Spin Seebeck Research Articles

Inkjet‐Printed Flexible Spin Seebeck Thermopile Device for Low‐Cost Spintronics Device Fabrication

Spin‐momentum‐mediated heat–charge conversion technologies for developing thin thermoelectric generators and heat flow sensors have been extensively studied. The thermoelectric generation (TEG) based on the spin Seebeck (SSE) effect is a promising technology. In this study, a flexible SSE thermopile device is fabricated using inkjet printing with Fe3O4 and Ag nanoparticle (NP) ink. A square Fe3O4 NP and a wire‐shaped Ag NP layer are used as the first and second layers, respectively. These layers are overlaid using inkjet printing twice. To obtain the SSE voltage (VSSE), a thin Pt film is deposited on a printed thermopile device. SSE generators consisting of a Pt thin film/Fe3O4 NP layer are connected through a Ag NP wire to form a thermopile structure. The saturated VSSE of the SSE thermopile device increases with the number of SSE generators. In addition, the SSE‐TEG of an SSE thermopile device is observed using a heat source with a curved surface. In these results, it is indicated that the inkjet‐printing fabrication method holds great potential in both the SSE thermopile devices and the flexible spintronic devices because it is a simple and fast process that does not require lithography.

Vector spin Seebeck effect and spin swapping effect in antiferromagnetic insulators with non-collinear spin structure

Antiferromagnets (AFs) are prospective for next-generation high-density and high-speed spintronic applications due to their negligible stray field and ultrafast spin dynamics, notwithstanding the challenges in detecting and manipulating AF order with no magnetization (M = 0). Among the AFs, non-collinear AFs are of particular interest because of their unique properties arising from the non-collinear spin structure and the small magnetization M. In this work, we describe the recently observed vector spin Seebeck effect in non-collinear LuFeO3, where the magneto-thermovoltage under an in-plane temperature gradient, not previously observed, is consistent with the predicted spin swapping effect. Our results shed light on the importance of the non-collinear spin structure in the emerging spin phenomena in non-collinear AFs and offer a new class of materials for AF spintronics and spin caloritronics.

Spin and spin current—From fundamentals to recent progress

Along with the progress of spin science and spintronics research, the flow of electron spins, i.e., spin current, has attracted interest. New phenomena and electronic states were explained in succession using the concept of spin current. Moreover, as many of the conventionally known spintronics phenomena became well organized based on spin current, it has rapidly been recognized as an essential concept in a wide range of condensed matter physics. In this article, we focus on recent developments in the physics of spin, spin current, and their related phenomena, where the conversion between spin angular momentum and different forms of angular momentum plays an essential role. Starting with an introduction to spin current, we first discuss the recent progress in spintronic phenomena driven by spin-exchange coupling: spin pumping, topological Hall torque, and emergent inductor. We, then, extend our discussion to the interaction/interconversion of spins with heat, lattice vibrations, and charge current and address recent progress and perspectives on the spin Seebeck and Peltier effects. Next, we review the interaction between mechanical motion and electron/nuclear spins and argue the difference between the Barnett field and rotational Doppler effect. We show that the Barnett effect reveals the angular momentum compensation temperature, at which the net angular momentum is quenched in ferrimagnets.

Oxygen nonstoichiometry effects in spin Seebeck insulating Y3−xPrxFe5O12+ δ materials

Yttrium iron garnet (Y3Fe5O12) and its derivatives are ferrimagnetic spin Seebeck insulating materials crucial for the spin transport based phenomena such as the spin Seebeck effect (SSE) and spin Hall magnetoresistance. Structure–property correlation studies of such materials under different conditions are useful for optimizing the relevant constraint in the existed phenomena. The usage of Y3Fe5O12 type materials over the broad range of temperature conditions (27–450 °C) in SSE is under study. We report here the structure–property correlation in spin Seebeck insulating Y3−xPrxFe5O12+δ oxides as a representative material and introduce the additional degrees of freedom in the crystal system relevant to the spin transport based phenomena under high temperature conditions. The natural tendency of having oxygen nonstoichiometry in an iron garnet family of materials strengthens the Fe–O–Fe superexchange interaction, which, in turn, tends to increase the spin voltage correlated magnetic parameters. The analysis of experimental high temperature neutron diffraction data (over 27–450 °C) reveals the oxide ion nonstoichiometry and excess oxide ion transport pathways at moderate temperature 150 °C in the crystal lattices of studied garnet materials. Oxide ion nonstoichiometry, ionic transport, and electron hopping in crystal lattices cause a tremendous variation of electrical conductivity (10−11–10−2 S cm−1) over a moderate change of temperature (27–450 °C). The occurrence of electrical transport in the required thermal gradient over the garnet material in SSE can evoke the additional degrees of freedom in the usage of such materials at high temperatures. The present work provides a new outlook in terms of structure–property correlation for spin transport based materials.

