Ferromagnetic Insulator Layer Research Articles

Spin dynamics in superconductor/ferromagnetic insulator hybrid structures with precessing magnetization.

The main goal of the present work is the description of the dynamics of spin current and induced magnetization inside a superconducting film S that is in contact with a ferromagnetic insulator layer FI. Spin current and induced magnetization are calculated not only at the interface of the S/FI hybrid structure, but also inside the superconducting film. The new and interesting predicted effect is the frequency dependence of the induced magnetization with a maximum appearing at high temperatures. It is also shown that the increase of the magnetization precession frequency can drastically change the spin distribution of quasiparticles at the S/FI interface.

Semiconductor-ferromagnet-superconductor planar heterostructures for 1D topological superconductivity

Hybrid structures of semiconducting (SM) nanowires, epitaxially grown superconductors (SC), and ferromagnetic-insulator (FI) layers have been explored experimentally and theoretically as alternative platforms for topological superconductivity at zero magnetic field. Here, we analyze a tripartite SM/FI/SC heterostructure but realized in a planar stacking geometry, where the thin FI layer acts as a spin-polarized barrier between the SM and the SC. We optimize the system’s geometrical parameters using microscopic simulations, finding the range of FI thicknesses for which the hybrid system can be tuned into the topological regime. Within this range, and thanks to the vertical confinement provided by the stacking geometry, trivial and topological phases alternate regularly as the external gate is varied, displaying a hard topological gap that can reach half of the SC one. This is a significant improvement compared to setups using hexagonal nanowires, which show erratic topological regions with typically smaller and softer gaps. Our proposal provides a magnetic field-free planar design for quasi-one-dimensional topological superconductivity with attractive properties for experimental control and scalability.

Optimizing the topological properties of semiconductor-ferromagnet-superconductor heterostructures

We study the electronic properties of a planar semiconductor-superconductor heterostructure, in which a thin ferromagnetic insulator layer lies in between and acts as a spin filtering barrier. We find that in such a system one can simultaneously enhance the strengths of all the three important induced physical quantities, i.e., Rashba spin-orbit coupling, exchange coupling, and superconducting pairing potential, for the hybrid mode by external gating. Our results show the specific advantage of this stacked device geometry compared to conventional devices. We further discuss how to optimize geometrical parameters for the heterostructure and complement our numerical simulations with analytic calculations.

Tunable proximity effects and topological superconductivity in ferromagnetic hybrid nanowires

Hybrid semiconducting nanowire devices combining epitaxial superconductor and ferromagnetic insulator layers have been recently explored experimentally as an alternative platform for topological superconductivity at zero applied magnetic field. In this proof-of-principle work we show that the topological regime can be reached in actual devices depending on some geometrical constraints. To this end, we perform numerical simulations of InAs wires in which we explicitly include the superconducting Al and magnetic EuS shells, as well as the interaction with the electrostatic environment at a self-consistent mean-field level. Our calculations show that both the magnetic and the superconducting proximity effects on the nanowire can be tuned by nearby gates thanks to their ability to move the wavefunction across the wire section. We find that the topological phase is achieved in significant portions of the phase diagram only in configurations where the Al and EuS layers overlap on some wire facet, due to the rather local direct induced spin polarization and the appearance of an extra indirect exchange field through the superconductor. While of obvious relevance for the explanation of recent experiments, tunable proximity effects are of interest in the broader field of superconducting spintronics.

Topological superconductivity in tripartite superconductor-ferromagnet-semiconductor nanowires

Motivated by recent experiments searching for Majorana zero modes in tripartite semiconductor nanowires with epitaxial superconductor and ferromagnetic-insulator layers, we explore the emergence of topological superconductivity in such devices for paradigmatic arrangements of the three constituents. Accounting for the competition between magnetism and superconductivity, we treat superconductivity self consistently and describe the electronic properties, including the superconducting and ferromagnetic proximity effects, within a direct wave-function approach. We conclude that the most viable mechanism for topological superconductivity relies on a superconductor-semiconductor-ferromagnet arrangement of the constituents, in which spin splitting and superconductivity are independently induced in the semiconductor by proximity and superconductivity is only weakly affected by the ferromagnetic insulator.

