Spin-dependent Seebeck Effect Research Articles

Bipolar Magnetic and Thermospin Transport Properties of Graphene Nanoribbons with Zigzag and Klein Edges

Magnetic nanoribbons based on one‐dimensional materials are potential candidates for spin caloritronics devices. Here, we constructed ferromagnetic graphene nanoribbons with zigzag and Klein edges (N‐ZKGNRs, N = 4–21) and found that the N‐ZKGNRs are in the indirect‐gap bipolar magnetic semiconducting state (BMS). Moreover, when a temperature difference is applied through the nanoribbons, spin‐dependent currents with opposite flow directions and opposite spin directions are generated, indicating the occurrence of the spin‐dependent Seebeck effect (SDSE). In addition, the spin‐dependent Seebeck diode effect (SDSD) also appeared in these devices. More importantly, we found that the BMS with a larger bandgap is promising for generating the SDSD, while the BMS with a smaller bandgap is promising for generating the SDSE. These findings show that ZKGNRs are promising candidates for spin caloritronics devices.

Spin-dependent seebeck effect and pure spin current in ferromagnetic fluorinated boron nitride nanotubes

Based on the density functional theory combined with non-equilibrium Green's function formalism, we investigate the electronic structure and spin caloritronic transport properties of ferromagnetic fluorinated boron nitride nanotubes (F-BNNTs). The results show that these systems exhibit robust half-metallic feature and obvious spin-dependent Seebeck effect . Especially, pure spin current without charge current can be realized by tuning the temperature and temperature difference or by tuning the chirality of F-BNNT. The underlying mechanism is analyzed by the Fermi-Dirac distribution function, spin-resolved transmission spectra, band structures and current spectra. These results demonstrate that the ferromagnetic F-BNNTs hold great potential for spin caloritronics and ultra-low-energy-consumption nanoelectronic devices. • Electronic structure and spin caloritronic transport properties of F-BNNTs are investigated. • F-BNNTs exhibit robust half-metallic feature and obvious Spin-dependent Seebeck effect. • Pure spin current is realized by tuning the temperature and temperature difference. • Pure spin current is realized by tuning the chirality of F-BNNT.

Spin Seebeck effect in the 2D ferromagnetic CrPbTe3

Spin Seebeck effect has been mostly explored in bulk magnetic materials, but the efficiency is not large enough for device application. Recently, we have found that the 2D ferromagnetic CrPbTe3 layer has rather a high Curie temperature of 110 K. Thus, we have investigated the spin dependent Seebeck effect using the Boltzmann transport approach. We have found the largest carrier mobility and relaxation time in the minority spin electron carrier system due to the spin dependent effective mass and deformation potential constant. The onset energy of the electrical conductivity shows spin and carrier type dependency. Thus, both effective spin and effective charge Seebeck coefficients are originated from the majority spin carrier in the hole doped system whereas the minority spin carrier controls the effective spin and effective charge Seebeck coefficient in the electron doped system. We have obtained the maximum spin Seebeck coefficient of 1320 μV/K at 50 K. This value is suppressed at 100 K, but we still find a large spin Seebeck coefficient of 715 μV/K.

Spin-dependent Seebeck and Nernst effects in an ideal skyrmion gas

We theoretically and numerically study spin-dependent Seebeck and Nernst effects in 2D ferromagnetic materials with the topological spin texture (skyrmion and vortex) ideal gas. From the numerical solution of the matrix Boltzmann equation for a nonequilibrium distribution function and the Lippmann–Schwinger equation for a T-matrix we find the strong nonlinear behaviors in the thermoelectric coefficients depending on skyrmion/vortex diameters and electron concentrations. In particular, the dramatic dependences in the Seebeck and Nernst coefficients take place at larger magnetic texture sizes where the abrupt sign flip in the vortex Seebeck and Nernst coefficients occurs in the narrow region of electron concentrations. In this case the normalized Nernst coefficient changes from +5 to -7. The spin-dependent thermoelectric coefficients are proportional to T at low temperatures for all skyrmion/vortex sizes.