Spin-thermoelectric effects in a quantum dot hybrid system with magnetic insulator

We investigate spin thermoelectric properties of a hybrid system consisting of a single-level quantum dot attached to magnetic insulator and metal electrodes. Magnetic insulator is assumed to be of ferromagnetic type and is a source of magnons, whereas metallic lead is reservoir of electrons. The temperature gradient set between the magnetic insulator and metallic electrodes induces the spin current flowing through the system. The generated spin current of magnonic (electric) type is converted to electric (magnonic) spin current by means of quantum dot. Expanding spin and heat currents flowing through the system, up to linear order, we introduce basic spin thermoelectric coefficients including spin conductance, spin Seebeck and spin Peltier coefficients and heat conductance. We analyse the spin thermoelectric properties of the system in two cases: in the large ondot Coulomb repulsion limit and when these interactions are finite.

Promising spin caloritronics and spin diode effects based on 1T-FeCl2 nanotube devices

The spin filtering effect, negative differential resistance, spin Seebeck effect and spin diode effect are found in homogeneous and heterogeneous 1T-FeCl2 nanotubes, which suggest their potential applications in spintronic devices.

Spintronic Thermal Management

In spin caloritronics, a branch of spintronics, the spin degree of freedom is exploited for thermoelectric conversion and thermal transport. Since the inception of spin caloritronics, many experimental and theoretical studies have focused on clarifying the fundamental physics of the heat-to-spin and heat-to-charge current conversion phenomena in magnetic materials and magnetic hybrid structures, such as the spin Seebeck and anomalous Nernst effects. While research on these phenomena is progressing, there are also many spin-caloritronic phenomena that output heat currents. The observations of such phenomena have recently been accomplished through cutting-edge heat detection techniques. The recent developments in spin caloritronics have revealed that the generation, conversion, and transport of heat can be actively controlled by spins and/or magnetism. In this review article, we propose a new concept called spintronic thermal management. With proof-of-concept demonstrations, we introduce the basic principles, behaviors, measurement methods, and heat control functionalities of spin-caloritronic phenomena and discuss potential applications of spintronic thermal management.

Spin current at a magnetic junction as a probe of the Kondo state

We investigate the spin Seebeck effect and spin pumping in a junction between a ferromagnetic insulator and a magnetic impurity deposited on a normal metal. By the numerical renormalization group calculation, we show that spin current is enhanced by the Kondo effect. This spin current is suppressed by increase of the temperature or the magnetic field which is comparable with the Kondo temperature. Our results indicate that spin transport can be a direct probe of spin excitation in strongly correlated systems.

Progress of microscopic thermoelectric effects studied by micro- and nano-thermometric techniques

Heat dissipation is one of the most serious problems in modern integrated electronics with the continuously decreasing devices size. Large portion of the consumed power is inevitably dissipated in the form of waste heat which not only restricts the device energy-efficiency performance itself, but also leads to severe environment problems and energy crisis. Thermoelectric Seebeck effect is a green energy-recycling method, while thermoelectric Peltier effect can be employed for heat management by actively cooling overheated devices, where passive cooling by heat conduction is not sufficiently enough. However, the technological applications of thermoelectricity are limited so far by their very low conversion efficiencies and lack of deep understanding of thermoelectricity in microscopic levels. Probing and managing the thermoelectricity is therefore fundamentally important particularly in nanoscale. In this short review, we will first briefly introduce the microscopic techniques for studying nanoscale thermoelectricity, focusing mainly on scanning thermal microscopy (SThM). SThM is a powerful tool for mapping the lattice heat with nanometer spatial resolution and hence detecting the nanoscale thermal transport and dissipation processes. Then we will review recent experiments utilizing these techniques to investigate thermoelectricity in various nanomaterial systems including both (two-material) heterojunctions and (single-material) homojunctions with tailored Seebeck coefficients, and also spin Seebeck and Peltier effects in magnetic materials. Next, we will provide a perspective on the promising applications of our recently developed Scanning Noise Microscope (SNoiM) for directly probing the non-equilibrium transporting hot charges (instead of lattice heat) in thermoelectric devices. SNoiM together with SThM are expected to be able to provide more complete and comprehensive understanding to the microscopic mechanisms in thermoelectrics. Finally, we make a conclusion and outlook on the future development of microscopic studies in thermoelectrics.

