Ferromagnetic Tunnel Junctions Research Articles

Spin-selective transport in a correlated double quantum dot-Majorana wire system

In this work we investigate the spin-dependent transport through a double quantum dot embedded in a ferromagnetic tunnel junction and side attached to a topological superconducting nanowire hosting Majorana zero-energy modes. We focus on the transport regime when the Majorana mode leaks into the double quantum dot competing with the two-stage Kondo effect and the ferromagnetic-contact-induced exchange field. In particular, we determine the system’s spectral properties and analyze the temperature dependence of the spin-resolved linear conductance by means of the numerical renormalization group method. Our study reveals unique signatures of the interplay between the spin-resolved tunneling, the Kondo effect and the Majorana modes, which are visible in the transport characteristics. In particular, we uncover a competing character of the coupling to topological superconductor and that to ferromagnetic leads, which can be observed already for very low spin polarization of the electrodes. This is signaled by an almost complete quenching of the conductance in one of the spin channels which is revealed through perfect conductance spin polarization. Moreover, we show that the conductance spin polarization can change sign depending on the magnitude of spin imbalance in the leads and strength of interaction with topological wire. Thus, our work demonstrates that even minuscule spin polarization of tunneling processes can have large impact on the transport properties of the system.

Giant tunnel magnetoresistance induced by thermal bias

We analyze the spin-resolved transport and, in particular, the tunnel magnetoresistance of an asymmetric ferromagnetic tunnel junction with an embedded quantum dot or molecule subject to thermal and voltage bias in the nonlinear response regime. We demonstrate that such system exhibits a giant tunnel magnetoresistance effect that can be tuned by gate and bias voltages. Large values of magnetoresistance are associated with the interplay between the Kondo correlations and the ferromagnetic-contact-induced exchange field. In particular, we show that the nonequilibrium current in the parallel and antiparallel magnetic configuration of the system changes sign at different values of the voltage and thermal bias. This gives rise to giant values of magnetoresistance, the sign of which can be controlled by the applied sources.

Perspectives on field-free spin–orbit torque devices for memory and computing applications

The emergence of embedded magnetic random-access memory (MRAM) and its integration in mainstream semiconductor manufacturing technology have created an unprecedented opportunity for engineering computing systems with improved performance, energy efficiency, lower cost, and unconventional computing capabilities. While the initial interest in the existing generation of MRAM—which is based on the spin-transfer torque (STT) effect in ferromagnetic tunnel junctions—was driven by its nonvolatile data retention and lower cost of integration compared to embedded Flash (eFlash), the focus of MRAM research and development efforts is increasingly shifting toward alternative write mechanisms (beyond STT) and new materials (beyond ferromagnets) in recent years. This has been driven by the need for better speed vs density and speed vs endurance trade-offs to make MRAM applicable to a wider range of memory markets, as well as to utilize the potential of MRAM in various unconventional computing architectures that utilize the physics of nanoscale magnets. In this Perspective, we offer an overview of spin–orbit torque (SOT) as one of these beyond-STT write mechanisms for the MRAM devices. We discuss, specifically, the progress in developing SOT-MRAM devices with perpendicular magnetization. Starting from basic symmetry considerations, we discuss the requirement for an in-plane bias magnetic field which has hindered progress in developing practical SOT-MRAM devices. We then discuss several approaches based on structural, magnetic, and chiral symmetry-breaking that have been explored to overcome this limitation and realize bias-field-free SOT-MRAM devices with perpendicular magnetization. We also review the corresponding material- and device-level challenges in each case. We then present a perspective of the potential of these devices for computing and security applications beyond their use in the conventional memory hierarchy.

The influence of the magnetic configuration on the charge current generated by the temperature gradient in the double planar ferromagnetic tunnel junctions

The charge current generated by the temperature gradient applied to the double planar tunnel junctions with ferromagnetic external electrodes and central layer is investigated in the free-electron model. The situation when the bias voltage is applied to the junction is also considered. It has been shown that the analyzed current may depend on the orientation of magnetic moments in the ferromagnetic external electrodes and the central layer. The influence of the magnetic configuration on the tunnel current can be enhanced due to the resonant tunneling by the resonant states. Especially strong modifications of the current flowing through the junction can be observed as a result of changing the orientation of the magnetic moments in the central layer in junctions where the tunneling by the resonant states located below the bottom for the spin-down electrons in the source electrode is observed. The studied effects are strongly depended on the thickness of the central layer, spin splitting of the electron bands in the external electrodes and in the central layer, and the bias voltage applied to the junctions, but are also sensitive to the average temperature of the junction and the height and width of the barriers, whereas they are less sensitive to the temperature difference between the electrodes.

