Thermospin Effects Research Articles

Influence of epitaxial growth on spin current transmission of NiO films

Abstract This letter reports the effect of the epitaxial growth of NiO films on its spin current transmission. Two sample series of Pt/NiO/CoFeB, including epitaxial and polycrystalline NiO, were prepared using different deposition conditions. The spin current transmission of NiO is evaluated using the thermo-spin effects in the trilayers. Epitaxial NiO exhibits a non-monotonic behavior in the NiO thickness dependence of spin current transmission, while this behavior is smeared out in polycrystalline NiO. The non-monotonic behavior implies the presence of multiple spin transport mechanisms, which becomes apparent with epitaxial growth.

Deposition temperature dependence of thermo-spin and magneto-thermoelectric conversion in Co2MnGa films on Y3Fe5O12 and Gd3Ga5O12

We have characterized Co2MnGa (CMG) Heusler alloy films grown on Y3Fe5O12 (YIG) and Gd3Ga5O12 (GGG) substrates at different deposition temperatures and investigated thermo-spin and magneto-thermoelectric conversion properties by means of a lock-in thermography technique. X-ray diffraction, magnetization, and electrical transport measurements show that the deposition at high substrate temperatures induces the crystallized structures of CMG, while the resistivity of the CMG films on YIG (GGG) prepared at and above 500 °C (550 °C) becomes too high to measure the thermo-spin and magneto-thermoelectric effects due to large roughness, highlighting the difficulty of fabricating highly ordered continuous CMG films on garnet structures. Our lock-in thermography measurements show that the deposition at high substrate temperatures results in an increase in the current-induced temperature change for CMG/GGG and a decrease in that for CMG/YIG. The former indicates the enhancement of the anomalous Ettingshausen effect in CMG through crystallization. The latter can be explained by the superposition of the anomalous Ettingshausen effect and the spin Peltier effect induced by the positive (negative) charge-to-spin conversion for the amorphous (crystallized) CMG films. These results provide a hint to construct spin-caloritronic devices based on Heusler alloys.

Spontaneous symmetry breaking induced thermospin effect in superconducting tunnel junctions

We discuss the charge and the spin tunneling currents between two Bardeen-Cooper-Schrieffer (BCS) superconductors, where one density of states is spin-split. In the presence of a large temperature bias across the junction, we predict the generation of a spin-polarized thermoelectric current. This thermo-spin effect is the result of a spontaneous particle-hole symmetry breaking in the absence of a polarizing tunnel barrier. The two spin components, which move in opposite directions, generate a spin current larger than the purely polarized case when the thermo-active component dominates over the dissipative one.

Enhancement of charge-to-spin current conversion in a Ni/Pt bilayer film detected by spin Peltier effect

We demonstrate the enhancement of charge-to-spin current conversion by a ferromagnet/paramagnet interface. The enhancement is confirmed by comparing the spin Peltier effect (SPE) between Ni/Pt/yttrium-iron-garnet (YIG) and Pt/YIG hybrid structures, where the SPE signal is proportional to the spin current generated in the Ni/Pt or Pt layer. The SPE signal normalized by charge current density for the Ni/Pt/YIG system increases by 65% compared to the Pt/YIG system, which cannot be explained by other thermoelectric and thermo-spin effects. This result suggests the presence of additive charge-to-spin current conversion at the Ni/Pt interface, providing simple ways to improve spin-injection efficiency.

Carbon atomic chains in a spin thermoelectric device

We investigated spin-dependent thermoelectric properties of a carbon atomic chain (CAC) between two zigzag-edged triangular graphene nanoflakes (GNFs), with two semi-infinite armchair graphene nanoribbons (AGNRs) as the electrodes. Using the non-equilibrium Green’s function method combined with the effective self-consistent mean-field Hubbard approximation, we found that the designed configuration shows large thermo-spin effects that could be tuned by the gate voltage, chemical potential as well as temperature. Our results show that the non-magnetic AGNRs with the edge magnetism of the GNFs and CAC located between them induces a notable spin Seebeck coefficient (SSC), comparable to or more than that in zigzag graphene nanoribbon junctions, for all sizes of CAC. Finally, applying a gate voltage and a temperature gradient across the chain-based caloritronic device could induce a pure spin current, a perfect spin filtering effect, and an unrivaled negative differential thermal resistance in a broad range of temperatures close to room temperature.

