Spin-dependent Seebeck Coefficient Research Articles

Giant spin caloritronic properties in spin-semiconductor graphene nanoribbons via zigzag edge extensions

In this work, we theoretically studied the spin caloritronic properties of 7-width armchair graphene nanoribbons with isolated zigzag edge extension (D-system), cove-to-zigzag edge extensions (D1-system), cove-to-cove edge extensions (D2-system), and zigzag-to-zigzag edge extensions (D3-system), respectively, by combining first-principles calculations with a non-equilibrium Green's function method. The results illustrate that the D-system and D1-system with sublattice imbalance show spin-semiconductor properties and obtain thermally induced pure spin current devoid of charge current due to the symmetric spin-up and spin-down channels around the Fermi level. Additionally, it observes substantial spin-dependent Seebeck coefficients Ssp, approximately −2.5 mV/K for the D-system and −3.0 mV/K for the D1-system, near chemical potential ±0.5 eV. More than that, the D1-system showcases a remarkable spin-dependent thermoelectric figure of merit, ZspT, at room temperature, approximately approaching 8 near the Fermi level. In contrast, the D2-system and D3-system only achieved charge-dependent thermoelectric figure of merit of about 0.5 due to the preservation of sublattice balance. Our findings provide important suggestions for designing spin caloritronic devices with high efficiency.

Interplay between diffusion and magnon-drag thermopower in pure iron and dilute iron alloy nanowire networks

Results of measurements on the thermoelectric power of 45 nm diameter interconnected nanowire networks consisting of pure Fe, dilute FeCu and FeCr alloys and Fe/Cu multilayers are presented. The thermopower values of Fe nanowires are very close to those found in bulk materials, at all temperatures studied between 70 and 320 K. For pure Fe, the diffusion thermopower at room temperature, estimated to be around − 15 upmuV/K from our data, is largely supplanted by the estimated positive magnon-drag contribution, close to 30 upmuV/K. In dilute FeCu and FeCr alloys, the magnon-drag thermopower is found to decrease with increasing impurity concentration to about 10 upmuV/K at 10% impurity content. While the diffusion thermopower is almost unchanged in FeCu nanowire networks compared to pure Fe, it is strongly reduced in FeCr nanowires due to pronounced changes in the density of states of the majority spin electrons. Measurements performed on Fe(7 nm)/Cu(10 nm) multilayer nanowires indicate a dominant contribution of charge carrier diffusion to the thermopower, as previously found in other magnetic multilayers, and a cancellation of the magnon-drag effect. The magneto-resistance and magneto-Seebeck effects measured on Fe/Cu multilayer nanowires allow the estimation of the spin-dependent Seebeck coefficient in Fe, which is about − 7.6 upmuV/K at ambient temperature.

Spin Seebeck effect driven by thermal flux in two-dimensional ferromagnets

The spin Seebeck effect is a novel indirect thermoelectric conversion phenomenon, in which magnetic materials play an important role, that is distinct from the traditional direct thermoelectric conversion method. In this paper, we studied the spin Seebeck effect driven by thermal flux in two-dimensional ferromagnets and derived the spin-dependent Seebeck coefficient and the spin Seebeck coefficient contributed by conduction electrons in a ferromagnet based on the non-equilibrium linear irreversible thermodynamics and the Boltzmann linear theory. The spin Seebeck coefficients of six two-dimensional ferromagnetic materials (including manganese halides and transition metal chalcogenides) were numerically calculated. A largest spin Seebeck coefficient is found for MnCl3 to be 1600 μV/K in the range of temperature from 50 to 120 K, which is even larger than that of known CrI3 and CrGeTe3. The present study on the heat flow-spin current transport properties of ferromagnets could have great significance for the thermoelectric applications in spintronics.

The spin-dependent properties of silicon carbide/graphene nanoribbons junctions with vacancy defects

We have designed high-efficient spin-filtering junctions composed of graphene and silicon carbide nanoribbons. We have calculated the spin and charge transport in the junction by non-equilibrium Green’s function formalism combined with the density functional theory to find its spin-dependent electrical conductance, thermal conductance and Seebeck coefficient. In addition, the effect of Si and C atoms vacancies on the transport properties of the junction has been carefully investigated. The enhanced spin-filtering is clearly observed due to the edge and vacancy effects. On the other hand, vacancy defects increase the electrical and spin conductances of the junctions. The results show that the considered junctions are half-metal with reduced thermal conductance which makes them a suitable spin-dependent thermoelectric device. Our results predict the promising potential of the considered junctions for application in spintronic devices.