Heat transport in insulator/ferromagnetic-insulator/insulator heterogeneous nanostructures at low temperatures

A kinetic approach to the heat transport by phonons and magnons through a ferromagnetic insulator (FI) layer located between two massive insulators $({I}_{1}$ and ${I}_{2})$ is analytically considered. The effective transverse heat conductivity of such a layered system with an arbitrary thickness of the FI layer is calculated, and the thickness at which the size effect is manifested in the thermal conductivity is found.

Recent progress in the research of quantum anomalous <?A3B2 ACK?>Hall effect

<p indent="0mm">The quantum anomalous Hall (QAH) effect is a quantum Hall effect that can occur without external magnetic field. The experimental realization of this effect in magnetically doped topological insulator thin films not only provides an ideal platform for research and application on chiral quantum Hall state, but also paves the way for constructing many other exotic topological quantum phases. But the realization temperature of QAH is quite low. The non-uniform magnetic disorder could be one of main reasons. To increase the uniformity of magnetic order, different element was doped, like V instead of Cr, or Cr-V co-doping, which enhance the magnetic anisotropy in the system. In Cr-V co-doping system, the realization temperature of the QAH was ten times enhanced. Also, a penta-layer structure with heavily doped sub-surface layer was formed to effectively increase the QAH temperature. In this structure, the modulation-doping originates from the suppression of the disorder in the surface state interacting with the rich magnetic moments. In the future, with combined element doping and modified structure, we may get much improved QAH temperature. To elevate the working temperature of the QAH effect, a TI sandwiched by two ferromagnetic insulator layers is also applied because of the more ordered structure and potentially high Curie temperature. Although several works on FMI/TI heterostructure reports the proximity ferromagnetic in TI, the QAH was observed at quite a low temperature in an all-telluride base heterostructure. A strong coupling between the surface state of TI and ferromagnetic layer could be required. In a magnetically doped TI system, it is natural to ask how a QAH system evolves in a magnetization reversal process. Two groups observed the zero plateau of Hall conductance around the coercive field independently, indicating transition between two QAH phases occurs via an ordinary insulator phase, which could be understood in a multi-magnetic domain structure. A similar zero plateau of Hall conductance have been observed in magnetic-doped TI (MTI) sandwich structure MTI/TI/MTI but with opposite out-of-plane magnetization direction on top and bottom surface. The system behaves like a normal insulator in a <italic>DC</italic> transport measurement but will have topological magnetoelectric effect in <italic>AC</italic> measurement, which is still a challenge. The QAH insulator is important not only as a rare topological quantum phase that has been unambiguously experimentally realized, but also as a solid and versatile building block to construct many other topological quantum states. The latter role requires a QAH system to be engineered or incorporated into various kinds of heterostructures, on which several exciting experimental progresses have been made in the past years: The QAH insulator-normal insulator heterostructures simulated high Chern number QAH insulator has been experimental reported. But it is still quite challenging because the multilayer structure acquires accurate growth parameter control; a QAH insulator acquiring proximity superconductivity from an adjacent <italic>s</italic> wave superconductor (SC) layer can show the properties of a chiral topological SC. Majorana bound states (MBSs) are expected to appear in the magnetic vortices of the topological SC and could be used to compose topological qubits which are presumably robust against decoherence with topological protection. A better-defined QAH-SC interface is crucial for further explorations in this direction to avoid disturbance of interface disorders; the first intrinsic anti-ferromagnetic TI material MnBi₂Te₄ with intercalation ferromagnetic layer MnTe was found. By alternating MBE growth of Bi₂Te₃ and MnTe, MnBi₂Te₄ thin film was formed and gapless Dirac surface state from ARPES in over 2 SL films indicate the “topology” of this material. Recently the quantized Hall resistance was reported in exfoliated thin flake of MnBi₂Te₄ with high magnetic field. The results establish that MnBi₂Te₄ could be an idea platform for further exploring topological phenomena. The recent progresses on QAH show us big opportunities to reach various novel topological states of matter experimentally. The explorations call for more exquisitely designed heterostructures and higher sample quality. With these efforts, the fantastic world of topological quantum matter is getting more and more realistic.

Spin-dependent electronic lenses based on hybrid graphene nanostructures

We study electronic transport in graphene/ferromagnetic insulator hybrid devices. The system comprises an armchair graphene nanoribbon with a lens-shaped EuO ferromagnetic insulator layer deposited on top of it. When the device supports a large number of propagating modes, the proximity exchange interaction of electrons with the magnetic ions of the ferromagnetic insulator results in electrons being spatially localised at different spots depending on their spin. We found the spin-dependent electron focusing is robust under moderate edge disorder. A spin-polarised electric current can be generated by placing a third contact in the proper place. This opens the possibility to use these effects for fabricating tunable sources of polarized electrons.

