All-oxide magnetic tunnel junctions (MTJs) incorporating functional materials as insulating barriers have the potential of becoming the founding technology for novel multi-functional devices. We investigate, by first-principles density functional theory, the bias-dependent transport properties of an all-oxide SrRuO3/BaTiO3/SrRuO3 MTJ. This incorporates a BaTiO3 barrier which can be found either in a non-ferroic or in a ferroelectric state. In such an MTJ not only can the tunneling magnetoresistance reach enormous values, but also, for certain voltages, its sign can be changed by altering the barrier electric state. These findings pave the way for a new generation of electrically-controlled magnetic sensors.
Magnetoelectric coupling has been a trending research topic in both organic and inorganic materials and hybrids. The concept of controlling magnetism using an electric field is particularly appealing in energy efficient applications. In this spirit, ferroelectricity has been introduced to organic spin valves to manipulate the magneto transport, where the spin transport through the ferromagnet/organic spacer interfaces (spinterface) are under intensive study. The ferroelectric materials in the organic spin valves provide a knob to vary the interfacial energy alignment and the interfacial crysta…
Organic spintronic devices have been appealing because of the long spin life time of the charge carriers in the organic materials and their low cost, flexibility and chemical diversity. In previous studies, the control of resistance of organic spin valves is generally achieved by the alignment of the magnetization directions of the two ferromagnetic electrodes, generating magnetoresistance.1 Here we employ a new knob to tune the resistance of organic spin valves by adding a thin ferroelectric interfacial layer between the ferromagnetic electrode and the organic spacer. We show that the resista…
Using magnetron sputtering, we have prepared Co-Fe-B/tunnel barrier/Co-Fe-B magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia, and magnesia-alumina bilayer systems. The highest tunnel magnetoresistance ratios we found were 73% for alumina and 323% for magnesia-based tunnel junctions. Additionally, tunnel junctions with a unified layer stack were prepared for the three different barriers. In these systems, the tunnel magnetoresistance ratios at optimum annealing temperatures were found to be 65% for alumina, 173% for magnesia, and 78% for the composite tunnel barrie…
We created epitaxial magnetic tunnel junctions of FeCo/MgO/EuS on MgO buffered Si (100). Tunnel magnetoresistance reached up to 64% at 4.2 K. An unexpected fast drop of magnetoresistance was recorded for MgO thickness above 1 nm, which is attributed to the forced nonspecular conductance across the EuS conduction band minimum located at the X point, rather than the desired Delta_1 conductance centered around the Gamma point.
We report a large, quasi-reversible tunnel magnetoresistance in exchange-biased ferromagnetic semiconductor tunnel junctions wherein a soft ferromagnetic semiconductor (\gma) is exchange coupled to a hard ferromagnetic metal (MnAs). Our observations are consistent with the formation of a region of inhomogeneous magnetization (an "exchange spring") within the biased \gma layer. The distinctive tunneling anisotropic magnetoresistance of \gma produces a pronounced sensitivity of the magnetoresistance to the state of the exchange spring.
