Temperature dependence of the interface spin Seebeck effect
F. N. Kholid, D. Hamara, M. Terschanski, F. Mertens, D. Bossini, M. Cinchetti, L. McKenzie-Sell, J. Patchett, D. Petit, R. Cowburn, J. Robinson, J. Barker, C. Ciccarelli
We performed temperature-dependent optical pump - THz emission measurements in Y3Fe5O12 (YIG)|Pt from 5 K to room temperature in the presence of an externally applied magnetic field. We study the temperature dependence of the spin Seebeck effect and observe a continuous increase as temperature is decreased, opposite to what is observed in electrical measurements where the spin Seebeck effect is suppressed as 0K is approached. By quantitatively analysing the different contributions we isolate the temperature dependence of the spin-mixing conductance and observe features that are correlated to the bands of magnon spectrum in YIG.
The longitudinal spin-Seebeck effect (LSSE) has been investigated for Pt/yttrium iron garnet (YIG) bilayer systems. The magnitude of the voltage induced by the LSSE is found to be sensitive to the Pt/YIG interface condition. We observed large LSSE voltage in a Pt/YIG system with a better crystalline interface, while the voltage decays steeply when an amorphous layer is introduced at the interface artificially.
The temperature dependence of the Seebeck coefficient of homoepitaxial metal organic vapor phase (MOVPE) grown, silicon doped $β$-Ga$_2$O$_3$ thin films was measured relative to aluminum. For room temperature we found the relative Seebeck coefficient of $S_{β\text{-Ga}_2\text{O}_3\text{-Al}}=(-300\pm20)\;μ$V/K. At high bath temperatures $T>240$ K, the scattering is determined by electron-phonon-interaction. At lower bath temperatures between $T=100$ K and $T=300$ K, an increase in the magnitude of the Seebeck coefficient is explained in the frame of Strattons formula. The influence of the diff…
We study the nonequilibrium Seebeck spin transport across metal-magnetic insulator interfaces. The conjugate-converted thermal-spin transport is assisted by the exchange interaction at the interface, between conduction electrons in the metal lead and localized spins in the insulating magnet lead. We predict the rectification and negative differential spin Seebeck effect and resolve their microscopic mechanism, as a consequence of the strongly-fluctuated electronic density of states in the metal lead. The rectification of spin Peltier effect is also discussed. The phenomena predicted here are r…
We study the Seebeck effect in the $τ$-type organic conductors, $τ$-(EDO-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$ and $τ$-(P-$S$,$S$-DMEDT-TTF)$_{2}$(AuBr$_{2}$)$_{1+y}$, where EDO-$S$,$S$-DMEDT-TTF and P-$S$,$S$-DMEDT-TTF are abbreviated as OOSS and NNSS, respectively, both experimentally and theoretically. Theoretically in particular, we perform first-principles band calculation for the two materials and construct a two-orbital model, on the basis of which we calculate the Seebeck coefficient. We show that the calculated temperature dependence of the Seebeck coefficient $S$ is semi-quant…
We investigate the thermoelectric properties of Fe/MgO/Fe(001) magnetic tunnel junctions (MTJs) by means of the linear-response theory combined with a first-principles-based Landauer-Büttiker approach. We find that the Seebeck coefficient of Fe/MgO/Fe(001) MTJs strongly depends on the barrier thickness and the tetragonal distortion. A compressive tetragonal distortion of the in-plane lattice parameter in the MTJs provides interface resonant states just above the Fermi energy. This causes resonant tunneling in the MTJs and significantly enhances the Seebeck coefficient when the thickness of the…
We report on the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in multiferroic CoCr2O4 (CCO) spinel thin films with Pt contacts. We observe a large enhancement of both signals below the spin-spiral (Ts = 28 K) and the spin lock-in transitions (T_{lock_in} = 14 K). The SMR and SSE response in the spin lock-in phase are one order of magnitude larger than those observed at the ferrimagnetic transition temperature (Tc = 94 K), which indicates that the interaction between spins at the Pt|CCO interface is more efficient in the non-collinear magnetic state below Ts and T_{lock-in}. …
We have investigated the influence of the interface quality on the spin Seebeck effect (SSE) of the bilayer system yttrium iron garnet (YIG) - platinum (Pt). The magnitude and shape of the SSE is strongly influenced by mechanical treatment of the YIG single crystal surface. We observe that the saturation magnetic field H_{sat} for the SSE signal increases from 55.3 mT to 72.8 mT with mechanical treatment. The change in the magnitude of H_{sat} can be attributed to the presence of a perpendicular magnetic anisotropy due to the treatment induced surface strain or shape anisotropy in the Pt/YIG s…
The observation of the spin Seebeck effect in insulators has meant a breakthrough for spin caloritronics due to the unique ability to generate pure spin currents by thermal excitations in insulating systems without moving charge carriers. Since the recent first observation, the underlying mechanism and the origin of the observed signals have been discussed highly controversially. Here we present a characteristic dependence of the longitudinal spin Seebeck effect amplitude on the thickness of the insulating ferromagnet (YIG). Our measurements show that the observed behavior cannot be explained …