Spin thermoelectric transport of Co-salophene with borophene nanoribbon electrodes

As a new member of the two-dimensional material family, borophene exhibits extremely low thermal conductivity which is beneficial to the efficiency of the Seebeck effect in thermoelectric devices. We have investigated the spin-dependent thermoelectric transport in a Co-salophene molecule sandwiched between two semi-infinite borophene nanoribbon electrodes based on the density functional theory and nonequilibrium Green's function method. The electric currents show excellent spin-filter efficiency when the applied bias exceeds 0.3 V. When a temperature gradient is applied between two leads, the spin-up and spin-down thermoelectric currents flow in opposite directions and both of them increase linearly with the increasing temperature gradient. Moreover, large Seebeck polarization and spin figure of merit can be obtained in a wide range of reference temperatures, which suggests the potential application of magnetic molecular devices with borophene electrodes in spin caloritronics.

Magnetic Coupling in Y3Fe5O12/Gd3Fe5O12 Heterostructures

Ferrimagnetic Y$_3$Fe$_5$O$_{12}$ (YIG) is the prototypical material for studying magnonic properties due to its exceptionally low damping. By substituting the yttrium with other rare earth elements that have a net magnetic moment, we can introduce an additional spin degree of freedom. Here, we study the magnetic coupling in epitaxial Y$_3$Fe$_5$O$_{12}$/Gd$_3$Fe$_5$O$_{12}$ (YIG/GIG) heterostructures grown by pulsed laser deposition. From bulk sensitive magnetometry and surface sensitive spin Seebeck effect (SSE) and spin Hall magnetoresistance (SMR) measurements, we determine the alignment of the heterostructure magnetization through temperature and external magnetic field. The ferromagnetic coupling between the Fe sublattices of YIG and GIG dominates the overall behavior of the heterostructures. Due to the temperature dependent gadolinium moment, a magnetic compensation point of the total bilayer system can be identified. This compensation point shifts to lower temperatures with increasing thickness of YIG due the parallel alignment of the iron moments. We show that we can control the magnetic properties of the heterostructures by tuning the thickness of the individual layers, opening up a large playground for magnonic devices based on coupled magnetic insulators. These devices could potentially control the magnon transport analogously to electron transport in giant magnetoresistive devices.

The Strain-Tuned Spin Seebeck Effect, Spin Polarization, and Giant Magnetoresistance of a Graphene Nanobubble in Zigzag Graphene Nanoribbons.

By using first-principle calculations combined with the non-equilibrium Green’s function approach, we studied the spin caloritronic properties of zigzag graphene nanoribbons with a nanobubble at the edge (NB-ZGNRs). The thermal spin-polarized currents can be induced by a temperature difference, and the spin Seebeck effect is found in the nanoribbon. The spin polarization, magnetoresistance, and Seebeck coefficients are discussed, which are strongly affected and can be tuned by the geometrical strain. Moreover, some novel spin caloritronic devices are designed, such as a device that generates bidirectional perfect spin currents and thermally induced giant magnetoresistances. Our results open up the possibility of tuning the spin caloritronic properties of the NB-ZGNR-based devices by changing the elastic strain on the graphene nanobubble.

G–C4N3–graphene-g-C4N3: A useful spin thermoelectric material

Present investigation establishes simple 2D tunnelling nanostructure created by an assembly of ferromagnetic g-C4N3 electrodes separated by insulating graphene sheet; as a potential spincaloritronic device. The system shows large departure in up and down spin component of Seebeck coefficient and spin thermoelectric figure of merit is 103 times larger than charge part. Emergence of spin polarized Seebeck coefficient may be attributed to intrinsic ferromagnetism of g-C4N3 and spin-injection from ferromagnetic electrode to insulating graphene. This widely varying spin polarized Seebeck coefficient is attributed to large negative slope of logarithmic transmission coefficient of spin up channel compared to spin down counterpart.