We propose a tunneling heterostructure by replacing one of the metal electrodes in a metal/ferroelectric/metal ferroelectric tunnel junction with a heavily doped semiconductor. In this metal/ferroelectric/semiconductor tunnel diode, both the height and the width of the tunneling barrier can be electrically modulated due to the ferroelectric field effect, leading to a colossal tunneling electroresistance. This idea is implemented in Pt/BaTiO3/Nb:SrTiO3 heterostructures, in which an ON/OFF conductance ratio above 10$^4$ can be readily achieved at room temperature. The colossal tunneling electror…
Tunneling electroresistance (TER) effect is the change in the electrical resistance of a ferroelectric tunnel junction (FTJ) associated with polarization reversal in the ferroelectric barrier layer. Here we predict that a FTJ with a composite barrier that combines a functional ferroelectric film and a thin layer of a non-polar dielectric can exhibit a significantly enhanced TER. Due to the change in the electrostatic potential with polarization reversal the non-polar dielectric barrier acts as a switch that changes its barrier height from a low to high value. The predicted values of TER are gi…
Electron transport through fully depleted ferroelectric tunnel barriers sandwiched between two metal electrodes and its dependence on ferroelectric polarization direction are investigated. The model assumes a polarization direction dependent ferroelectric barrier. The transport mechanisms, including direct tunneling, Fowler-Nordheim tunneling and thermionic injection, are considered in the calculation of the electroresistance as a function of ferroelectric barrier properties, given by the properties of the ferroelectric, the barrier thickness, and the metal properties, and in turn of the polar…
Perpendicular magnetic tunnel junction with MgAl2O4 barrier is investigated. It is found that reactive RF sputtering with O2 is essential to obtain strong perpendicular magnetic anisotropy and large tunneling magnetoresistance in MgAl2O4-based junctions. An interfacial perpendicular magnetic anisotropy energy density of 2.25 mJ/m2 is obtained for the samples annealed at 400C. An enhanced magnetoresistance of 60% has also been achieved. The Vhalf, bias voltage at which tunneling magnetoresistance drops to half of the zero-bias value, is found to be about 1V, which is substantially higher than t…
We report on resonant tunneling magnetoresistance via localized states through a ZnSe semiconducting barrier which can reverse the sign of the effective spin polarization of tunneling electrons. Experiments performed on Fe/ZnSe/Fe planar junctions have shown that positive, negative or even its sign-reversible magnetoresistance can be obtained, depending on the bias voltage, the energy of localized states in the ZnSe barrier and spatial symmetry. The averaging of conduction over all localized states in a junction under resonant condition is strongly detrimental to the magnetoresistance.
Stable and switchable polarization of ferroelectric materials opens a possibility to electrically control their functional behavior. A particularly promising approach is to employ ferroelectric tunnel junctions where the polarization reversal in a ferroelectric barrier changes the tunneling current across the junction. Here, we demonstrate the reproducible tunneling electroresistance effect using a combination of Piezoresponse Force Microscopy (PFM) and Conducting Atomic Force Microscopy (C-AFM) techniques on nanometer-thick epitaxial BaTiO3 single crystal thin films on SrRuO3 bottom electrode…
The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in alfa-FeRh-based junctions driven by the magnetic phase transition of alfa-FeRh and resultantly large variation of the density of states in the vicin…
2D Ferroelectric materials are promising for designing low-dimensional memory devices. Here, we explore strain tunable ferroelectric properties of group-IV monochalcogenides MX (M=Ge, Sn; X=S, Se) and their potential application in lateral field tunnel junction devices. We find that these monolayers have in-plane ferroelectricity, with their ferroelectric parameters being on par with other known 2D ferroelectric materials. Amongst SnSe, SnS, GeSe, and GeS, we find that GeS has the best ferroelectric parameters for device applications, which can be improved further by applying uniaxial tensile …
Magnetic tunnel junctions with the layer sequence Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al/MgO/CoFe were fabricated by magnetron sputtering at room temperature (RT). The samples exhibit a large inverse tunneling magnetoresistance (TMR) effect of up to -66% at RT. The largest value of -84% at 20 K reflects a rather weak influence of temperature. The dependence on the voltage drop shows an unusual behavior with two almost symmetric peaks at $\pm600$ mV with large inverse TMR ratios and small positive values around zero bias.
Ferroelectric tunnel junctions, in which ferroelectric polarization and quantum tunneling are closely coupled to induce the tunneling electroresistance (TER) effect, have attracted considerable interest due to their potential in non-volatile and low-power consumption memory devices. The ferroelectric size effect, however, has hindered ferroelectric tunnel junctions from exhibiting robust TER effect. Here, our study proposes doping engineering in a two-dimensional in-plane ferroelectric semiconductor as an effective strategy to design a two-dimensional ferroelectric tunnel junction composed of …
A magnetic tunnel junction Fe3O4/MgO/Fe with (001) layer orientation is considered. The junction magnetic energy is analyzed as a function of the angle between the layer magnetization vectors under various magnetic fields. The tunnel magnetoresistance is calculated as a function of the external magnetic field. In contrast with junctions with unidirectional anisotropy, a substantially lower magnetic field is required for the junction switching.