The spin-Seebeck effect refers to voltage signals induced in metals by thermally driven spin currents in adjacent magnetic systems. We present a theory of the spin-Seebeck signal in the case where the conductor that supports the voltage signal is the topologically protected two-dimensional surface-state system at the interface between a ferromagnetic insulator (FI) and a topological insulator (TI). Our theory uses a Dirac model for the TI surface-states and assumes Heisenberg exchange coupling between the TI quasiparticles and the FI magnetization. The spin-Seebeck voltage is induced by the TI…
The acute magnetic proximity effects in Pt/YIG compromise the suitability of Pt as a spin current detector. We show that Au/YIG, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in Pt/YIG. The experiment results are consistent with the spin-polarized density-functional calculations for Pt with a sizable and Au with a negligible magnetic moment near the interface with YIG.
The temperature dependence of the charge carrier density, mobility and Seebeck coefficient of melt-grown, bulk ZnGa2O4 single crystals was measured between 10 K and 310 K. The electrical conductivity at room temperature is about s = 286 S/cm due to a high electron concentration of n = 3.26*10^(19) cm^(-3), caused by unintenional doping. The mobility at room temperature is mu = 55 cm^2/Vs, whereas the scattering on ionized impurities limits the mobility to mu =62 cm^2/Vs for temperatures lower than 180 K. The Seebeck coefficient relative to aluminum at room temperature is S_(ZnGa2O4-Al) = (-125…
Based on the relaxation time approximation, the mean collision time for magnon scattering $τ_c(T)$ is computed from the experimental spin Seebeck coefficient of a bulk YIG / Pt bilayer. The scattering results to be composed by two processes: the low temperature one, with a $T^{-1/2}$ dependence, is attributed to the scattering by defects and provides a mean free path around 10 $μ$m; the high temperature one, depending on $T^{-4}$, is associated to the scattering by other magnons. The results are employed to predict the thickness dependence of the spin Seebeck coefficient for thin films.
NaTaO3-50wt% Fe2O3 composite ceramics showed a large Seebeck voltage of -300 mV at a temperature gradient of 650 K yielding a constant Seebeck coefficient of more than -500 microV/K over a wide temperature range. We report for the first time that SPS sintering at low temperature 870K could maintain the short-circuit current of -80 microA, which makes this thermoelectric material a possible candidate for high-temperature applications up to 1623 K. The reason for the good performance is the interface between Fe2O3 and surrounding NaTaO3 perovskite. When spark-plasma sintering (SPS) is used, cons…
The longitudinal spin Seebeck effect refers to the generation of a spin current when heat flows across a normal metal/magnetic insulator interface. Until recently, most explanations of the spin Seebeck effect use the interfacial temperature difference as the conversion mechanism between heat and spin fluxes. However, recent theoretical and experimental works claim that a magnon spin current is generated in the bulk of a magnetic insulator even in the absence of an interface. This is the so-called intrinsic spin Seebeck effect. Here, by utilizing a non-local spin Seebeck geometry, we provide ad…
We investigate the generation of magnonic thermal spin currents and their mode selective spin transport across interfaces in insulating, compensated ferrimagnet/normal metal bilayer systems. The spin Seebeck effect signal exhibits a non-monotonic temperature dependence with two sign changes of the detected voltage signals. Using different ferrimagnetic garnets, we demonstrate the universality of the observed complex temperature dependence of the spin Seebeck effect. To understand its origin, we systematically vary the interface between the ferrimagnetic garnet and the metallic layer, and by us…
We study the temperature dependence of the longitudinal spin-Seebeck effect (LSSE) in a yttrium iron garnet Y3Fe5O12 (YIG) / Pt system for samples of different thicknesses. In this system, the thermal spin torque is magnon-driven. The LSSE signal peaks at a specific temperature that depends on the YIG sample thickness. We also observe freeze-out of the LSSE signal at high magnetic fields, which we attribute to the opening of an energy gap in the magnon dispersion. We observe partial freeze-out of the LSSE signal even at room temperature, where kBT is much larger than the gap. This suggests tha…
Here we report on measurements of the spin-Seebeck effect of GaMnAs over an extended temperature range alongside the thermal conductivity, specific heat, magnetization, and thermoelectric power. The amplitude of the spin-Seebeck effect in GaMnAs scales with the thermal conductivity of the GaAs substrate and the phonon-drag contribution to the thermoelectric power of the GaMnAs, demonstrating that phonons drive the spin redistribution. A phenomenological model involving phonon-magnon drag explains the spatial and temperature dependence of the measured spin distribution.