Thermospin effects in a T-shaped spin valves with spin-flip scattering

With the help of the nonequilibrium Green's function technique, we theoretically analyze the thermospin property through a typical T-shaped spin valve with spin-flip scattering in the linear regime. The influences of spin-flip coefficient of interdot λ, spin-flip coefficient of intradot η and interdot hopping coefficient t+δσΔt on thermospin property are discussed. As interdot hopping coefficient t is equal to energy level ε, the spectrum of Gs shows Fano-like effect with ε variation. Antiresonance position of Gs is almost unchanged and its width becomes narrower with ε increasing. Spin thermopower Ss is close to the maximum of the peak and charge thermopower Sc is equal to zero for t=ε. As a result, the pure spin thermopower Ss can be obtained, which means that a pure spin current may be produced by a temperature gradient in our system. It is found that spin figure of merit ZTs can reach a considerable value by adjusting key parameters of the system, such as Δt, β, α, ϕ. The typical T-shaped spin valve can be treated as a stable thermospin battery which allows to convert the heat energy to spin voltage, thus produces the pure spin current in the device.

Large Thermospin Effects in Carbon Nanotubes with Vacancy Defects

We systematically investigated the possibility of improving thermospin effects in single-walled carbon nanotubes (SWCNTs) by vacancy engineering. The proposed method possesses several advantages: (I) both the armchair and zigzag SWCNTs with vacancy defects exhibit a spin-semiconducting phase with a localized band structure near the Fermi level; (II) applying a thermal gradient can generate spin-up and spin-down currents with the same magnitudes but opposite directions, demonstrating the existence of a pure spin current without any applied bias; (III) a giant spin Seebeck coefficient SS, comparable with those of other carbon-based structures, can be achieved; (IV) the phononic thermal conductance in defective SWCNTs is reduced by about 50%, thus strongly enhancing the thermospin conversion efficiency ZST; and (V) varying the nanotube diameter can tune the spin gap, such that the spin gap increases as the nanotube becomes narrower, indicating the possible application of SWCNTs in thermospintronic.

Enhanced thermo-spin effects in iron-oxide/metal multilayers

Since the discovery of the spin Seebeck effect (SSE), much attention has been devoted to the study of the interaction between heat, spin, and charge in magnetic systems. The SSE refers to the generation of a spin current upon the application of a thermal gradient and detected by means of the inverse spin Hall effect. Conversely, the spin Peltier effect (SPE) refers to the generation of a heat current as a result of a spin current induced by the spin Hall effect. Here we report a strong enhancement of both the SSE and SPE in Fe3O4/Pt multilayered thin films at room temperature as a result of an increased thermo-spin conversion efficiency in the multilayers. These results open the possibility to design thin film heterostructures that may boost the application of thermal spin currents in spintronics.

Valley-locked thermospin effect in silicene and germanene with asymmetric magnetic field induced by ferromagnetic proximity effect

Silicene and germanene, as graphenelike materials with observable spin-orbit couplings and two distinctive valleys, have potential applications in future low-dissipation spintronics and valleytronics. We here propose a magnetic system of silicene or germanene intercalated between two ferromagetic (FM) dielectric layers, and find that the system with a proximity-induced asymmetric magnetic field supports an attractive phenomenon named the valley-locked spin-dependent Seebeck effect (VL-SSE) driven by a thermal gradient. The VL-SSE indicates that the carries from only one valley could be thermally excited, with opposite spin polarization counterpropagating along the thermal gradient direction, while nearly no carrier from the other insulating valley is excited due to the relatively wide band gap. It is also illustrated that the VL-SSE here does not survive in the usual FM or anti-FM systems, and can be destroyed by the overlarge temperature broadening. Moreover, we prove that the signal for VL-SSE can be weakened gradually with the enhancement of the local interlayer electric field, and be strengthened lineally by increasing the source-drain temperature difference in a caloritronic field effect transistor. Further calculations indicate that the VL-SSE is robust against many perturbations, including the global and local Fermi levels as well as the magnetic strength. These findings about the valley-locked thermospin effect provide a nontrivial and convenient dimension to control the quantum numbers of spin and valley and are expected to be applied in future spin-valley logic circuits and energy-saving devices.