The contribution of a built-in electric field to ferroelectric phase transition in asymmetric ferroelectric tunnel junctions is studied using a multiscale thermodynamic model. It is demonstrated in details that there exists a critical thickness at which an unusual ferroelectric-\'\' polar non-ferroelectric\rq\rq phase transition occurs in asymmetric ferroelectric tunnel junctions. In the \'\' polar non-ferroelectric\rq\rq phase, there is only one non-switchable polarization which is caused by the competition between the depolarizing field and the built-in field, and closure-like domains are pr…
Very recently, ferroelectric polarization in staggered bilayer hexagonal boron nitride (BBN) and its novel sliding inversion mechanism was reported experimentally (Science 2021, 372, 1458; 2021, 372, 1462), which paves a new way to realize van der Waals (vdW) ferroelectric devices with new functionalities. Here, we develop vdW sliding ferroelectric tunnel junctions (FTJs) using the sliding ferroelectric BBN unit as ultrathin barriers and explore their transport properties with different ferroelectric states and metal contacts via the first principles. It is found that the electrode/BBN contact…
We report tunnel magnetoresistance (TMR) ratios as high as 472% at room temperature and 804% at 5 K in pseudo-spin valve (SV) CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) annealed at 450oC, which is approaching the theoretically predicted value. By contrast, the TMR ratios for exchange-biased (EB) SV MTJs with a MnIr antiferromagnetic layer are found to drop when they are annealed at 450oC. Energy dispersive X-ray analysis shows that annealing at 450oC induces interdiffusion of Mn and Ru atoms into the MgO barrier and ferromagnetic layers in EB-SV MTJs. Mechanisms behind the different anne…
Polar metals with ferroelectric-like displacements in metals have been achieved recently, half century later than Anderson and Blount's prediction. However, the genuine ferroelectricity with electrical dipolar switching has not yet been attained experimentally in the conducting materials, especially the ones possessing magnetic responses. Here we report the coexistence of ferroelectricity and magnetoresistance (MR) in the metallic PbNb0.12Ti0.88O3 (PNTO) thin films. We found that the conducting and magnetic responses of PNTO films are highly asymmetric. Negative MR up to 50% is observed under …
We investigated the relationship between the tunnel magnetoresistance (TMR) ratio and the electrode structure in MgO-barrier magnetic tunnel junctions (MTJs). The TMR ratio in a MTJ with Co40Fe40B20 reference and free layers reached 355% at the post-deposition annealing temperature of Ta=400 degree C. When Co50Fe50 or Co90Fe10 is used for the reference layer material, no high TMR ratio was observed. The key to have high TMR ratio is to have highly oriented (001) MgO barrier/CoFeB crystalline electrodes. The highest TMR ratio obtained so far is 450% at Ta = 450 degree C in a pseudo spin-valve M…
We report on systematic ab-initio investigations of Co and Cr interlayers embedded in Fe(001)/MgO/Fe(001) magnetic tunnel junctions, focusing on the changes of the electronic structure and the transport properties with interlayer thickness. The results of spin-dependent ballistic transport calculations reveal options to specifically manipulate the tunnel magnetoresistance ratio. The resistance area products and the tunnel magnetoresistance ratios show a monotonous trend with distinct oscillations as a function of the Cr thickness. These modulations are directly addressed and interpreted by mea…
A tunnel magnetic junction is considered with magnetic hard and magnetic soft layers of cubic symmetry. The magnetic switching is analyzed of the layers by a magnetic field perpendicular to the initial magnetizations. In such a situation, an additional peak of the TMR ratio appears at the magnetic field value lower substantially than the anisotropy energy of the soft layer.