We present temporal evolution of the spin Seebeck effect in a YIG|Pt bilayer system. Our findings reveal that this effect is a sub-microseconds fast phenomenon governed by the temperature gradient and the thermal magnons diffusion in the magnetic materials. A comparison of experimental results with the thermal-driven magnon-diffusion model shows that the temporal behavior of this effect depends on the time development of the temperature gradient in the vicinity of the YIG|Pt interface. The effective thermal-magnon diffusion length for YIG|Pt systems is estimated to be around 700nm.
We perform a quantitative, comparative study of the spin pumping, spin Seebeck and spin Hall magnetoresistance effects, all detected via the inverse spin Hall effect in a series of over 20 yttrium iron garnet/Pt samples. Our experimental results fully support present, exclusively spin current-based, theoretical models using a single set of plausible parameters for spin mixing conductance, spin Hall angle and spin diffusion length. Our findings establish the purely spintronic nature of the aforementioned effects and provide a quantitative description in particular of the spin Seebeck effect.
The spin Seebeck effect is a spin-motive force generated by a temperature gradient in a ferromagnet that can be detected via normal metal contacts through the inverse spin Hall effect [K. Uchida {\it et al.}, Nature {\bf 455}, 778-781 (2008)]. We explain this effect by spin pumping at the contact that is proportional to the spin-mixing conductance of the interface, the inverse of a temperature-dependent magnetic coherence volume, and the difference between the magnon temperature in the ferromagnet and the electron temperature in the normal metal [D. J. Sanders and D. Walton, Phys. Rev. B {\bf …
We report on temperature dependent measurements of the Longitudinal Spin Seebeck Effect (LSSE) in the mixed valent manganite La$_{0.7}$Ca$_{0.3}$MnO$_3$. By disentangling the contribution arising due to the Anisotropic Nernst effect, we observe that these two thermally driven phenomena vary disparately with temperature. In a narrow low temperature regime, the LSSE exhibits a $T^{0.55}$ dependence, which matches well with that predicted by the magnon-driven spin current model. Across the double exchange driven paramagnetic-ferromagnetic transition, the LSSE exponent is significantly higher than…
We study the Seebeck effect in the three-dimensional Dirac electron system based on the linear response theory with Luttinger's gravitational potential. The Seebeck coefficient $S$ is defined by $S = L_{12} / L_{11} T$, where $T$ is the temperature, and $L_{11}$ and $L_{12}$ are the longitudinal response coefficients of the charge current to the electric field and to the temperature gradient, respectively; $L_{11}$ is the electric conductivity and $L_{12}$ is the thermo-electric conductivity. We consider randomly-distributed impurity potentials as the source of the momentum relaxation of elect…
We have studied the longitudinal spin Seebeck effect in a polar antiferromagnet $α$-Cu$_{2}$V$_{2}$O$_{7}$ in contact with a Pt film. Below the antiferromagnetic transition temperature of $α$-Cu$_{2}$V$_{2}$O$_{7}$, spin Seebeck voltages whose magnetic field dependence is similar to that reported in antiferromagnetic MnF$_{2}$$\mid$Pt bilayers are observed. Though a small weak-ferromagnetic moment appears owing to the Dzyaloshinskii-Moriya interaction in $α$-Cu$_{2}$V$_{2}$O$_{7}$, the magnetic field dependence of spin Seebeck voltages is found to be irrelevant to the weak ferromagnetic moment…
The spin Seebeck effect (SSE) signal of magnon polarons in bulk-Y3Fe5O12 (YIG)/Pt heterostructures is found to drastically change as a function of temperature. It appears as a dip in the total SSE signal at low temperatures, but as the temperature increases, the dip gradually decreases before turning to a peak. We attribute the observed dip-to-peak transition to the rapid rise of the four-magnon scattering rate. Our analysis provides important insights into the microscopic origin of the hybridized excitations and the overall temperature dependence of the SSE anomalies.