Giant magnetoresistance in antiferromagnetic Mn2Au-based tunnel junction

Recent studies on the electrical switching of tetragonal antiferromagnet (AFM) via Neel spin-orbit torque have paved the way for the economic use of antiferromagnetic materials. The most difficult obstacle that presently limits the application of antiferromagnetic materials in spintronics, especially in memory storage applications, could be the small and fragile magnetoresistance (MR) in the AFM-based nanostructure. In this study, we investigated the spin transports in Mn$_2$Au-based tunnel junctions based on the first-principle scattering theory. Giant MRs more than $1000%$ are predicted in some Fe/MgO/Ag/Mn$_2$Au/Ta junctions that are about the same order as that in an MgO-based ferromagnetic tunnel junction with same barrier thickness. The interplay of the spin filtering effect, the quantum well resonant states, and the interfacial resonant states could be responsible for the unusual giant and robust MRs observed in these Mn$_2$Au-based junctions.

Spin Hall effect induced Néel order switching in the tetragonal Mn2Au

Tetragonal Mn2Au is a metallic antiferromagnet with high Néel temperature well above 1000 K. The collinear magnetic sublattice with broken inversion symmetry in company with stable in-plane anisotropy in this material favors to control it via the Néel spin–orbital effect and spin-Hall effect. In this paper, we study the spin–orbital torque induced Néel order dynamics in the heavy-metal(HM)/Mn2Au system based on coupled Landau–Lifshitz-Gilbert-Slonczewski equations. We demonstrate robust picosecond switching of the Néel order of the Mn2Au in the presence of two charge currents, and close resemblance of the Néel order switching of the HM/Mn2Au bilayer to short-term plasticity and long-term potentiation observed in the biological synapses. Orders in magnitude reduction of the power consumption in the Pt/Mn2Au bilayer comparing with the ferromagnetic tunnel junction is demonstrated also. The ultra-high speed terahertz dynamics, ultra-lower power consumption accompanying with feasible electrical manipulation make HM/Mn2Au system a promising candidate in memory storage and neuromorphic computing applications.

Ferroic tunnel junctions and their application in neuromorphic networks

Brain-inspired neuromorphic computing has been intensively studied due to its potential to address the inherent energy and throughput limitations of conventional Von-Neumann based computing architecture. Memristors are ideal building blocks for artificial synapses, which are the fundamental components of neuromorphic computing. In recent years, the emerging ferroic (ferroelectric and ferromagnetic) tunnel junctions have been shown to be able to function as memristors, which are potential candidates to emulate artificial synapses for neuromorphic computing. Here, we provide a review on the ferroic tunnel junctions and their applications as artificial synapses in neuromorphic networks. We focus on the development history of ferroic tunnel junctions, their physical conduction mechanisms, and the intrinsic dynamics of memristors. Their current applications in neuromorphic networks will also be discussed. Finally, a conclusion and future outlooks on the development of ferroic tunnel junctions will be given. Our goal is to give a broad review of ferroic tunnel junction based artificial synapses that can be applied to neuromorphic computing and to help further ongoing research in this field.

Nonlocal thermoelectric effects in high-field superconductor-ferromagnet hybrid structures

We report on the experimental observation of nonlocal spin-dependent thermoelectric effects in superconductor-ferromagnet multiterminal structures. Our samples consist of a thin superconducting aluminum wire with several ferromagnetic tunnel junctions attached to it. When a thermal excitation is applied to one of the junctions in the presence of a Zeeman splitting of the density of states of the superconductor, a thermoelectric current is observed in remote junctions at distances exceeding $10~\mathrm{\mu m}$. The results can be explained by recent theories of coupled spin and heat transport in high-field superconductors.