Recently, magnetic tunnel junctions with perpendicular magnetized electrodes combined with exchange bias films have attracted large interest. In this paper we examine the tunnel magnetoresistance of Ta/Pd/IrMn/Co-Fe/Ta/Co-Fe-B/MgO/Co-Fe-B/capping/Pd magnetic tunnel junctions in dependence on the capping layer, i.e., Hf or Ta. In these stacks perpendicular exchange bias fields of -500\,Oe along with perpendicular magnetic anisotropy are combined. A tunnel magnetoresistance of $(47.2\pm 1.4)\%$ for the Hf-capped sample was determined compared to the Ta one $(42.6\pm 0.7)\%$ at room temperature. …
We investigated dependence of tunnel magnetoresistance effect in CoFeB/MgO/CoFeB magnetic tunnel junctions on Ar pressure during MgO-barrier sputtering. Sputter deposition of MgO-barrier at high Ar pressure of 10 mTorr resulted in smooth surface and highly (001) oriented MgO. Using this MgO as a tunnel barrier, tunnel magnetoresistance (TMR) ratio as high as 355% at room temperature (578% at 5K) was realized after annealing at 325 C or higher, which appears to be related to a highly (001) oriented CoFeB texture promoted by the smooth and highly oriented MgO. Electron-beam lithography defined d…
Fully-relativistic first-principles calculations of the Fe(001) surface demonstrate that resonant surface (interface) states may produce sizeable tunneling anisotropic magnetoresistance in magnetic tunnel junctions with a single magnetic electrode. The effect is driven by the spin-orbit coupling. It shifts the resonant surface band via the Rashba effect when the magnetization direction changes. We find that spin-flip scattering at the interface is controlled not only by the strength of the spin-orbit coupling, but depends strongly on the intrinsic width of the resonant surface states.
Magnetite (Fe3O4) based tunnel junctions with turret/mesa structure have been investigated for different barrier materials (SrTiO3, NdGaO3, MgO, SiO2, and Al2O(3-x)). Junctions with a Ni counter electrode and an aluminium oxide barrier showed reproducibly a tunneling magnetoresistance (TMR) effect at room temperature of up to 5% with almost ideal switching behavior. This number only partially reflects the intrinsic high spin polarization of Fe3O4. It is considerably decreased due to an additional series resistance within the junction. Only SiO2 and Al2O(3-x) barriers provide magnetically decou…
The influence of depolarizing field on the magnitude and stability of a uniform polarization in ferroelectric capacitors and tunnel junctions is studied using a nonlinear thermodynamic theory. It is predicted that, in heterostructures involving strained epitaxial films and metal electrodes, the homogeneous polarization state may remain stable against transformations into the paraelectric phase and into polydomain states down to the nanoscale. This result supports the possibility of depolarizing-field-related resistive switching in ferroelectric tunnel junctions with dissimilar electrodes. The …
We demonstrate with first-principles electron transport calculations that large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist in an all-oxide device. The TMR originates from the symmetry-driven spin filtering provided by the insulating BaTiO3 barrier to the electrons injected from SrRuO3. In contrast the TER is possible only when a thin SrTiO3 layer is intercalated at one of the SrRuO3/BaTiO3 interfaces. As the complex band-structure of SrTiO3 has the same symmetry than that of BaTiO3, the inclusion of such an intercalated layer does not negatively…
Spinel MgAl2O4 and family oxides are emerging barrier materials useful for magnetic tunnel junctions (MTJs). We report large tunnel magnetoresistance (TMR) ratios up to 429% at room temperature (RT) and 1,034% at 10 K in a Fe/MgAl2O4/Fe(001)-based MTJ prepared using electron-beam evaporation of Mg4Al-Ox. Resistance oscillations with a MTJ barrier thickness of 0.3-nm were significantly enhanced compared to those of a Fe/MgO/Fe(001) MTJ, resulting in a large TMR oscillation peak-to-valley difference of 125% at RT. The differential conductance spectra were symmetric with bias polarity, and the sp…
Self-poled ultra-thin ferroelectric PbZr0.52Ti0.48O3 (PZT) (5 and 7 nm) films have been grown by pulsed laser deposition technique on ferromagnetic La0.67Sr0.33MnO3 (LSMO) (30 nm) to check the effect of polar capping on magnetization for ferroelectric tunnel junction (FTJ) devices. PZT/LSMO heterostructures with thick polar PZT (7 nm) capping show nearly 100% enhancement in magnetization compared with thin polar PZT (5 nm) films, probably due to excess hole transfer from the ferroelectric to the ferromagnetic layers. Core-level X-ray photoelectron spectroscopy studies revealed the presence of …