Efficient spin injection from epitaxial ferrimagnetic NiFe$_2$O$_4$ thin films into a Pd layer is demonstrated via spin Seebeck effect measurements in the longitudinal geometry. The NiFe$_2$O$_4$ films (60 nm to 1 $μ$m) are grown by pulsed laser deposition on isostructural spinel MgAl$_2$O$_4$, MgGa$_2$O$_4$, and CoGa$_2$O$_4$ substrates with lattice mismatch varying between 3.2% and 0.2%. For the thinner films ($\leq$ 330 nm), an increase in the spin Seebeck voltage is observed with decreasing lattice mismatch, which correlates well with a decrease in the Gilbert damping parameter as determin…
This article reports temperature-dependent measurements of longitudinal spin Seebeck effects (LSSEs) in Pt/Y$_3$Fe$_5$O$_{12}$ (YIG)/Pt systems in a high temperature range from room temperature to above the Curie temperature of YIG. The experimental results show that the magnitude of the LSSE voltage in the Pt/YIG/Pt systems rapidly decreases with increasing the temperature and disappears above the Curie temperature. The critical exponent of the LSSE voltage in the Pt/YIG/Pt systems at the Curie temperature was estimated to be 3, which is much greater than that for the magnetization curve of Y…
We report the experimental observation of the spin Seebeck effect (SSE) in Ag/CoFe noble metal/magnetic metal bilayers with a longitudinal structure. Thermal voltages jointly generated by the anomalous Nernst effect (ANE) and the SSE were detected across the Ag/CoFe/Cu strip with a perpendicular thermal gradient. To effectively separate the SSE and the ANE part of the thermal voltages, we compared the experimental results between the Ag/CoFe/Cu strip and Cu/CoFe/Cu strip, where two samples processed with the heating power instead of the temperature difference through the thin CoFe film. The re…
The atomic variations of electronic wavefunctions at the surface and electron scattering near a defect have been detected unprecedentedly by tracing thermoelectric voltages given a temperature bias [Cho et al., Nature Mater. 12, 913 (2013)]. Because thermoelectricity, or Seebeck effect, is associated with heat-induced electron diffusion, how the thermoelectric signal is related to the atomic-scale wavefunctions and what the role of the temperature is at such a length scale remain very unclear. Here we show that coherent electron and heat transport through a point-like contact produces an atomi…
The temperature dependent longitudinal spin Seebeck effect (SSE) in heavy metal (HM)/ Y3Fe5O12 (YIG) bilayers is investigated as a function of different magnetic field strength, different HM detection material, and YIG thickness ranging from nm to mm. A large enhancement of the SSE signal is observed at low temperatures leading to a peak of the signal amplitude. We demonstrate that this enhancement shows a clear dependence on the film thickness, being more pronounced for thicker films and vanishing for films thinner than 600 nm. The peak temperature depends on the applied magnetic field streng…
Thermoelectric generation is an essential function of future energy-saving technologies. However, this generation has been an exclusive feature of electric conductors, a situation which inflicts a heavy toll on its application; a conduction electron often becomes a nuisance in thermal design of devices. Here we report electric-voltage generation from heat flowing in an insulator. We reveal that, despite the absence of conduction electrons, a magnetic insulator LaY2Fe5O12 converts a heat flow into spin voltage. Attached Pt films transform this spin voltage into electric voltage by the inverse s…
The electronic transport and spincaloric properties of epitaxial magnetic tunnel junctions with half-metallic Co$_2$MnSi Heusler electrodes, MgO tunneling barriers, and different interface terminations are investigated by using first-principles calculations. A new approach to spincaloric properties is presented that circumvents the linear response approximation inherent in the Seebeck coefficient and compared to the method of Sivan and Imry. This approach supports two different temperatures in the two electrodes and provides the exact current and/or voltage response of the system. Moreover, it…
Recently, Seebeck coefficients of ferromagnetic conductors are found to be spin-dependent. However straightforward method of accurately determining its spin polarization is still to be developed. Here, we have derived a linear dependence of anomalous Nernst coefficient on anomalous Hall angle with scaling factor related to spin polarization of Seebeck coefficient, which has been experimentally verified in [Co/Pt]n superlattices. Based on the dependence, we have also evaluated spin polarization of Seebeck coefficient of some ferromagnetic conductors. Besides, we have also found a new mechanism …