Thermospin effects in superconducting heterostructures

Recently thermally created pure spin currents were predicted for Zeeman-split superconductor/normal metal heterostructures. Here it is shown that this "thermospin" current can lead to an accumulation of a pure spin imbalance in a system. The thermally induced spin imbalance can reach the value of Zeeman splitting of the superconducting DOS and strongly influences superconductivity in the heterostructure. Depending on the temperature difference between the superconductor and the normal reservoir it can enhance the critical temperature of the superconductor or additionally suppress the zero-temperature superconducting state. The last possibility gives rise to an unusual superconducting state, which starts to develop at finite temperature.

Thermoelectric and thermospin properties in a quantum ring with an embedded protein-like single-helical molecule

We investigate the thermoelectric and thermospin effects in a quantum ring whose arms are embedded by one protein-like single-helical molecule and one quantum dot, respectively. It shows that both the thermoelectric and thermospin effects are very distinct in this system. If one local magnetic flux is introduced through this ring, the thermospin effect can be efficiently enhanced, accompanied by the weakness of thermoelectric effect. Therefore, this work can be helpful for the achievement of the thermoelectric and thermospin effects, respectively.

Enhanced Spin Figure of Merit in a Molecular Junction

We study the spin-dependent thermoelectric effect in a molecular junction where a semiconductor quantum dot in a magnetic field is trapped between two conducting polymer leads. The Green’s function method along with the Landauer formula based on the Holstein small polaron model is applied to study the thermo-spin effects in this system. The narrow bandwidth of the small polaron band in the polymer leads results in sharp carrier density of states, which offers the opportunity to enhance the spin Seebeck coefficient. At the same time, the spin conductance and electron thermal conductance can be tuned by tuning the affinity mismatch and chemical potential flexibly. Consequently, significant enhancement of spin figure of merit can be achieved. Detailed calculation of the influence of band mismatch, width of a small polaron band, chemical potential, temperature, and magnetic field on the thermo-spin effects is shown in this work. Our results show that the value of spin figure of merit is larger than 1 at a wid...

Adatom-induced local reconstructions in zigzag silicene nanoribbons: Spin semiconducting properties and large spin thermopowers

Using first-principles methods, we have investigated magnetic properties and thermospin effects of zigzag silicene nanoribbons (ZSiNRs) absorbed by a single Si atom. After a relaxation, a steady dumbbell-like structure is formed, which induces a weaker antiferromagnetic (AFM) coupling between two zigzag edges. Therefore, a band gap is opened, meanwhile the adsorbed ZSiNRs show a spin semiconducting property. A large spin thermopower and weak charge thermopower in adsorbed ZSiNR-based devices can be simultaneously achieved, which is attributed to a nearly perfect mirror symmetry of spin-up and spin-down transmission spectra relative to the Fermi level.

Enhancement of thermospin effect in ZGNRs via p-n co-doping on edge

Electronic and thermoelectric properties are studied for ferromagnetic zigzag graphene nanoribbons (ZGNRs) co-doped by a pair of boron and nitrogen atoms on one edge. In case of the 1st nearest neighbor co-doping, the two dopants are partly neutralized by each other and the corresponding impurity states approach to the Fermi energy symmetrically from high and low energies. As a result, the electronic properties are effectively manipulated and the conductances of different spin show opposite behaviours. Over a large range from zero temperature to higher-than-room temperature, the system shows a strong spin thermopower with a spin Seebeck coefficient much bigger than the charge one. These findings suggest that doped ZGNRs can be promising materials for spintronic and thermospintronic devices.

Enhancement of thermospin effect in germanene based normal/ferromagnetic stub/normal junction

Spin thermoelectric effects in ferromagnetic (FM) germanene are theoretically investigated by using the nonequilibrium Green's function method. It is found that the spin Seebeck effect can be generated by temperature bias ΔT when a FM germanene is considered in the central region. However, the obtained spin resolved Seebeck coefficients is quite low with maximum value of Sσ≃700μV/K. The spin Seebeck effect is shown to increase enormously in different energy states with the assistance of electric field or stub structure. By modulating the geometric parameters of stub, the spin thermopower Ss has distinct peak values in the bulk states. Moreover, varying the Fermi energy within the bulk gap by the gate, Ss can be significantly enhanced by increasing the strength of electric field. The spin thermopower obtained by each method is predicted to be 2500μV/K, which is more than 300% larger relative to the case without electric field or stub. In addition, the magnitude and sign of spin thermopower can be manipulated by adjusting the Fermi energy. The results show that such FM germanene stub device exhibits much better thermoelectric performance and may be used as a wide energy range tunable spin thermoelectric generator.