Vacancy tuned thermoelectric properties and high spin filtering performance in graphene/silicene heterostructures

The main contribution of this paper is to study the spin caloritronic effects in defected graphene/silicene nanoribbon (GSNR) junctions. Each step-like GSNR is subjected to the ferromagnetic exchange and local external electric fields, and their responses are determined using the nonequilibrium Green’s function (NEGF) approach. To further study the thermoelectric (TE) properties of the GSNRs, three defect arrangements of divacancies (DVs) are also considered for a larger system, and their responses are re-evaluated. The results demonstrate that the defected GSNRs with the DVs can provide an almost perfect thermal spin filtering effect (SFE), and spin switching. A negative differential thermoelectric resistance (NDTR) effect and high spin polarization efficiency (SPE) larger than 99.99% are obtained. The system with the DV defects can show a large spin-dependent Seebeck coefficient, equal to Ss ⁓ 1.2 mV/K, which is relatively large and acceptable. Appropriate thermal and electronic properties of the GSNRs can also be obtained by tuning up the DV orientation in the device region. Accordingly, the step-like GSNRs can be employed to produce high efficiency spin caloritronic devices with various features in practical applications.

Electrical and thermal generation of spin currents by magnetic bilayer graphene.

Ultracompact spintronic devices greatly benefit from the implementation of two-dimensional materials that provide large spin polarization of charge current together with long-distance transfer of spin information. Here spin-transport measurements in bilayer graphene evidence a strong spin-charge coupling due to a large induced exchange interaction by the proximity of an interlayer antiferromagnet (CrSBr). This results in the direct detection of the spin polarization of conductivity (up to 14%) and a spin-dependent Seebeck effect in the magnetic graphene. The efficient electrical and thermal spin-current generation is the most technologically relevant aspect of magnetism in graphene, controlled here by the antiferromagnetic dynamics of CrSBr. The high sensitivity of spin transport in graphene to the magnetization of the outermost layer of the adjacent antiferromagnet, furthermore, enables the read-out of a single magnetic sublattice. The combination of gate-tunable spin-dependent conductivity and Seebeck coefficient with long-distance spin transport in a single two-dimensional material promises ultrathin magnetic memory and sensory devices based on magnetic graphene.

Giant Magnetoresistance and Magneto-Thermopower in 3D Interconnected NixFe1-x/Cu Multilayered Nanowire Networks.

The versatility of the template-assisted electrodeposition technique to fabricate complex three-dimensional networks made of interconnected nanowires allows one to easily stack ferromagnetic and non-magnetic metallic layers along the nanowire axis. This leads to the fabrication of unique multilayered nanowire network films showing giant magnetoresistance effect in the current-perpendicular-to-plane configuration that can be reliably measured along the macroscopic in-plane direction of the films. Moreover, the system also enables reliable measurements of the analogous magneto-thermoelectric properties of the multilayered nanowire networks. Here, three-dimensional interconnected NiFe/Cu multilayered nanowire networks (with ) are fabricated and characterized, leading to large magnetoresistance and magneto-thermopower ratios up to 17% and −25% in NiFe/Cu, respectively. A strong contrast is observed between the amplitudes of magnetoresistance and magneto-thermoelectric effects depending on the Ni content of the NiFe alloys. In particular, for the highest Ni concentrations, a strong increase in the magneto-thermoelectric effect is observed, more than a factor of 7 larger than the magnetoresistive effect for NiFe/Cu multilayers. This sharp increase is mainly due to an increase in the spin-dependent Seebeck coefficient from −7 µV/K for the NiFe/Cu and NiFe/Cu nanowire arrays to −21 µV/K for the NiFe/Cu nanowire array. The enhancement of the magneto-thermoelectric effect for multilayered nanowire networks based on dilute Ni alloys is promising for obtaining a flexible magnetic switch for thermoelectric generation for potential applications in heat management or logic devices using thermal energy.