Switchable Josephson junction based on interfacial exchange field

We demonstrate exchange field switchable Josephson junctions where the Josephson tunneling between two superconducting layers (Nb and NbN) could be controlled by the relative magnetic alignment of two GdN ferromagnetic insulator layers sandwiching the Nb layer. The junction's working principle is based on the control of the superconducting state of one of the layers by means of the interfacial exchange field of the magnetic GdN layers. At zero field and low temperatures, the ground state of the junctions corresponds to an antiferromagnetic configuration of the two GdN layers that coexist with the superconducting state of the Nb layer. By applying an external field, the GdN layers are switched to a parallel configuration, thereby suppressing the superconductivity in Nb and hence the Josephson current via interfacial exchange. This switchable Josephson junction may be useful for integrated superconducting spintronics and quantum circuits.

Giant magneto-spin-Seebeck effect and magnon transfer torques in insulating spin valves

We theoretically study magnon transport in an insulating spin valve (ISV) made of an antiferromagnetic insulator sandwiched between two ferromagnetic insulator (FI) layers. In the conventional metal-based spin valve, the electron spins propagate between two metallic ferromagnetic layers, giving rise to giant magnetoresistance and spin transfer torque. Here, the incoherent magnons in the ISV serve as angular momentum carriers and are responsible for the angular momentum transport between two FI layers across the antiferromagnetic spacer. We predict two transport phenomena in the presence of the temperature gradient: a giant magneto-spin-Seebeck effect in which the output voltage signal is controlled by the relative orientation of the two FI layers and magnon transfer torque that can be used for switching the magnetization of the FI layers with a temperature gradient of the order of 0.1 Kelvin per nanometer.

Clocked single-spin source based on a spin-split superconductor

We propose an accurate clocked single-spin source for ac-spintronic applications. Our device consists of a superconducting island covered by a ferromagnetic insulator (FI) layer through which it is coupled to superconducting leads. Single-particle transfer relies on the energy gaps and the island's charging energy, and is enabled by a bias and a time-periodic gate voltage. Accurate spin transfer is achieved by the FI layer which polarizes the island, provides spin-selective tunneling barriers and improves the precision by suppressing Andreev reflection. We analyze realistic material combinations and experimental requirements which allow for a clocked spin current in the MHz regime.

Spin and Heat Transport in Ferromagnetic Insulators (FI)/Normal Metal (NM) Mesoscopic Devices — A Boltzmann Approach

We build self-consistent spin-dependent Boltzmann equation in ferromagnetic insulator (FI) and metal layers. The effect of temperature gradient is considered. In this theoretical framework, we focus on the transport phenomena in FI/normal metal (NM) layers, including spin Seebeck effect (SSE). A temperature gradient can generate magnon current in FI layer, and it can converse to the spin current in NM layer through the interaction between magnon and electron at the interface. We also study the different factors of spin-dependent scattering at the interface, regarded as the boundary conditions. By adjusting the FI/NM interface parameters and a temperature gradient, we also obtain influence of diffusion length and [Formula: see text] on the final spin currents.

Bulk magnon spin current theory for the longitudinal spin Seebeck effect

The longitudinal spin Seebeck effect (LSSE) consists in the generation of a spin current parallel to a temperature gradient applied across the thickness of a bilayer made of a ferromagnetic insulator (FMI), such as yttrium iron garnet (YIG), and a metallic layer (ML) with strong spin orbit coupling, such as platinum. The LSSE is usually detected by a DC voltage generated along the ML due to the conversion of the spin current into a charge current perpendicular to the static magnetic field by means of the inverse spin Hall effect. Here we present a model for the LSSE that relies on the bulk magnon spin current created by the temperature gradient across the thickness of the FMI. We show that the spin current pumped into the metallic layer by the magnon accumulation in the FMI provides continuity of the spin current at the FMI/ML interface and is essential for the existence of the LSSE. The results of the theory are in good agreement with experimental LSSE data in YIG/Pt bilayers on the variation of the DC voltage with the sample temperature, with the FMI layer thickness and with the intensity of high magnetic fields.