Results of first-principles calculations of the Fe/GaAs/Ag(001) epitaxial tunnel junctions reveal that hybridization of interface resonances formed at both interfaces can enhance the tunnelling anisotropic magnetoresistance (TAMR) of the systems. This mechanism is manifested by a non-monotonic dependence of the TAMR effect on the thickness of the tunnel barrier, with a maximum for intermediate thicknesses. A detailed scan of k-resolved transmissions over the two-dimensional Brillouin zone proves an interplay between a few hybridization-induced hot spots and a contribution to the tunnelling fro…
Recently, van der Waals (vdW) magnetic heterostructures have received increasing research attention in spintronics. However, the lack of room-temperature magnetic order of vdW material has largely impedes its development in practical spintronics devices. Inspired by the recently discovered vdW ferromagnet Fe3GaTe2, which has been shown to have magnetic order above room temperature and sizable perpendicular magnetic anisotropy, we investigate the basic electronic structure and magnetic properties of Fe3GaTe2 as well as tunneling magnetoresistance effect in magnetic tunnel junctions (MTJs) with …
We investigate the dependence of perpendicular and parallel spin transfer torque (STT) and tunneling magnetoresistance (TMR) on the insulator barrier energy in the magnetic tunnel junction (MTJ). We employed single orbit tight binding model combined with the Keldysh non-equilibrium Green's function method in order to calculate the perpendicular and parallel STT, and TMR in MTJ with the finite bias voltages. The dependences of STT and TMR on the insulator barrier energy are calculated for the semi-infinite half metallic ferromagnetic electrodes. We find that perfect linear relation between the …
In double spin filter (SF) tunnel junctions, the spin information is generated and analyzed purely from the SF effect with nonmagnetic electrodes. In this article we numerically evaluate the bias dependence of magnetoresistance in such tunnel junctions (nonmagnetic metal / SF / nonmagnetic insulator / SF / nonmagnetic metal), particularly in cases when different SF materials are utilized. A large magnetoresistance with nonmonotonic and asymmetric bias dependence is expected within the frame of WKB approximation. We illustrate the systematic influence of tunnel barrier height, tunnel barrier th…
Manipulation of tunneling spin-polarized electrons via a ferroelectric interlayer sandwiched between two ferromagnetic electrodes, dubbed Multiferroic Tunnel Junctions (MFTJs), can be achieved not only by the magnetic alignments of two ferromagnets but also by the electric polarization of the ferroelectric interlayer, providing great opportunities for next-generation multi-state memory devices. Here we show that a La0.67Sr0.33MnO3 (LSMO)/PbZr0.2Ti0.8O3(PZT)/Co structured MFTJ device can exhibit multilevel resistance states in the presence of gradually reversed ferroelectric domains via tunneli…
We report the observation of a very large negative magnetoresistance effect in a van der Waals tunnel junction incorporating a thin magnetic semiconductor, CrI3, as the active layer. At constant voltage bias, current increases by nearly one million percent upon application of a 2 Tesla field. The effect arises from a change between antiparallel to parallel alignment of spins across the different CrI3 layers. Our results elucidate the nature of the magnetic state in ultrathin CrI3 and present new opportunities for spintronics based on two-dimensional materials.
The effects of the spin-orbit interaction on the tunneling magnetoresistance of ferromagnet/semiconductor/normal metal tunnel junctions are investigated. Analytical expressions for the tunneling anisotropic magnetoresistance (TAMR) are derived within an approximation in which the dependence of the magnetoresistance on the magnetization orientation in the ferromagnet originates from the interference between Bychkov-Rashba and Dresselhaus spin-orbit couplings that appear at junction interfaces and in the tunneling region. We also investigate the transport properties of ferromagnet/semiconductor/…
We have carried out calculations of current-voltage characteristics for the electron tunnel current through a junction with a thin insulating ferroelectric barrier assuming that interface transmissions for the left and right interfaces noticeably differ due to dissimilarity of the interfaces. Obtained conductance versus voltage and current versus voltage curves exhibit well distinguishable asymmetric hysteresis. We show that the asymmetry in the hysteretic effects could originate from the asymmetric bias voltage profile inside the junction. In particular, we analyze the hysteresis asymmetries …
Materials that demonstrate large magnetoresistance have attracted significant interest for many decades. Recently, extremely large tunnel magnetoresistance (TMR) has been reported by several groups across ultrathin CrI$_3$ by exploiting the weak antiferromagnetic coupling between adjacent layers. Here, we report a comparative study of TMR in all three chromium trihalides (CrX$_3$, X= Cl, Br, or I) in the two-dimensional limit. As the materials exhibit different transition temperatures and interlayer magnetic ordering in the ground state, tunneling measurements allow for an easy determination o…