Spin Seebeck coefficients of Fe, Co, Ni, and Ni80Fe20 3d-metallic thin films

It is challenge to quantitatively determine the spin Seebeck effect (SSE) of ferromagnetic metals (FM) due to many other thermoelectric signals including the anomalous Nernst effect (ANE), magnetic proximity-induced ANE, and inverse spin Hall effect (ISHE) in FM. By comparing the thermoelectric signals of single FM layer and FM/Pt bilayer and model-assisted analyses of ISHE contribution in FM, we are able to determine the pure SSE signals and the spin Seebeck coefficients of Fe, Co, Ni, and Ni80Fe20 over a temperature range from 90 to 300 K. The spin-dependent Seebeck coefficients S↑ and S↓, which is related to the density of states of FM, were also determined experimentally by combining the conventional Seebeck coefficient and the SSE coefficient. We further suggest the calculation of the SSE coefficient should include the spin-dependent conductivities via Ss≡σ↑S↑-σ↓S↓σ↑+σ↓.

Spin Caloritronics in 3D Interconnected Nanowire Networks.

Recently, interconnected nanowire networks have been found suitable as flexible macroscopic spin caloritronic devices. The 3D nanowire networks are fabricated by direct electrodeposition in track-etched polymer templates with crossed nano-channels. This technique allows the fabrication of crossed nanowires consisting of both homogeneous ferromagnetic metals and multilayer stack with successive layers of ferromagnetic and non-magnetic metals, with controlled morphology and material composition. The networks exhibit extremely high, magnetically modulated thermoelectric power factors. Moreover, large spin-dependent Seebeck coefficients were directly extracted from experimental measurements on multilayer nanowire networks. This work provides a simple and cost-effective way to fabricate large-scale flexible and shapeable thermoelectric devices exploiting the spin degree of freedom.

Even–odd effect of spin-dependent transport and thermoelectric properties for ferromagnetic zigzag phosphorene nanoribbons under an electric field

We study the spin-dependent transport and thermoelectric properties of ferromagnetic zigzag-edge phosphorene nanoribbon with N chains (N-ZPNR) modulated by a perpendicular electric field (PEF). By adopting the Hubbard model Hamiltonian combined with the self-consistent calculation, and using the nonequilibrium Green’s function method, we obtain the band structure, spin-dependent transmission coefficients and Seebeck coefficients. It is demonstrated that the bands are split with spin-up and -down ones for both 12- and 13-ZPNR in the ferromagnetic state, and both ZPNRs show spin-semiconducting behavior with the spin gap of value 1 eV. Interestingly, there is an even–odd effect on the spin-dependent transport and thermoelectric properties when a PEF is applied, which originates from the structure symmetry of N-ZPNR and can result in different behavior of edge states for even and odd cases. The electric field lifts the degeneracy of the two edge bands for both spin-up and -down channels in 12-ZPNR, however, it only bends the spin-up edge bands and shifts the spin-down ones upward without splitting in 13-ZPNR. In addition, we find that the spin Seebeck coefficients exhibit wide plateaus and their maximum values can reach unprecedented giant 5 mV K−1 for both ZPNRs. The spin Seebeck coefficients for both ZPNRs decrease gradually as temperature increasing. As the electric field increasing, the value of the plateau in the spin Seebeck coefficient for 12-ZPNR decreases gradually, however, the plateau almost has no variation for 13-ZPNR in this case. This different responses of spin Seebeck coefficients to the PEF between even and odd cases originate from the different edge state responses to the electric field, and we provide the explanation from the corresponding spin-up and spin-down Seebeck coefficients. Our results reveal that ferromagnetic ZPNRs are perfectly spin-polarized, and are promising candidates for spin caloritronics with high efficiency.

Modulation of spin thermoelectric properties in transition metal porphyrin single-molecule spin caloritronic devices by Fano resonance

Abstract Single-molecule spin caloritronic devices, which convert the heat into electricity by using spin degree of freedom of electron, are attracting attention with the inspiration of finally shrinking electronic components by utilizing molecules as elementary blocks. In this work, one kind of molecular-scale spin caloritronic devices is theoretically proposed, which consists of a transition metal porphyrin (TM(Pr), TM=V, Mn, Fe, Co) molecule sandwiched between single-walled carbon nanotube (SWCNT) electrodes via carbon atomic chains (CACs). The corresponding spin transport properties are calculated. The results show there is robust Fano resonance in the transmission spectra around the Fermi energy for one spin component in each device, which is attributed to the quantum interference between continuous spin states originating from delocalized states over the pristine SWCNT-porphyrin device and “quasi-discrete” state mainly localized on the TM(Pr) molecule. Utilizing Fano resonance, spin-dependent Seebeck coefficients of the devices for one spin component can be strengthened. Interestingly, it is revealed the energy, spatial distributions, and parity of molecular orbitals of TM(Pr) molecules play an important role in the generation of Fano resonance, which is dependent on the kind of TM atom. Therefore, this further provides the possibility to optimize spin-dependent Seebeck coefficient by TM atoms. This work is helpful for the future design of spin caloritronic devices.