Large spin to charge conversion in antiferromagnetic Weyl semimetal Mn3Sn

The Weyl antiferromagnet Mn3Sn has recently attracted significant attention as it is not only a novel magnetic quantum material of fundamental interest, but it also opens opportunities to investigate a number of exotic spin-dependent transports for practical antiferromagnetic devices. Here, we report the large spin to charge conversion observed in YIG/Mn3Sn. Evidenced by both spin Seebeck and spin pumping measurements, the spin to charge conversion efficiency of Mn3Sn is found ∼2.5 times of that for the conventional heavy metal Ta. Our results suggest a promising potential for employing a topological non-trivial antiferromagnet to achieve more efficient spin to charge conversion than conventional metallic materials.

Nanophotonic structures with optical surface modes for tunable spin current generation.

We propose a novel type of photonic-crystal (PC)-based nanostructures for efficient and tunable optically-induced spin current generation via the spin Seebeck and inverse spin Hall effects. It has been experimentally demonstrated that optical surface modes localized at the PC surface covered by ferromagnetic layer and materials with giant spin-orbit coupling (SOC) notably increase the efficiency of the optically-induced spin current generation, and provides its tunability by modifying the light wavelength or angle of incidence. Up to 100% of the incident light power can be transferred to heat within the SOC layer and, therefore, to the spin current. Importantly, the high efficiency becomes accessible even for ultra-thin SOC layers. Moreover, the surface patterning of the PC-based spintronic nanostructure allows for the local generation of spin currents at the pattern scales rather than the diameter of the laser beam.

Graphene-based nano-devices: high spin Seebeck and pure spin photogalvanic effects.

We investigate the magnetic, thermoelectric transport, and photogalvanic effect (PGE) properties of two nano-devices based on sawtooth edged graphene nanoribbons (SGNRs). It is found that a robust spin-semiconducting property exists in SGNRs. When SGNRs are arranged in a configuration, a large spin Seebeck coefficient is obtained, indicating a high Seebeck effect under a temperature difference. In addition, we also propose a new spatial inversion symmetry nano-device, which is constructed by two head to head semi-infinite SGNRs in a configuration. The results show that spin-up and spin-down currents are generated by the PGE with opposite flowing directions and the same magnitude. As a result, only a finite pure spin current arises without an accompanying charge current. More importantly, the pure spin current is robustly induced by photons and is independent of the photon energy, polarization angle and the model of polarization (linear or elliptical polarization), which is attributed to the symmetry of the spatial inversion and anti-symmetry of the spin density inversion. The results presented here provide a useful insight into the real application of both spin caloritronics and photoelectric carbon-based nano-devices.

Simultaneously Enhanced Spin Hall Effect and Spin-Mixing Conductance in a Y3Fe5O12 /bcc- W1−xCrx Heterostructure by Bulk Extrinsic Scattering and Interfacial Electric Field