Thermospin effects in a quantum dot connected to normal leads

We study thermospin effects through a quantum dot (QD) with a magnetic field attached to normal leads. In the stationary case, we obtain a tunable pure spin current and highly spin-polarized current. The spin Seebeck coefficient can be as large as the charge counterpart and a pure spin Seebeck coefficient is found. In the presence of microwave (MW) fields, the spin and charge current can be controlled by the amplitude of MW fields. Due to the photon-assisted tunneling, the behavior of current spin polarization SP becomes complicated and we can find a large SP in a larger range of the QD level Vg. At zero Vg, we can modulate both amplitude and direction of the spin current by the phase ϕ of MW fields. More peaks and valleys appear in the Seebeck coefficient owing to more sub-channels opened by the photons. Like the current, the spin Seebeck coefficient can be also tuned by ϕ.

Anomalous thermospin effect in the low-buckled Dirac materials

A strong spin Nernst effect with nontrivial dependences on the carrier concentration and electric field applied is expected in silicene and other low-buckled Dirac materials. These Dirac materials can be considered as being made of two independent electron subsystems of the two-component gapped Dirac fermions. For each subsystem, the gap breaks a time-reversal symmetry and thus plays the role of an effective magnetic field. Accordingly, the standard Kubo formalism has to be altered by including the effective magnetization in order to satisfy the third law of thermodynamics. We explicitly demonstrate this by calculating the magnetization and showing how the correct thermoelectric coefficient emerges.

External magnetic field on the thermoelectric and thermospin effect in a quantum dot

The interactions of spin angular momentum and the external magnetic field in a quantum dot coupled to ferromagnetic lead have been considered here. The results show that the magnetic moment of spin precession caused by the direction of the external field changing could be used to adjust the coulomb gaps of quantum dots, and to control thermal-to-electron conversion efficiency or thermal-to-spin transformation efficiency. When the polarized directions of the ferromagnetic leads are opposite, it is possible to obtain a better thermal spin quality factor. So the system symmetry and Coulomb blockade effect limit the spin current of a quantum dot system. Our results are important to the design of quantum dot devices.

Perfect spin filtering and large spin thermoelectric effects in organic transition-metal molecular junctions.

We present ab initio studies of spin-polarized transport properties and thermospin effects in cyclopentadienyl-iron molecular junctions. It is found that the spin-up transmission coefficient at the Fermi level shows an odd-even oscillating behaviour, while the spin-down transmission coefficient has an exponential decay with the molecule length. The spin polarization at the Fermi level rapidly tends toward a saturation value close to 100% with the molecule length. This is ascribed to the existence of different orbital states for different spin components at the Fermi level. In addition, we find that the spin-up Seebeck coefficient oscillates between positive and negative values, while the spin-down Seebeck coefficient always has a positive value and monotonically increases with the molecule length. Therefore in some cases, the spin Seebeck coefficient is even larger than the corresponding charge Seebeck effect. Finally, we also provide a possibility of utilizing cyclopentadienyl-iron molecular junctions to achieve the pure spin current without an accompanying charge current at about room temperature.

Anisotropic spin-dependent thermopower and current in ferromagnetic graphene junctions

We study the spin-dependent thermoelectric transport through two-dimensional normal/ferromagnetic/normal/ferromagnetic/normal graphene (NG/FG/NG/FG/NG) junctions. It is found that both charge and spin thermopowers depend on the FGʼs magnetization direction and exhibit an anisotropic behavior. Interestingly, the spin thermopower can be as large as the charge thermopower and even can exceed the latter in magnitude. Moreover, the pure spin thermopower and spin current emerge in this device. The results obtained here suggest a feasible way of enhancing thermospin effects and generating the pure spin current in two-dimensional graphene. • Spin-dependent thermopower in ferromagnetic graphene junctions is studied. • Spin and charge thermopowers are anisotropic. • Spin thermopowers can be as large as the charge thermopower. • A pure spin thermopower (spin current) is obtained.