Bipolar magnetic semiconductor properties and spin-dependent Seebeck effects induced by nanoscale graphene domains doped into armchair boron nitride nanoribbons

Inspired by the synthesis of nanometer-scale graphene quantum dots via a doping process in boron nitride (BN) sheets (Adv. Mater. 2019, 1805778), we investigated the magnetic and thermal spin transport properties of armchair BN nanoribbons containing nanoscale graphene domains. It was found that both M-QB and M-QN structures were bipolar magnetic semiconductors (BMSs). Two spin caloritronics devices were designed on the basis of the 15-6B and 15-6N nanoribbons. Spin-dependent Seebeck effects, spin-dependent Seebeck diode effects and thermoelectric diode effects were observed in these devices. This work provides theoretical guidance for the design of new spin caloritronics devices.

Thermal transport and spin-dependent Seebeck effect in parallel step-like zigzag graphene nanoribbon junctions.

By using nonequilibrium molecular dynamics, thermal transport through a series of parallel step-like graphene nanoribbon (GNR) junctions is investigated. The theoretical results show that the thermal current flows preferentially from wide GNRs to narrow ones, displaying a pronounced thermal rectification effect. Moreover, several step-like GNR-based devices are designed, and the thermally driven spin-dependent currents are calculated by using density functional theory combined with the nonequilibrium Green's function approach. We find that thermal spin-dependent currents with opposite flow directions are generated when a temperature gradient is applied along the GNRs, indicating the occurrence of a spin-dependent Seebeck effect (SDSE). More interestingly, a negative differential SDSE occurs in the thermal spin currents, and the odd and even law appears in the spin-dependent currents, thermopowers and thermoelectric conversion efficiencies. Our theoretical results indicate that the parallel step-like GNRs are potential candidates to design spin caloritronics devices hosting thermal rectification and multiple thermal-spin transport functionalities.

Spin-dependent Seebeck effect, and spin-filtering and diode effects in magnetic boron–nitrogen nanotube heterojunctions

The spin-Seebeck effect (SSE) has long been considered as an effective route to generate pure spin currents by using a temperature gradient in magnetic materials.

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.

Effect of Co and Fe stoichiometry on terahertz emission from Ta/(CoxFe1−x)80B20/MgO thin films

Laser pulse induced terahertz (THz) emission from Ta/${({\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x})}_{80}{\mathrm{B}}_{20}$/MgO thin films, with varying compositions $x$ and annealing temperatures, is investigated. With increasing annealing temperature, the THz emission intensity exhibits significant dependence on $x$, with maxima at $x$ of 0.1--0.3. The $x$ dependence of the THz emission originated from the composition dependence of the spin currents induced in ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}$ formed by the crystallization of amorphous CoFeB after the annealing. The origin of the laser-induced spin current is qualitatively discussed in terms of a ballistic transport of hot electrons and a spin-dependent Seebeck effect with different compositions.

Spin-valley filter effect and Seebeck effect in a silicene based antiferromagnetic/ferromagnetic junction

The presence of the coupled spin and valley degrees of freedom makes silicene an important material for spintronics and valleytronics. Here we report a spin-valley filter effect in a silicene based antiferromagnetic/ferromagnetic junction. It is found that at zero Fermi level a valley locked bipolar spin filter effect is observed, where in a broad gate voltage range in one valley one spin (the other spin) electrons contribute to the current under the positive (negative) bias, but in the other valley the transport is forbidden. At the finite Fermi level a valley locked fully spin-polarized current can exist under both the positive and negative biases. Furthermore, at the high Fermi level by reversing the bias direction, the spin filter effect can switch to the valley filter effect. In addition, by changing the sign of the Fermi level, the spin polarization direction of the current can be reversed. If a temperature bias is applied, the spin-dependent Seebeck effect (SSE) always exists. With increasing the temperature bias, the system undergoes three regions: valley locked SSE, normal SSE and valley Seebeck effect. Moreover, by tuning the interlayer electric field, three phases: thermally induced valley locked spin filter effect, valley Seebeck effect and valley mixed Seebeck effect are observed.