Control of spin-polarised currents in graphene nanorings

We study electronic transport in systems comprising square graphene nanorings with a ferromagnetic insulator layer on top of them. The rings are connected symmetrically or asymmetrically to contacts. The proximity exchange interaction of electrons with magnetic ions results in spin-dependent transport properties. When a nanoring is connected asymmetrically, the occurrence of Fano-like antiresonances in the transmission coefficient can induce abrupt changes in the spin polarisation under minute variations of the Fermi energy. We also demonstrate that the spin polarisation can be efficiently controlled by a side-gate voltage. This opens a possibility to use these effects for fabricating tunable sources of polarised electrons.

Magnon spin-current theory for the longitudinal spin-Seebeck effect

We present a theoretical model for the longitudinal spin-Seebeck effect (LSSE) in bilayers made of a ferromagnetic insulator (FMI), such as yttrium iron garnet (YIG), and a normal metal (NM), such as platinum (Pt), that relies on the bulk magnon spin current created by the temperature gradient across the thickness of the FMI. We show that the spin current pumped into the NM layer by the magnon accumulation in the FMI provides continuity of the spin current at the FMI/NM interface and is essential for the existence of the longitudinal spin-Seebeck effect. The results of the theory are in good agreement with experimental data for the variation of the LSSE with the sample temperature and with the FMI layer thickness in YIG/Pt bilayers.

Spin transport in normal metal/insulator/topological insulator coupled to ferromagnetic insulator structures

In this study, we investigate the spin transport in normal metal (NM)/insulator (I)/topological insulator (TI) coupled to ferromagnetic insulator (FI) structures. In particular, we focus on the barrier thickness dependence of the spin transport inside the bulk gap of the TI with FI. The TI with FI is described by two-dimensional (2D) Dirac Hamiltonian. The energy profile of the insulator is assumed to be a square with barrier height V and thickness d along the transport-direction. This structure behaves as a tunnel device for 2D Dirac electrons. The calculation is performed for the spin conductance with changing the barrier thickness and the components of magnetization of FI layer. It is found that the spin conductance decreases with increasing the barrier thickness. Also, the spin conductance is strongly dependent on the polar angle θ, which is defined as the angle between the axis normal to the FI and the magnetization of FI layer. These results indicate that the structures are promising candidates for novel tunneling magnetoresistance devices.

Electron Transport at the Interface Between a Ferromagnetic Insulator and a Topological Insulator

We perform a theoretical study of the electron transport through a normal metal-topological insulator-normal metal (NM-TI-NM) system, where a ferromagnetic insulator (FI) layer is deposited on top of the TI. The spin conductance of the system is analyzed as a function of parameters such as the strength of exchange coupling between the surface states of the TI and the magnetic moments of the FI layer, as well as the dimension of the TI channel. We find that the strength of the spin conductance can be optimized by tuning the parameters.

Spin current generator based on topological insulator coupled to ferromagnetic insulators

We propose a spin current generator based on a topological insulator current-in-plane spin valve, consisting of a 3D topological insulator sandwiched between two ferromagnetic insulator layers. The “on” and “off” states of the spin current can be toggled by switching the magnetization configuration of the two ferromagnetic insulator layers which are coupled to the surface states of the topological insulator.

Magnon Mediated Electric Current Drag Across a Ferromagnetic Insulator Layer

In a semiconductor heterostructure, the Coulomb interaction is responsible for the electric current drag between two 2D electron gases across an electron impenetrable insulator. For two metallic layers separated by a ferromagnetic insulator (FI) layer, the electric current drag can be mediated by a nonequilibrium magnon current of the FI. We determine the drag current by using the semiclassical Boltzmann approach with proper boundary conditions of electrons and magnons at the metal-FI interface.

Spin filtering in a ferromagnetic graphene superlattice

We have studied spin transport in monolayer and bilayer ferromagnetic graphene superlattice, based on transfer matrix method. The ferromagnetic insulator layers with exchange energy E ex , are in contact with the gate electrode. The magnetization in ferromagnetic insulator layers aligned in the z or negative z-direction. We find that the conductivity of the systems has an oscillatory behavior with respect to the gate voltage and it depends on the spin state of electron. It has been shown that the spin current can be controlled by gate voltage and in proper width of the wells and barriers it is possible to filter electron with a specific spin.