We report the nonlocal spin Seebeck effect (nlSSE) in a lateral configuration of Pt/Y$_3$Fe$_5$O$_{12}$(YIG)/Pt systems as a function of the magnetic field $B$ (up to 10 T) at various temperatures $T$ (3 K < $T$ < 300 K). The nlSSE voltage decreases with increasing $B$ in a linear regime with respect to the input power (the applied charge-current squared $I^2$). The reduction of the nlSSE becomes substantial when the Zeeman energy exceeds thermal energy at low temperatures, which can be interpreted as freeze-out of magnons relevant for the nlSSE. Furthermore, we found the non-linear power depe…
We investigate picosecond spin-currents across Au/iron-garnet interfaces in response to ultrafast laser heating of the electrons in the Au film. In the picoseconds after optical heating, interfacial spin currents occur due to an interfacial temperature difference between electrons in the metal and magnons in the insulator. We report measurements of this interfacial longitudinal spin Seebeck effect between Au and rare-earth iron-garnet insulators, i.e. RE$_3$ Fe$_5$O$_{12}$, where RE is Y, Eu, Tb, Tm. We use time domain thermoreflectance (TDTR) measurements to characterize the thermal response …
Spin Seebeck effect (SSE), the generation of spin current from heat, has been extensively studied in a large variety of magnetic materials, including ferromagnets, antiferromagnets, paramagnets, and quantum spin liquids. In this paper, we report the study of the SSE in the single crystalline Pb2V3O9, a spin-gapped quantum magnet candidate with quasi-one-dimensional spin-1/2 chain. Detailed temperature and magnetic field dependences of the SSE are investigated, and the temperature-dependent critical magnetic fields show a strong correlation to the Bose-Einstein condensation phase of the quantum…
Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium-iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current $j_\textrm{s}$ arises on the same ~100fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transf…
For longitudinal spin Seebeck effect (LSSE) devices, a multilayer structure comprising ferromagnetic and nonmagnetic layers is expected to improve their thermoelectric power. In this study, we developed the fabrication method for alternately stacked yttrium-iron-garnet (YIG)/Pt multilayer films on a gadolinium gallium garnet (GGG) (110) substrate, GGG/[YIG(49 nm)/Pt(4 nm)]$_n$ ($n =$ 1 - 5) based on room-temperature sputtering and $ex$-$situ$ post-annealing method and we evaluated their structural and LSSE properties. The fabricated [YIG/Pt]$_n$ samples show flat YIG/Pt interfaces and almost i…
Antiferromagnets are beneficial for future spintronic applications due to their zero magnetic moment and ultrafast dynamics. But gaining direct access to their antiferromagnetic order and identifying the properties of individual magnetic sublattices, especially in thin films and small-scale devices, remains a formidable challenge. So far, the existing read-out techniques such as anisotropic magnetoresistance, tunneling anisotropic magnetoresistance, and spin-Hall magnetoresistance, are even functions of sublattice magnetization and thus allow us to detect different orientations of the Néel ord…
We report the observation of longitudinal spin Seebeck effects (LSSE) in an all-oxide bilayer system comprising an IrO$_2$ film and an Y$_3$Fe$_5$O$_{12}$ film. Spin currents generated by a temperature gradient across the IrO$_2$/Y$_3$Fe$_5$O$_{12}$ interface were detected as electric voltage via the inverse spin Hall effect in the conductive IrO$_2$ layer. This electric voltage is proportional to the magnitude of the temperature gradient and its magnetic field dependence is well consistent with the characteristic of the LSSE. This demonstration may lead to the realization of low-cost, stable,…
We theoretically study the thermoelectric transport properties of strained HgTe in the topological insulator phase. We developed a model for the system using a Dirac Hamiltonian including the effect of strain induced by the interface between HgTe and the CdTe substrate. The conductivity tensor was explored assuming the electrons are scattered by charge impurities, while the thermopower tensor was addressed using the Mott relation. Seebeck and Nernst responses exhibit remarkable enhancements in comparison with other two-dimensional Dirac materials, such as graphene, germanane, prosphorene and s…
The low temperature monoclinic, insulating phase of vanadium dioxide is ordinarily considered nonmagnetic, with dimerized vanadium atoms forming spin singlets, though paramagnetic response is seen at low temperatures. We find a nonlocal spin Seebeck signal in VO2 films that appears below 30 K and which increases with decreasing temperature. The spin Seebeck response has a non-hysteretic dependence on in-plane external magnetic field. This paramagnetic spin Seebeck response is discussed in terms of prior findings on paramagnetic spin Seebeck effects and expected magnetic excitations of the mono…