Electrical-field tuned thermoelectric performance of graphene nanoribbon with sawtooth edges

By using first-principle calculations combined with the non-equilibrium Green’s function approach, we report that a vertical electrical field can modulate the thermoelectric performance of a graphene nanoribbon with sawtooth edges. The results show that the sawtooth graphene nanoribbon exhibits the spin-dependent Seebeck effect under the temperature gradients, and is strengthened by increasing the width of the sawtooth graphene nanoribbon. When the vertical electrical field is applied to the device, the spin-dependent Seebeck effect can also be enhanced. The vertical electrical field can modulate the device transferring from hole-conducting to electron-conducting. This opens up the possibility of tuning the thermal transport properties of the device by the electrical field.

Steering of magnetic domain walls by single ultrashort laser pulses

We present a magnetic domain-imaging study by x-ray magnetic circular dichroism photoelectron emission microscopy on a $\mathrm{Co}/{\mathrm{Fe}}_{75}{\mathrm{Gd}}_{25}$ bilayer under exposure to single focused ultrashort (100 fs) infrared laser pulses. Magnetic domain walls experience a force in the intensity gradient of the laser pulses away from the center of the pulse, which can be used to steer domain walls to move in a certain direction. Maximum domain-wall displacements after individual laser pulses close to $1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in zero external field are observed. Quantitative estimates show that electronic spin currents from the spin-dependent Seebeck effect are not strong enough to explain the effect, which we thus attribute to the torque exerted by magnons from the spin Seebeck effect that are reflected at the domain wall.

Generating pure spin current with spin-dependent Seebeck effect in ferromagnetic zigzag graphene nanoribbons

Pure spin current is of great importance in spintronics and may be achieved by spin dependent Seebeck effect (SDSE) in magnetic systems. Zigzag-edged graphene nanoribbons (ZGNRs) are very well-known 2D magnetic nanostructures. However, perfect and pristine ZGNRs either in the anti-ferromagnetic ground state or in the ferromagnetic (FM) state are not capable of producing pure spin current by SDSE at low temperature. In this work, by density functional theory calculations, we propose a scheme for generating pure spin current using SDSE in FM-ZGNRs by introducing antidots. Specifically, by creating a hexagonal antidot with either armchair edges or zigzag edges in the scattering region, we can get finite Seebeck thermopower for both spin channels with opposite signs, leading to the opposite flow directions of the two spin channels. The mechanism is well explained by the cooperation of the varying localization features of states around the Fermi level and the antidot induced scattering potential. By slightly tuning the chemical potential, pure spin current can be achieved. The size and edge shape effects have also been systematically studied. The findings indicate a novel scheme for thermally generating pure spin current in zigzag graphene nanoribbons and may find important application in graphene based spintronics.

Comment on “Optical detection of transverse spin-Seebeck effect in permalloy film using Sagnac interferometer microscopy”

Measuring spin currents and spin accumulations is challenging. This is mainly because of their small size and the lack of a standard spin detector comparable to conventional ampere meters or voltage detectors. It is all the more pleasing to hear about new methods for detecting spin currents and spin accumulations. A recent example is the paper of McLaughlin et al. [Phys. Rev. B 95, 180401 (2017)], who report novel measurements of the transverse spin-Seebeck effect (TSSE) in metals by optical means using a Sagnac interferometer microscope. However, because no standard procedures or reference measurements are available for verification, critical discussions are crucial for evaluation of such novel experimental approaches. In our comment, we estimate the size of the effect observed by McLaughlin et al. To illustrate that the estimated size is exceptionally large, we use a comparison with another spin-caloritronic effect: the spin-dependent Seebeck effect. Such a large TSSE is in contradiction with previous experiments that could not resolve a TSSE signal. Therefore, we suggest that signal measured by McLaughlin et al. is not or not only caused by the TSSE.

Thermal spin transport properties in a hybrid structure of single-walled carbon nanotubes and zigzag-edge boron nitride nanoribbons