Signature of spin-dependent Seebeck effect in dynamical spin injection of metallic bilayer structures

The dynamical spin injection in a ferromagnetic/paramagnetic bilayer with various paramagnetic layers has been examined by using the inverse spin Hall effect. We adapt a CoFeB film as a ferromagnetic layer, which has a large spin dependent Seebeck coefficient. The contribution of the spin pumping was evaluated from the line-width change of the ferromagnetic resonant spectra while that of the thermal spin injection was evaluated from the heat conductivity for the paramagnetic layer. We find that the spin Hall voltage does not show the systematic change with respect to the line-width change. However, the normalized spin Hall voltage is found to increase with the heat conductivity for the paramagnetic layer. These results suggest that the thermal spin injection is a major contribution for the dynamical spin injection in CoFeB/paramagnetic bilayer systems.

Making flexible spin caloritronic devices with interconnected nanowire networks.

Spin caloritronics has recently emerged from the combination of spintronics and thermoelectricity. Here, we show that flexible, macroscopic spin caloritronic devices based on large-area interconnected magnetic nanowire networks can be used to enable controlled Peltier cooling of macroscopic electronic components with an external magnetic field. We experimentally demonstrate that three-dimensional CoNi/Cu multilayered nanowire networks exhibit an extremely high, magnetically modulated thermoelectric power factor up to 7.5 mW/K2m and large spin-dependent Seebeck and Peltier coefficients of -11.5 μV/K and -3.45 mV at room temperature, respectively. Our investigation reveals the possibility of performing efficient magnetic control of heat flux for thermal management of electronic devices and constitutes a simple and cost-effective pathway for fabrication of large-scale flexible and shapeable thermoelectric coolers exploiting the spin degree of freedom.

Substantial enhancement of thermal spin polarization in Py/Cu interface

We investigated the temperature dependence of thermally excited spin current properties of ferromagnet Py $({\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20})$ by using a lateral spin-valve structure. The spin-dependent Seebeck coefficient at the Py/Cu interface was found to show a significant increase at low temperature while the charge Seebeck coefficient showed the ordinal reduction with decreasing temperature. This produces a crossover between the charge- and spin-dependent Seebeck coefficients, resulting in the thermal spin polarization greater than $100%$ below 125 K. From the first-principles calculation specially developed for the Py/Cu interfaces, the spin-dependent Seebeck coefficient is found to be highly susceptible to the magnetic disorder-induced scattering at the interface. The calculation with considering the temperature dependence of the magnetic disorder provides a consistent description for the significant enhancement of spin-dependent Seebeck coefficient in the Py/Cu interface. Our demonstration shows the importance of interface disorder and paves the way toward the development of conventional ferromagnetic metal for the practical application of thermo-spin energy conversion.

Temperature dependence of the electrical and thermal transport in FeCo/Cu/Ni80Fe20 spin valves

Creating spin current under temperature gradient in magnetic devices has resulted in a new emerging field of spin caloritronics, but extraction of material parameters i.e. Seebeck coefficient and interpretation of thermal transport characteristics are still great challenges due to the thermal contact effect, especially in a wide temperature range. Because the heat-driven voltages do not depend on the change of magnetic states in spin valve, this could be used to obtain the accurate temperature difference applied on sample and exclude the thermal contact effect. Based on this calibration, the electrical and thermal spin transport behaviors in the in-plane FeCo/Cu/FeNi spin valve were systematically studied with various temperature. We observed that the Seebeck coefficients of spin valve was negative and spin-dependent Seebeck coefficient was proportional to the asymmetry parameter in a wide temperature range from 100 to 300 K, indicating that the spin-dependent thermal transports are closely related with electrical transports in the in-plane spin valve.

Thermoelectric effects of resonant magnetic tunnel junctions

Thermoelectric effects of CoFeB/MgO/InP/MgO/CoFeB double-barrier magnetic tunnel junctions (DBMTJs) are presented in the linear response regime. The nonequilibrium Green’s function (NEGF) formalism is used within the effective mass approximation. There are descriptions of temperature and angular dependence for thermoelectric properties. The results show an enhancement in the DBMTJs thermoelectric compared to single-barrier MTJs due to the resonant tunneling effect through the DBMTJs. It is also found that the Seebeck coefficients may be increase in asymmetric DBMTJs. Effects of the temperature and magnetization alignments on the spin-dependent Seebeck coefficients are also described. Finally, the Seebeck coefficients are investigated considering the Rashba spin-orbit coupling at the insulator/semiconductor interfaces.