We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in manganite/Nb:SrTiO3 pn junctions in relation to the special interface electronic structure; magnetoelectric coupling in manganite/ferroelectric structures that takes advantage of strain, carrier density, and magnetic field sensitivity; tunneling magnetoresistance in tunnel junctions with dielectric, ferroelectric, and organic semiconductor spacers using the fully spin polarized nature…
Giant tunnel magnetoresistance (TMR) ratios of 417% at room temperature (RT) and 914% at 3 K were demonstrated in epitaxial Fe/MgO/Fe(001) exchanged-biased spin-valve magnetic tunnel junctions (MTJs) by tuning growth conditions for each layer, combining sputter deposition for the Fe layers, electron-beam evaporation of the MgO barrier, and barrier interface tuning. Clear TMR oscillation as a function of the MgO thickness with a large peak-to-valley difference of ~80% was observed when the layers were grown on a highly (001)-oriented Cr buffer layer. Specific features of the observed MTJs are s…
Inverse magnetoresistance has been observed in magnetic tunnel junctions with pinhole nanocontacts over a broad temperature range. The tunnel magnetoresistance undergoes a change of sign at higher bias and temperature. This phenomenon is attributed to the competition between the spin conserved ballistic transport through the pinhole contact where the transmission probability is close to unity and spin polarized tunneling across the insulating spacer with weak transmittivity.
We report the observation of tunneling anisotropic magnetoresistance effect (TAMR) in the epitaxial metal-semiconductor system Fe/GaAs/Au. The observed two-fold anisotropy of the resistance can be switched by reversing the bias voltage, suggesting that the effect originates from the interference of the spin-orbit coupling at the interfaces. Corresponding model calculations reproduce the experimental findings very well.
We report on a theoretical study on the tunneling anomalous Hall effect (TAHE) in a ferroelectric tunnel junction (FTJ), resulting from spin-orbit coupling (SOC) in the ferroelectric barrier. For ferroelectric barriers with large SOC, such as orthorhombic HfO2 and BiInO3, we predict values of the tunneling anomalous Hall conductivity (TAHC) measurable experimentally. We demonstrate strong anisotropy in TAHC depending on the type of SOC. For the SOC with equal Rashba and Dresselhaus parameters, we predict the perfect anisotropy with zero TAHC for certain magnetization orientations. The TAHC cha…
We examine the spin torque (ST) response of magnetic tunnel junctions (MTJs) with ultra-thin MgO tunnel barrier layers to investigate the relationship between the spin-transfer torque and the tunnel magnetoresistance (TMR) under finite bias. We find that the spin torque per unit current exerted on the free layer decreases by less than 10% over a bias range where the TMR decreases by over 40%. We examine the implications of this result for various spin-polarized tunneling models and find that it is consistent with magnetic-state-dependent effective tunnel decay lengths.
The magnetic tunnel junction is a cornerstone of spintronic devices and circuits, providing the main way to convert between magnetic and electrical information. In state-of-the-art magnetic tunnel junctions, magnesium oxide is used as the tunnel barrier between magnetic electrodes, providing a uniquely large tunnel magnetoresistance at room temperature. However, the wide bandgap and band alignment of magnesium oxide-iron systems increases the resistance-area product and causes challenges of device-to-device variability and tunnel barrier degradation under high current. Here, we study using fir…
We have fabricated a spin-polarized tunneling device based on half metallic manganites incorporating $Ba_{2}LaNbO_{6}$ as insulating barrier. An anomalous bias dependence of tunnel magnetoresistance (TMR) has been observed, the first of its kind in a symmetric electrode tunnel junction with single insulating barrier. The bias dependence of TMR shows an extremely sharp zero bias anomaly, which can be considered as a demonstration of the drastic density of states variation around the Fermi level of the half metal. This serves as a strong evidence for the existence of minority spin tunneling stat…
We investigated spin dependent transport through Fe/GaAs/Fe tunnel junctions. The tunneling magnetoresistance effect (TMR) was probed for different types of Fe/GaAs interfaces. For interfaces cleaned by hydrogen plasma the TMR effect is increased and observable at room temperature. If an epitaxial Fe/GaAs(001) interface is involved, the tunnel junction exhibits a bias dependent inversion of the TMR effect. This is a first experimental signature for band structure effects at a Fe/GaAs interface and relevant for spin injection experiments.