The spin caloritronics device, because of the characteristics of spintronics and thermoelectronics, plays an important role in human sustainable development. A lot of spin caloritronic devices based carbon materials (such as graphene nanoribbons, carbon nanotubes) have been reported. However, there are few studies of the thermal spin transport properties in a hybrid structure of single-walled carbon nanotubes and zigzag-edge BN nanoribbons, and the thermal spin transport mechanism of this structure is still unclear. In this paper, using the nonequilibrium Green’s function (NEGF) combined with the first principle calculations, the electronic structures and the thermal spin transport properties of the zigzag edge BN nanoribbons functionalized single-walled carbon nanotubes are studied. It is shown that the ZBNRs-N-(6, 6)SWCNT is a half-metal, while the <i>n</i>ZBNRs-N-(6, 6)SWCNT are magnetic metals (<i>n</i> = 2−8), and the <i>n</i>ZBNRs-B-(6, 6)SWCNT are bipolar magnetic semiconductors (<i>n</i> = 1−8). The 4ZBNRs-N-(4, 4)SWCNT and 4ZBNRs-B-(4, 4)SWCNT are half-metals, while the 4ZBNRs-B-(<i>m</i>, <i>m</i>)SWCNT (<i>m</i> = 5−9)are magnetic metals, and the 4ZBNRs-N-(<i>m</i>, <i>m</i>)SWCNT (<i>m</i> = 5−9) are bipolar magnetic semiconductors. Then, some novel spin caloritronicdevices are designed based on <i>n</i>ZBNRs-N-(6, 6)SWCNT and <i>n</i>ZBNRs-B-(6, 6)SWCNT (<i>n</i> = 1, 8). For the ZBNRs-B-(6, 6)SWCNT, when the temperature of the left electrode is increased above a critical value, the thermal spin-up current then increases remarkably from zero. Meanwhile the thermal spin-down current remains approximately equal to zero in the entire temperature region, thus indicating the formation of a thermal spin filter. For the 8ZBNRs-N-(6, 6)SWCNT and <i>n</i>ZBNRs-B-(6, 6)SWCNT (<i>n</i> = 1, 8), when a temperature gradient is produced between two electrodes, the spin-up and spin-down currents are driven in the opposite directions, which indicates that the spin-dependent Seebeck effect (SDSE) appears. In order to obtain the fundamental mechanism of thermal spin filter effect and SDSE, the Landauer-Büttiker formalism is adopted. It is found that the currents (<i>I</i><sub>up</sub> and <i>I</i><sub>dn</sub>) mainly depend on two factors: 1)the transport coefficient; 2) the difference between the Fermi-Dirac distributions of the left and right electrode. Additionally, the electron current <i>I</i><sub>e</sub> and the hole current <i>I</i><sub>h</sub> will be generated when a temperature gradient is produced between the left and right lead. Furthermore, the <i>I</i><sub>up</sub> and <i>I</i><sub>dn</sub> have the opposite directions for the spin up transmission peaksbelow the Fermi level while they have the opposite directions for the spin down transmission peaks above the Fermi level in the transmission spectrum, which demonstrates the presence of the SDSE in the 8ZBNRs-B-(6, 6)SWCNT and <i>n</i>ZBNRs-N-(6, 6)SWCNT (<i>n</i> = 1, 8). Finally, the results indicate that <i>n</i>ZBNR-N-(<i>m</i>, <i>m</i>)SWCNT and <i>n</i>ZBNR-B-(<i>m</i>, <i>m</i>)SWCNT can have potential applications in thermospin electronic devices.

Modulation of spin transport in DNA-based nanodevices by temperature gradient: A spin caloritronics approach

Spin caloritronics, a novel and rich research field, combining spin, charge and heat transport in materials, provides alternative strategies for thermoelectric waste heat recovery, and the information technologies. Understanding of the spin transport properties of materials has important implications for spin-based devices. Flexibility, low cost, and adjustable conductance are the important factors to consider the spin transport properties of DNA chains. The spin current corresponding to spin-up and spin-down electrons in the presence of a temperature gradient along the DNA chain are calculated. ∇T=1K/Å is the most appropriate temperature gradient in which the maximum spin current flows. The spin current of different sequences determines the best sequence for spin transport in different situations. Applying the external magnetic field can amplify the spin current along the DNA chain, and one can choose the most appropriate field. Is−V characteristic diagrams corresponding to different sequences are distinctive and determine the spin-dependent negative differential resistance (SNDR) regions corresponding to the negative slope intervals. Eventually, the spin-dependent Seebeck effect (SDSE) in DNA and the SDSE coefficients as a function of temperature are studied. The peak of the SDSE diagrams is different for different species of sequences. Poly(AT), hc1, and poly(CG) show maximum peaks of SDSE coefficient at 300, 310, and 320 K, respectively. Therefore, one can design a DNA based spin nanodevice with maximal efficiency in different situations.