Spin Seebeck effect (SSE) has been investigated in thin films of two Y-hexagonal ferrites Ba$_2$Zn$_{2}$Fe$_{12}$O$_{22}$ (Zn2Y) and Ba$_2$Co$_{2}$Fe$_{12}$O$_{22}$ (Co2Y) deposited by a spin-coating method on SrTiO$_3$(111) substrate. The selected hexagonal ferrites are both ferrimagnetic with similar magnetic moments at room temperature and both exhibit easy magnetization plane normal to $c$-axis. Despite that, SSE signal was only observed for Zn2Y, whereas no significant SSE signal was detected for Co2Y. We tentatively explain this different behavior by a presence of two different magnetic …
The longitudinal spin Seebeck effect (LSSE) in Pt/Y$_3$Fe$_5$O$_{12}$ (YIG) junction systems has been investigated at various magnetic fields and temperatures. We found that the LSSE voltage in a Pt/YIG-slab system is suppressed by applying high magnetic fields and this suppression is critically enhanced at low temperatures. The field-induced suppression of the LSSE in the Pt/YIG-slab system is too large at around room temperature to be explained simply by considering the effect of the Zeeman gap in magnon excitation. This result requires us to introduce magnon-frequency-dependent mechanism in…
A thorough understanding of the mean-free-path (MFP) distribution of the energy carriers is crucial to engineer and tune the transport properties of materials. In this context, a significant body of work has investigated the phonon and electron MFP distribution, however, similar studies of the magnon MFP distribution have not been carried out so far. In this work, we used thickness-dependence measurements of the longitudinal spin Seebeck (LSSE) effect of yttrium iron garnet films to reconstruct the cumulative distribution of a SSE related effective magnon MFP. By using the experimental data re…
We report on spin Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in single crystal of the rare-earth antiferromagnet DyFeO$_{3}$ with a thin Pt film contact. The angular shape and symmetry of the SMR at elevated temperatures reflect the antiferromagnetic order of the Fe$^{3+}$ moments as governed by the Zeeman energy, the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction. We interpret the observed linear dependence of the signal on the magnetic field strength as evidence for field-induced order of the Dy$^{3+}$ moments up to room temperature. At and below the …
Spin-momentum locking in protected surface states enables efficient electrical detection of magnon decay at a magnetic-insulator/topological-insulator heterojunction. Here we demonstrate this property using the spin Seebeck effect, i.e. measuring the transverse thermoelectric response to a temperature gradient across a thin film of yttrium iron garnet, an insulating ferrimagnet, and forming a heterojunction with (BixSb1-x)2Te3, a topological insulator. The non-equilibrium magnon population established at the interface can decay in part by interactions of magnons with electrons near the Fermi e…
We report temperature-dependent spin-Seebeck measurements on Pt/YIG bilayers and Pt/NiO/YIG trilayers, where YIG (Yttrium iron garnet, Y$_3$Fe$_5$O$_{12}$) is an insulating ferrimagnet and NiO is an antiferromagnet at low temperatures. The thickness of the NiO layer is varied from 0 to 10 nm. In the Pt/YIG bilayers, the temperature gradient applied to the YIG stimulates dynamic spin injection into the Pt, which generates an inverse spin Hall voltage in the Pt. The presence of a NiO layer dampens the spin injection exponentially with a decay length of $2 \pm 0.6$ nm at 180 K. The decay length i…
The switching of the magnetization configurations of giant magnetoresistance multilayer stacks not only changes the electric and thermal conductivities, but also the thermopower. We study the magnetotransport and the magneto-thermoelectric properties of Co/Cu multilayer devices in a lateral thermal gradient. We derive values of the Seebeck coefficient, the thermoelectric figure of merit, and the thermoelectric power factor. The Seebeck coefficient reaches values up to -18 microvolt/K at room temperature and shows a magnetic field dependence up to 28.6 % upon spin reversal. In combination with …
The principles of the thermoelectric phenomenon, including Seebeck effect, Peltier effect, and Thomson effect are discussed. The dependence of the thermoelectric devices on the figure of merit, Seebeck coefficient, electrical conductivity, and thermal conductivity are explained in details. The paper provides an overview of the different types of thermoelectric materials, explains the techniques used to grow thin films for these materials, and discusses future research and development trends for this technology.