The influence of insertion of an ultra-thin NiO layer between the MgO barrier and ferromagnetic electrode in magnetic tunnel junctions has been investigated by measuring the tunneling magnetoresistance and the X-ray magnetic circular dichroism (XMCD). The magnetoresistance shows a high asymmetry with respect to bias voltage, giving rise to a negative value of -16% at 2.8 K. We attribute this to the formation of non-collinear spin structures in the NiO layer as observed by XMCD. The magnetic moments of the interface Ni atoms tilt from the easy axis due to exchange interaction and the tilting an…
We investigate transport through 6 to 10 nm thin epitaxial GaAs(001) barriers sandwiched between polycrystalline iron films. Apart from a pronounced tunneling magnetoresistance effect (TMR) at low magnetic fields we observe a distinct negative magnetoresistance (MR) at low and a positive MR at higher temperatures. We show that the negative MR contribution is only observed for the ferromagnetic iron contacts but is absent if iron is replaced by copper or gold electrodes. Possible explanations of the negative MR involve suppression of spin-flip scattering or Zeeman splitting of the tunneling bar…
My research is dedicated to the electronic properties of functional oxides. My activity specifically focuses on ferroelectric tunnel junctions in which an ultrathin layer of ferroelectric material is intercalated between two metallic electrodes. In these devices, polarization reversal induces large modifications of the tunnel resistance, leading to a non-destructive readout of the information. After a demonstration of the concept with scanning probe microscopy techniques, I have been exploring the properties of ferroelectric tunnel junctions with various ferroelectric materials (BaTiO3, BiFeO3…
Recently, it has been shown that magnetic tunnel junctions with thin MgO tunnel barriers exhibit extraordinarily high tunneling magnetoresistance (TMR) values at room temperature1, 2. However, the physics of spin dependent tunneling through MgO barriers is only beginning to be unravelled. Using planar magnetic tunnel junctions in which ultra-thin layers of magnetic metals are deposited in the middle of a MgO tunnel barrier here we demonstrate that the TMR is strongly modified when these layers are discontinuous and composed of small pancake shaped nanodots. At low temperatures, in the Coulomb …
Ferroelectric tunnel junction devices based on ferroelectric thin films of solid solutions of hafnium dioxide can enable CMOS integration of ultra-low power ferroelectric devices with potential for memory and emerging computing schemes such as in-memory computing and neuromorphic applications. In this work, we present ferroelectric tunnel junctions based on Hf$_{0.5}$Zr$_{0.5}$O$_{2}$ with materials and processes compatible with CMOS back-end-of-line integration. We show a device architecture based on W-Hf$_{0.5}$Zr$_{0.5}$O$_{2}$-Al$_{2}$O$_{3}$-TiN stacks featuring low temperature annealing …
Based on the structure predicted in a ferroelectric tunnel junction in the resent density functional theory study, we investigate the electron transport through the FTJ with asymmetric interfaces, i.e., one interface dipole is pinned and the other interface dipole is switchable. Tuneling electroresistance can be induced due to the nonswitchable interface dipole in FTJs with symmetric electrodes. Compared with the dependence relationship between TER and the polarization of switchable interface, TER is not sensitive to the variation of the polarization of pinned interface. A large TER can be ach…
We investigated the relationship between tunnel magnetoresistance (TMR) ratio and the crystallization of CoFeB layers through annealing in magnetic tunnel junctions (MTJs) with MgO barriers that had CoFe/Ru/CoFeB synthetic ferrimagnet pinned layers with varying Ru spacer thickness (tRu). The TMR ratio increased with increasing annealing temperature (Ta) and tRu, reaching 361% at Ta = 425C, whereas the TMR ratio of the MTJs with pinned layers without Ru spacers decreased at Ta over 325C. Ruthenium spacers play an important role in forming an (001)-oriented bcc CoFeB pinned layer, resulting in a…