Spin-dependent Seebeck effect, spin-dependent Seebeck diode, thermal spin filtering and figure of merit of nitrophenyl diazonium functionalized graphene

Spin caloritronic devices are multifunctional devices that combine spintronics with caloritronics and are set to play significant roles in low-power-consumption systems. In this work, the magnetic properties of nitrophenyl diazonium (NPD)-functionalized graphene (NPDG) sheets with NPD coverage ratios of 1:8, 1:18 and 1:32 are presented. The stable ground states of these sheets have ferromagnetic (FM) configurations and their Curie temperature (TC) values are 540 K, 444 K and 157 K, respectively. The thermal spin transport properties of the NPDG sheets (with two transport directions: zigzag edge and armchair edge) with the NPD coverage ratio of 1:8 are presented. For the zigzag edge, when a temperature gradient was produced between two electrodes, the spin-up and spin-down currents were driven in opposite directions, which indicated the appearance of the spin-dependent Seebeck effect (SDSE). In addition, a spin caloritronic device could be obtained in the form of a spin-dependent Seebeck diode (SDSD). For the armchair edge, when the temperature of the left contact increased beyond a critical value, the thermal spin-up current then increased remarkably from zero, while the thermal spin-down current remained approximately equal to zero over the entire temperature region, thus indicating the formation of a thermal spin filter. Finally, in terms of its behavior with respect to the charge current (Ich), the spin-up thermopower (S↑) was found to be nearly equal to the spin-down thermopower (S↓); consequently, the charge thermopower must be nearly zero, thus further demonstrating that the SDSE has been generated. Our results indicate that NPDG sheets can be designed to produce high-efficiency spin caloritronic devices for room temperature operation.

Magnons versus electrons in thermal spin transport through metallic interfaces

We develop a theory for spin transport in magnetic metals that treats the contribution of magnons and electrons on equal footing. As an application, we consider thermally-driven spin injection across an interface between a magnetic metal and a normal metal, i.e. the spin-dependent Seebeck effect. We show that the ratio between magnonic and electronic contribution scales as , with the Fermi temperature TF and the Curie temperature TC. Since, typically, , the magnonic contribution may dominate the thermal spin injection, even though the interface is more transparent for electronic spin current.

Spin caloritronics in armchair silicene nanoribbons with sp3 and sp2-type alternating hybridizations

Finite-layer nanoribbon materials have long been considered as potential candidates for nanodevices with novel quantum effects. Here we constructed a series of ferromagnetic armchair silicene nanoribbons (ASiNRs) with sp3 and sp2-type alternating hybridizations, and found that the ASiNRs with different widths are localized in different spin-resolved electronic states. As the width parameter N is increased from 5 to 22, the ASiNR transits from indirect-gap half metallicity (HM), to indirect-gap spin semiconductor (SC), then to direct-gap SC and finally to direct-gap HM. When a temperature gradient is produced along the nanoribbons, the spin-dependent currents with the opposite flow directions are driven and a nearly perfect spin-dependent Seebeck effect (SDSE) occurs. Moreover, attributing to symmetrical spin-resolved transport channels, nearly pure thermal spin current without any accompanying charge current can be generated. In addition, for some ASiNRs with proper widths, both the thermal spin-up current and spin-down one are contributed by the electrons in energy valleys, resulting in a well-defined valley-dependent SDSE. These theoretical findings suggest that the ASiNRs with the sp3 and sp2-type alternating hybridizations can be outstanding candidates for future spin caloritronic devices.

Thermally Generated Spin Signals in a Nondegenerate Silicon Spin Valve

Spincaloritronic signal generation due to thermal spin injection and spin transport is demonstrated in a non-degenerate Si spin valve. The spin-dependent Seebeck effect is used for the spincaloritronic signal generation, and the thermal gradient of about 200 mK at an interface of Fe and Si enables generating a spin voltage of 8 {\mu}V at room temperature. A simple expansion of a conventional spin drift-diffusion model with taking into account the spin-dependent Seebeck effect shows semiconductor materials are quite potential for the spincaloritronic signal generation comparing with metallic materials, which can allow efficient heat recycling in semiconductor spin devices.