The spin-current efficiency in ferromagnet (FM)/nonmagnetic metal (NM) heterostructures is a key issue for many subcategories of spintronics, and it is determined by both the spin Hall angle $({\ensuremath{\theta}}_{\mathrm{SH}})$ of the NM and the spin-mixing conductance $({g}_{\ensuremath{\uparrow}\ensuremath{\downarrow}})$ related to the constituents and, in particular, to the interface. Here, we study spin transport in the ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}(\mathrm{YIG})$/${\mathrm{W}}_{1\text{\ensuremath{-}}x}{\mathrm{Cr}}_{x}$ heterostructure over the full composition by means of the spin Seebeck effect and spin pumping. An appreciably enhanced inverse spin Hall voltage is observed for the substitutional alloy of bcc-$\mathrm{W}$ and $\mathrm{Cr}$, with the maximum obtained at an equiatomic composition. We find that the ${\ensuremath{\theta}}_{\mathrm{SH}}$ of bcc-${\mathrm{W}}_{0.5}{\mathrm{Cr}}_{0.5}$ is 1.3 times as large as that of \ensuremath{\beta}-$\mathrm{W}$, which mainly originates from intensively disordered atomic scattering. More significantly, $\mathrm{YIG}$/bcc-${\mathrm{W}}_{0.5}{\mathrm{Cr}}_{0.5}$ has a ${g}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}$ value of $1.42\phantom{\rule{0.25em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.25em}{0ex}}{10}^{18}\phantom{\rule{0.25em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$, which is more than twice that for $\mathrm{YIG}$/\ensuremath{\beta}-$\mathrm{W}$ ($5.98\phantom{\rule{0.25em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.25em}{0ex}}{10}^{17}\phantom{\rule{0.25em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$). Meanwhile, a sizable interfacial electric field is identified in $\mathrm{YIG}$/\ensuremath{\beta}-$\mathrm{W}$, but not in $\mathrm{YIG}$/bcc-${\mathrm{W}}_{0.5}{\mathrm{Cr}}_{0.5}$, showing strong correlation between ${g}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}$ and the interfacial electric field. This work not only provides a readily achievable alloy with ${\ensuremath{\theta}}_{\mathrm{SH}}$ exceeding that of the metastable \ensuremath{\beta}-$\mathrm{W}$, but also implies the possibility of manipulating ${g}_{\ensuremath{\uparrow}\ensuremath{\downarrow}}$ via Rashba-type spin-orbit coupling by engineering an interfacial electric field in insulating FM/NM heterostructures.

Negligible thermal contributions to the spin pumping signal in ferromagnetic metal–platinum bilayers

Spin pumping by ferromagnetic resonance is one of the most common techniques to determine spin Hall angles, Edelstein lengths, or spin diffusion lengths of a large variety of materials. In recent years, there have been increasing concerns over the interpretation of these experiments, underlining that the signal could arise purely from thermoelectric effects rather than coherent spin pumping. Here, we propose a method to evaluate the presence or absence of thermal effects in spin pumping signals, by combining bolometry and spin pumping by ferromagnetic resonance measurements and comparing their timescale. Using a cavity to perform the experiments on Pt\permalloy and La0.7Sr0.3MnO3\Pt samples, we conclude on the absence at resonance of any measurable thermoelectric contribution such as the spin Seebeck and anomalous Nernst effects.

Thermoelectric microscopy of magnetic skyrmions

The magnetic skyrmion is a nanoscale topological object characterized by the winding of magnetic moments, appearing in magnetic materials with broken inversion symmetry. Because of its low current threshold for driving the skyrmion motion, they have been intensely studied toward novel storage applications by using electron-beam, X-ray, and visible light microscopies. Here, we demonstrate another imaging method for skyrmions by using spin-caloritronic phenomena, that is, the spin Seebeck and anomalous Nernst effects, as a probe of magnetic texture. We scanned a focused heating spot on a Hall-cross shaped MgO/CoFeB/Ta/W multilayer film and mapped the magnitude as well as the direction of the resultant thermoelectric current due to the spin-caloritronic phenomena. Our experimental and calculation reveal that the characteristic patterns in the thermoelectric signal distribution reflect the skyrmions’ magnetic texture. The thermoelectric microscopy will be a complementary and useful imaging technique for the development of skyrmion devices owing to the unique symmetry of the spin-caloritronic phenomena.

First principle study of temperature-dependent spin transport in VSe2 monolayer

An applicable use of a linear combination of atomic orbitals (LCAO) based density functional theory (DFT) together with non-equilibrium Green’s function (NEGF) is performed for investigating the dependence of temperature on spin-resolved transport characteristics of electron in magnetic Vanadium Diselenide (VSe2) monolayer. To know the dependence of heat (temperature) on spin Seebeck effect and spin filtration, important factors like transmission spectrum and spin-resolved current are calculated. Approximately 100% spin injection efficiency is attained in the low-temperature gradient. Low temperature (TL) of cold (left) electrode with respect to high temperature (TR) of hot (right) electrode leads to a smaller spin injection efficiency. Giant thermal magnetoresistance around 3.936 × 103% is achieved and this giant thermal magnetoresistance is directly proportional to the difference between the temperature of right and left electrode (ΔT). This resultant thermal magnetoresistance is also dependent on TL. A large value of temperature gradient based magnetoresistance (MR) and magnificent temperature gradient based spin filtration have been achieved for VSe2 monolayer advocates the useful application of this compound in the area of spin caloritronic.