clearly inverted TMR at higher bias voltages. The TMR inversion depends sensitively on the thickness of LiF, and the tendency of inversion disappears when LiF gets thick enough and recovers its intrinsic properties.

Dual Control of Giant Field-like Spin Torque in Spin Filter Tunnel Junctions.

We predict a giant field-like spin torque, , in spin-filter (SF) barrier tunnel junctions in sharp contrast to existing junctions based on nonmagnetic passive barriers. We demonstrate that has linear bias behavior, is independent of the SF thickness, and has odd parity with respect to the SF’s exchange splitting. Thus, it can be selectively controlled via external bias or external magnetic field which gives rise to sign reversal of via magnetic field switching. The underlying mechanism is the interlayer exchange coupling between the noncollinear magnetizations of the SF and free ferromagnetic electrode via the nonmagnetic insulating (I) spacer giving rise to giant spin-dependent reflection at the SF/I interface. These findings suggest that the proposed field-like-spin-torque MRAM may provide promising dual functionalities for both ‘reading’ and ‘writing’ processes which require lower critical current densities and faster writing and reading speeds.

Strongly bias-dependent tunnel magnetoresistance in manganite spin filter tunnel junctions.

A highly unconventional bias-dependent tunnel magnetoresistance (TMR) response is observed in Sm0.75 Sr0.25 MnO3 -based nanopillar spin filter tunnel junctions (SFTJs) with two different behaviors in two different thickness regimes of the barrier layer. Thinner barrier devices exhibit conventional SFTJ behaviors; however, for larger barrier thicknesses, the TMR-bias dependence is more complex and reverses sign at higher bias.

Spin-transfer torque in spin filter tunnel junctions

Spin-transfer torque in a class of magnetic tunnel junctions with noncollinear magnetizations, referred to as spin filter tunnel junctions, is studied within the tight-binding model using the nonequilibrium Green's function technique within Keldysh formalism. These junctions consist of one ferromagnet (FM) adjacent to a magnetic insulator (MI) or two FM separated by a MI. We find that the presence of the magnetic insulator dramatically enhances the magnitude of the spin-torque components compared to conventional magnetic tunnel junctions. The fieldlike torque is driven by the spin-dependent reflection at the MI/FM interface, which results in a small reduction of its amplitude when an insulating spacer (S) is inserted to decouple MI and FM layers. Meanwhile, the dampinglike torque is dominated by the tunneling electrons that experience the lowest barrier height. We propose a device of the form FM/(S)/MI/(S)/FM that takes advantage of these characteristics and allows for tuning the spin-torque magnitudes over a wide range just by rotation of the magnetization of the insulating layer.

Effects of interface structure on spin filter tunnel junctions

In this article, the spin filter tunneling in the low biases has been investigated by considering the interfacial barrier roughness. In spite of the approaches that use an artificial periodic interface to find the effects of roughness on transport calculation, our method is based on implementing the height deviation from a mean surface as the roughness characteristics in the transfer matrix method. Moreover the spin-dependent transport properties have been calculated by using the height of a simulated interface in which the rough interface has been grown by ballistic deposition. The results show that the surface roughness suppresses the temperature dependence of spin polarization and reduces the maximum achievable tunneling magnetoresistance and spin-dependent transmission probability.

Nanopillar spin filter tunnel junctions with manganite barriers.

The potential of a manganite ferromagnetic insulator in the field of spin-filtering has been demonstrated. For this, an ultrathin film of Sm0.75Sr0.25MnO3 is integrated as a barrier in an epitaxial oxide nanopillar tunnel junction and a high spin polarization of up to 75% at 5 K has been achieved. A large zero-bias anomaly observed in the dynamic conductance at low temperatures is explained in terms of the Kondo scattering model. In addition, a decrease in spin polarization at low bias and hysteretic magneto-resistance at low temperatures are reported. The results open up new possibilities for spin-electronics and suggest exploration of other manganites-based materials for the room temperature spin-filter applications.

Interface states in CoFe2O4spin-filter tunnel junctions

Spin-filter tunneling is a promising way to generate highly spin-polarized current, a key component for spintronics applications. In this paper we explore the tunneling conductance across the spin-filter material CoFe${}_{2}$O${}_{4}$ interfaced with Au electrodes, a geometry which provides nearly perfect lattice matching at the CoFe${}_{2}$O${}_{4}$/Au(001) interface. Using density functional theory calculations we demonstrate that interface states play a decisive role in controlling the transport spin polarization in this tunnel junction. For a realistic CoFe${}_{2}$O${}_{4}$ barrier thickness, we predict a tunneling spin polarization of about $\ensuremath{-}60%$. We show that this value is lower than what is expected based solely on considerations of the spin-polarized band structure of CoFe${}_{2}$O${}_{4}$, and therefore that these interface states can play a detrimental role. We argue that this is a rather general feature of ferrimagnetic ferrites and could make an important impact on spin-filter tunneling applications.