On-chip cooling by heating with superconducting tunnel junctions

Heat management and refrigeration are key concepts for nanoscale devices operating at cryogenic temperatures. The design of an on-chip mesoscopic refrigerator that works thanks to the input heat is presented, thus realizing a solid-state implementation of the concept of cooling by heating. The system consists of a circuit featuring a thermoelectric element based on a ferromagnetic insulator-superconductor tunnel junction (N-FI-S) and a series of two normal metal-superconductor tunnel junctions (SINIS). The N-FI-S element converts the incoming heat in a thermovoltage, which is applied to the SINIS, thereby yielding cooling. The cooler's performance is investigated as a function of the input heat current for different bath temperatures. We show that this system can efficiently employ the performance of SINIS refrigeration, with a substantial cooling of the normal metal island. Its scalability and simplicity in the design makes it a promising building block for low-temperature on-chip energy management applications.

Robust spin transfer torque in antiferromagnetic tunnel junctions

We theoretically study the current-induced spin torque in antiferromagnetic tunnel junctions, composed of two semi-infinite antiferromagnetic layers separated by a tunnel barrier, in both clean and disordered regimes. We find that the torque enabling the electrical manipulation of the N\'eel antiferromagnetic order parameter is out of plane $\sim {\bf n}\times{\bf p}$, while the torque competing with the antiferromagnetic exchange is in-plane $\sim {\bf n}\times({\bf p}\times{\bf n})$. Here, ${\bf { p}}$ and ${\bf { n}}$ are the N\'eel order parameter direction of the reference and free layers, respectively. Their bias dependence shows similar behavior as in ferromagnetic tunnel junctions, the in-plane torque being mostly linear in bias while the out-of-plane torque is quadratic. Most importantly, we find that the spin transfer torque in antiferromagnetic tunnel junctions is much more robust against disorder than in antiferromagnetic metallic spin-valves due to the tunneling nature of spin transport.

Development of bio-magnetic sensors with a ferromagnetic tunnel junction

Development of bio-magnetic sensors with a ferromagnetic tunnel junction

Self-Oscillating Josephson Quantum Heat Engine

The design of a mesoscopic self-oscillating heat engine that works thanks to purely quantum effects is presented. The proposed scheme is amenable to experimental implementation with current state-of-the-art nanotechnology and materials. One of the main features of the structure is its versatility: The engine can deliver work to a generic load without galvanic contact. This makes it a promising building block for low-temperature on-chip energy management applications. The heat engine consists of a circuit featuring a thermoelectric element based on a ferromagnetic insulator-superconductor tunnel junction and a Josephson weak link that realizes a purely quantum DC/AC converter. This enables contactless transfer of work to the load (a generic RL circuit). The performance of the heat engine is investigated as a function of the thermal gradient applied to the thermoelectric junction. Power up to $1$ pW can be delivered to a load $R_L=10\ \Omega$.

Mapping the chemical potential dependence of current-induced spin polarization in a topological insulator

We report electrical measurements of the current-induced spin polarization of the surface current in topological insulator devices where contributions from bulk and surface conduction can be disen- tangled by electrical gating. The devices use a ferromagnetic tunnel junction (permalloy/Al2O3) as a spin detector on a back-gated (Bi,Sb)2Te3 channel. We observe hysteretic voltage signals as the magnetization of the detector ferromagnet is switched parallel or anti-parallel to the spin polariza- tion of the surface current. The amplitude of the detected voltage change is linearly proportional to the applied DC bias current in the (Bi,Sb)2Te3 channel. As the chemical potential is tuned from the bulk bands into the surface state band, we observe an enhancement of the spin-dependent voltages up to 300% within the range of the electrostatic gating. Using a simple model, we extract the spin polarization near charge neutrality (i.e. the Dirac point).

Present and perspective of bio-magnetic measurement using ferromagnetic tunnel junctions

Present and perspective of bio-magnetic measurement using ferromagnetic tunnel junctions

Crossover from diffusive to tunneling regime in NbN/DyN/NbN ferromagnetic semiconductor tunnel junctions