We investigate the antiferromagnetic spin Seebeck effect across the spin-flop transition in a numerical simulation based on the time-dependent Ginzburg-Landau equation for a bilayer of a uniaxial insulating antiferromagnet and an adjacent metal. By directly simulating the rate of change of the conduction-electron spin density ${\bf s}$ in the adjacent metal layer, we demonstrate that a sign reversal of the antiferromagnetic spin Seebeck effect across the spin-flop transition occurs when the interfacial coupling of ${\bf s}$ to the staggered magnetization ${\bf n}$ of the antiferromagnet domina…
Investigating exotic magnetic materials with spintronic techniques is effective at advancing magnetism as well as spintronics. In this work, we report unusual field-induced suppression of the spin-Seebeck effect (SSE) in a quasi one-dimensional frustrated spin-$\frac{1}{2}$ magnet LiCuVO$_4$, known to exhibit spin-nematic correlation in a wide range of external magnetic field $B$. The suppression takes place above $|B| > 2$ T in spite of the $B$-linear isothermal magnetization curves in the same $B$ range. The result can be attributed to the growth of the spin-nematic correlation while increas…
Realization of intrinsic surface dominant transport in a wide temperature region for a topological insulators (TIs) is an important frontier research in order to promote the progresses of TIs towards the future electronics. We report here systematic measurements of longitudinal electrical transport, Shubnikov-de-Haas (SdH) quantum oscillations, Hall coefficient (R_H^2D), and Seebeck coefficient as a function of film thickness (d) and temperature using high quality Bi2-xSbxTe3-ySey (BSTS) single crystal thin films grown by physical vapor-phase deposition. The thickness dependence of sheet condu…
We investigate the spin-dependent Seebeck coefficient and the tunneling magneto thermopower of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) in the presence of thermal gradients across the MTJ. Thermal gradients are generated by an electric heater on top of the nanopillars. The thermo power voltage across the MTJ is found to scale linearly with the heating power and reveals similar field dependence as the tunnel magnetoresistance. The amplitude of the thermal gradient is derived from calibration measurements in combination with finite element simulations of the heat flux. Based on this, larg…
We studied the thermoelectric coefficients (Seebeck and thermal conductance)of FeCo|MgO|FeCo(001) magnetic tunnel junctions (MTJs) from first principles using a generalized Landauer-Büttiker formalism. FeCo|MgO|FeCo(001) MTJs usually yield smaller thermoelectric effects compared with epitaxial Fe|MgO|Fe(001) MTJs. The (magneto-) Seebeck effect is sensitive to the details of the FeCo$|$MgO interfaces. Interfacial oxygen vacancies (OVs) can enhance the thermoelectric effects in MTJs greatly. We also compute angular dependent Seebeck coefficients that provide more information about the transport …
The spin-Seebeck effect (SSE) in ferromagnetic metals and insulators has been investigated systematically by means of the inverse spin-Hall effect (ISHE) in paramagnetic metals. The SSE generates a spin voltage as a result of a temperature gradient in a ferromagnet, which injects a spin current into an attached paramagnetic metal. In the paramagnet, this spin current is converted into an electric field due to the ISHE, enabling the electric detection of the SSE. The observation of the SSE is performed in longitudinal and transverse configurations consisting of a ferromagnet/paramagnet hybrid s…