Modification of the magnetization dynamics of a NiFe nanodot due to thermal spin injection

An array of NiFe nanodots has been prepared on a Cu/CoFeAl film. Since a thermal spin current is expected to be excited owing to a large spin-dependent Seebeck coefficient for the CoFeAl, we investigate the magnetization dynamics of the NiFe dots under the temperature gradient along the vertical direction. By using vector network analyzer measurements, we have demonstrated that the temperature gradient produces modulations of the frequency of ferromagnetic resonance and the linewidth of the resonance spectra. The observed parabolic dependences are well explained by the damping-like and field-like components of spin transfer torque.

Spin-dependent transport properties and Seebeck effects for a crossed graphene superlattice p-n junction with armchair edge

Using the nonequilibrium Green’s function method combined with the tight-binding Hamiltonian, we theoretically investigate the spin-dependent transmission probability and spin Seebeck coefficient of a crossed armchair-edge graphene nanoribbon (AGNR) superlattice p-n junction under a perpendicular magnetic field with a ferromagnetic insulator, where junction widths W1 of 40 and 41 are considered to exemplify the effect of semiconducting and metallic AGNRs, respectively. A pristine AGNR system is metallic when the transverse layer m = 3j + 2 with a positive integer j and an insulator otherwise. When stubs are present, a semiconducting AGNR junction with width W1 = 40 always shows metallic behavior regardless of the potential drop magnitude, magnetization strength, stub length, and perpendicular magnetic field strength. However, metallic or semiconducting behavior can be obtained from a metallic AGNR junction with W1 = 41 by adjusting these physical parameters. Furthermore, a metal-to-semiconductor transition can be obtained for both superlattice p-n junctions by adjusting the number of periods of the superlattice. In addition, the spin-dependent Seebeck coefficient and spin Seebeck coefficient of the two systems are of the same order of magnitude owing to the appearance of a transmission gap, and the maximum absolute value of the spin Seebeck coefficient reaches 370 µV/K when the optimized parameters are used. The calculated results offer new possibilities for designing electronic or heat-spintronic nanodevices based on the graphene superlattice p-n junction.

Spin and Charge Caloritronics in Bilayer Graphene Flakes with Magnetic Contacts

We investigate the coupling of spin and thermal currents as a means to rise the thermoelectric efficiency of nanoscale graphene devices. We consider nanostructures composed of overlapping graphene nanoribbons with ferromagnetic contacts in different magnetic configurations. Our results show that the charge Seebeck effect is greatly enhanced when the magnetic leads are in an antiparallel configuration, due to the enlargement of the transport gap. However, for the optimization of the charge figure of merit ZT it is better to choose a parallel alignment of the magnetization in the leads, because the electron-hole symmetry is broken in this magnetic configuration. We also obtain the spin-dependent Seebeck coefficient and spin figure of merit. In fact, the spin ZT can double its value with respect to the charge ZT for a wide temperature range, above 300 K. These findings suggest the potential value of graphene nanosystems as energy harvesting devices employing spin currents.

Efficient thermal spin injection in metallic nanostructures

Thermal spin injection is a unique and fascinating method for generating spin current. If magnetization can be controlled by thermal spin injection, various advantages will be provided in spintronic devices, through its wireless controllability. However, the generation efficiency of thermal spin injection is believed to be lower than that of electrical spin injection. Here, we explore a suitable ferromagnetic metal for an efficient thermal spin injection, via systematic experiments based on diffusive spin transport under temperature gradients. Since a ferromagnetic metal with strong spin splitting is expected to have a large spin-dependent Seebeck coefficient, a lateral spin valve based on CoFe electrodes has been fabricated. However, the superior thermal spin injection property has not been observed, because the CoFe electrode retained its crystalline signature—where s-like electrons dominate the transport property in the ferromagnet. To suppress the crystalline signature, we adopt a CoFeAl electrode, in which the Al impurity significantly reduces the contribution from s-like electrons. Highly efficient thermal spin injection has been demonstrated using this CoFeAl electrode. Further optimization for thermal spin injection has been demonstrated by adjusting the Co and Fe composition.