Electronic transport through EuO spin-filter tunnel junctions

Epitaxial spin filter tunnel junctions based on the ferromagnetic semiconductor europium monoxide, EuO, are investigated by means of density functional theory. In particular, we focus on the spin transport properties of Cu(100)/EuO(100)/Cu(100) junctions. The dependence of the transmission coefficient and the current-voltage curves on the interface spacing and on the EuO thickness is explained in terms of the EuO density of states and the complex band structure. Furthermore we also discuss the relation between the spin transport properties and the Cu-EuO interface geometry. The level alignment of the junction is sensitively affected by the interface spacing, since this determines the charge transfer between EuO and the Cu electrodes. Our calculations indicate that EuO epitaxially grown on Cu can act as a perfect spin filter, with a spin polarization of the current close to 100%, and with both the Eu-5d conduction band and the Eu-4f valence band states contributing to the coherent transport. For epitaxial EuO on Cu a symmetry filtering is observed, with the \Delta_1 states dominating the transmission. This leads to a transport gap larger than the fundamental EuO band gap. Importantly the high spin polarization of the current is preserved up to large bias voltages.

Spin-Filter Tunnel Junction with Matched Fermi Surfaces

Efficient injection of spin-polarized current into a semiconductor is a basic prerequisite for building semiconductor-based spintronic devices. Here, we use inelastic electron tunneling spectroscopy to show that the efficiency of spin-filter-type spin injectors is limited by spin scattering of the tunneling electrons. By matching the Fermi-surface shapes of the current injection source and target electrode material, spin injection efficiency can be significantly increased in epitaxial ferromagnetic insulator tunnel junctions. Our results demonstrate that not only structural but also Fermi-surface matching is important to suppress scattering processes in spintronic devices.

Spin-polarized Josephson and quasiparticle currents in superconducting spin-filter tunnel junctions

We present a theoretical study of the effect of spin filtering on the Josephson and dissipative quasiparticle currents in a superconducting tunnel junction. By combining the quasiclassical Green's functions and the tunneling Hamiltonian method, we describe the transport properties of a generic junction consisting of two superconducting leads with an effective exchange field $\mathbf{h}$ separated by a spin-filter insulating barrier. We show that in addition to the tunneling of Cooper pairs with total spin projection ${S}_{z}=0$ there is another contribution to the Josephson current due to triplet Cooper pairs with total spin projection ${S}_{z}\ensuremath{\ne}0$. The latter is finite and not affected by the spin-filter effect provided that the fields $\mathbf{h}$ and the magnetization of the barrier are noncollinear. We also determine the quasiparticle current for a symmetric junction and show that the differential conductance may exhibit peaks at different values of the voltage depending on the polarization of the spin filter, and the relative angle between the exchange fields and the magnetization of the barrier. Our findings provide a plausible explanation for existing experiments on Josephson junctions with magnetic barriers, predict further effects, and show how spin-polarized supercurrents in hybrid structures can be created.

Large Tunnel Magnetoresistance in Epitaxial Oxide Spin‐Filter Tunnel Junctions

AbstractA high‐performance spin filter tunnel junction composed of an epitaxial oxide heterostructure is reported. By independently controlling the magnetic orientations of ferromagnetic tunnel barrier and electrode layers, a tunnel magnetoresistance ratio exceeding 120% is obtained purely by the spin filtering effect. A newly introduced spin filter material, Pr0.8Ca0.2Mn1‐yCoyO3, is shown to be useful for building novel multibarrier spintronic tunnel devices due to its composition‐controlled magnetic hardness.

Spin filtering with EuO: Insight from the complex band structure

Spin-filter tunneling is a promising way to create highly-spin-polarized currents. So far the understanding of the spin-filtering effect has been limited to a free-electron description based on the spin-dependent tunneling barrier height. In this work we explore the complex of EuO as a representative ferromagnetic insulator used in spin-filter tunneling experiments and show that the mechanism of spin filtering is more intricate than it has been previously thought. We demonstrate the importance of the multiorbital band structure with an indirect band gap for spin-filter tunneling. By analyzing the symmetry of the complex bands and the decay rates for different wave vectors and energies we draw conclusions about spin-filter efficiency of EuO. Our results provide guidelines for the design of spin-filter tunnel junctions with enhanced spin polarization.