We have investigated NbN-DyN-NbN junctions with 1 to 10 nm thick DyN barriers. A crossover from diffusive (hopping) to tunneling-type transport was found in these junctions as the DyN thickness is reduced below $\ensuremath{\sim}$4 nm. We have also made a detailed study of magnetic and electrical properties of thicker DyN thin films deposited under similar conditions; DyN films were found to be ferromagnetic with ${T}_{\text{Curie}}\ensuremath{\sim}35\ifmmode\pm\else\textpm\fi{}5$ K. Electrical transport of the junctions with $\ensuremath{\sim}$10 nm DyN was understood in terms of Shklovskii-Efros (SE)-type variable range hopping (VRH) at low temperature between 90 and 35 K. We estimated localization length $\ensuremath{\xi}=5.6$ nm in this temperature range. Temperature dependence of resistance was found to deviate from SE-VRH below 35 K along with large suppression of resistance with magnetic field. This is correlated with onset of magnetism below 35 K. Large butterfly-shaped MR up to $\ensuremath{\sim}$40$%$ was found for the $\ensuremath{\sim}$10 nm thick DyN junction at 2 K. In the tunneling regime, barrier height of the tunnel junction was estimated $\ensuremath{\sim}$50 meV from the Simmons model. Signatures of spin filtering was found in temperature dependence of resistance in tunnel junction with $\ensuremath{\sim}$3 nm thick DyN. Cooper pair tunneling in these junctions below ${T}_{C}$ ($\ensuremath{\sim}$10.8 K) of NbN was understood according to S-I-S tunneling current model. We found coherent tunneling of Cooper pairs through a $\ensuremath{\sim}$1 nm thick DyN tunnel barrier with critical current ${I}_{C}\ensuremath{\sim}12$ $\ensuremath{\mu}$A. The critical current also showed modulation with magnetic field.

ChemInform Abstract: Bio‐Inspired Magnetic Carbon Materials

AbstractReview: 117 refs.

Tunnel conductance in GaN:Mn/AlN/GaN:Mn (0001) junction from first-principles calculations

The tunnel conductance of diluted ferromagnetic semiconductor tunnel junction GaN:Mn/AlN/GaN:Mn is calculated based on the first-principles calculations. The dependence of the conductance on AlN barrier thickness, doping site, and doping concentration of Mn is studied. The contributions from coherent tunneling and resonant tunneling are discussed by combining the complex band structure of AlN barrier layer and the interface states on both sides of the barrier.

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.

Reversible electrical switching of spin polarization in multiferroic tunnel junctions

Spin-polarized transport in ferromagnetic tunnel junctions, characterized by tunnel magnetoresistance, has already been proven to have great potential for application in the field of spintronics and in magnetic random access memories. Until recently, in such a junction the insulating barrier played only a passive role, namely to facilitate electron tunnelling between the ferromagnetic electrodes. However, new possibilities emerged when ferroelectric materials were used for the insulating barrier, as these possess a permanent dielectric polarization switchable between two stable states. Adding to the two different magnetization alignments of the electrode, four non-volatile states are therefore possible in such multiferroic tunnel junctions. Here, we show that owing to the coupling between magnetization and ferroelectric polarization at the interface between the electrode and barrier of a multiferroic tunnel junction, the spin polarization of the tunnelling electrons can be reversibly and remanently inverted by switching the ferroelectric polarization of the barrier. Selecting the spin direction of the tunnelling electrons by short electric pulses in the nanosecond range rather than by an applied magnetic field enables new possibilities for spin control in spintronic devices.

Fabrication of planar-type Ni/vacuum/Ni tunnel junctions based on ferromagnetic nanogaps using field-emission-induced electromigration

Planar-type Ni/vacuum/Ni tunnel junctions based on ferromagnetic nanogaps were fabricated by field-emission-induced electromigration in an approach that we call “activation.” In the activation method, we were simply and easily able to control the tunnel resistance of the nanogaps from 1.6 MΩ to 169 kΩ by adjusting only the magnitude of the field emission current. The resistance of the junction formed by the activation was varied by applying external magnetic fields, and the magnetoresistance (MR) ratio was approximately 12.2% at 16 K with a bias voltage of 0.72 mV. Furthermore, after the bias voltage was increased from 0.72 to 7.3 mV, the MR ratio decreased from 12.2% to 6.2%. When the applied bias voltage was fixed at 1.6 mV, the MR ratio was decreased from 11.6% to 1.2% by increasing the measurement temperature from 16 to 270 K. These results imply that it is possible to easily fabricate planar-type Ni/vacuum/Ni ferromagnetic tunnel junctions via activation.