Spin Caloritronic Devices Research Articles

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

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

Engineering the spin conversion in graphene monolayer epitaxial structures

Spin Hall and Rashba–Edelstein effects, which are spin-to-charge conversion phenomena due to spin–orbit coupling (SOC), are attracting increasing interest as pathways to manage rapidly and at low consumption cost the storage and processing of a large amount of data in spintronic devices as well as more efficient energy harvesting by spin-caloritronics devices. Materials with large SOC, such as heavy metals (HMs), are traditionally employed to get large spin-to-charge conversion. More recently, the use of graphene (gr) in proximity with large SOC layers has been proposed as an efficient and tunable spin transport channel. Here, we explore the role of a graphene monolayer between Co and a HM and its interfacial spin transport properties by means of thermo-spin measurements. The gr/HM (Pt and Ta) stacks have been prepared on epitaxial Ir(111)/Co(111) structures grown on sapphire crystals, in which the spin detector (i.e., top HM) and the spin injector (i.e., Co) are all grown in situ under controlled conditions and present clean and sharp interfaces. We find that a gr monolayer retains the spin current injected into the HM from the bottom Co layer. This has been observed by detecting a net reduction in the sum of the spin Seebeck and interfacial contributions due to the presence of gr and independent from the spin Hall angle sign of the HM used.

Spin transport properties in Dirac spin gapless semiconductors Cr2X3 with high Curie temperature and large magnetic anisotropic energy

2D honeycomb-Kagome (HK) lattices have attracted extensive attention in recent years due to the peculiar electronic and magnetic properties such as the Dirac band, the half-metallicity, and the high Curie temperature. In this Letter, we theoretically investigate the spin transport properties of a recently proposed 2D Dirac spin gapless semiconductor (also known as a Dirac half-metal with zero energy gap in one spin channel) of the Cr2S3 monolayer with the HK lattice. The excellent spin filtering effect and negative differential resistance effect are found at a bias voltage, and interestingly, a temperature difference can also drive the spin filtering effect. These peculiar transport properties can be understood from the Dirac spin gapless semiconductivity and the spin-dependent transmission spectrum. In addition, we predict that, similar to Cr2S3 and Cr2Se3, 2D Cr2Te3 is also a Dirac spin gapless semiconductor with the above room-temperature Curie temperature and a large magneto-crystalline anisotropic energy (MAE). Under a tensile biaxial strain, the MAE can be greatly increased, and the easy magnetization axis is still along the in-plane. All these results are achieved by the first-principles combined with nonequilibrium Green's function method. The present work will stimulate theoretical and experimental studies on spintronic devices and spin caloritronic devices based on more 2D Dirac HK lattices.

Spin caloritronics in two-dimensional CrI3/NiCl2 van der Waals heterostructures

Two-dimensional van der Waals (vdW) heterostructures have recently emerged as attractive candidates to work as spintronic and optoelectronic devices. Here, two types of magnetic ${\mathrm{CrI}}_{3}/{\mathrm{NiCl}}_{2}$ vdW heterostructures are constructed to design spin caloritronic devices. The first-principles calculations uncover that the magnetic configurations of ${\mathrm{CrI}}_{3}/{\mathrm{NiCl}}_{2}$ vdW heterostructures can be converted easily to a ferromagnetic, an antiferromagnetic, and even a bipolar magnetic semiconducting state by an external electric field. More interestingly, two thermal spin-dependent currents with opposite spin orientations can be driven by a temperature gradient to flow in opposite transport directions independently in the different layers of vdW heterostructures, demonstrating that the ${\mathrm{CrI}}_{3}/{\mathrm{NiCl}}_{2}$ vdW heterostructures can exhibit a nearly perfect thermal spin-filtering effect in each layer while generating a well-defined spin-Seebeck effect in the whole system. Our work puts forward a class of material candidates to design spin caloritronic devices characterized by multiple inspiring thermal-spin transport behaviors.

Spin-Seebeck effect and thermoelectric properties of one-dimensional graphene-like nanoribbons periodically embedded with four- and eight-membered rings.

The spin-Seebeck effect together with a high spin thermoelectric conversion efficiency has been regarded as one of the core topics in spin caloritronics. In this work, we propose a spin caloritronic device constructed on hydrogen-terminated sawtooth graphene-like nanoribbons periodically embedded with four- and eight-membered rings to investigate the thermal spin currents and thermoelectric properties by using density functional theory combined with the non-equilibrium Green's function method. Our theoretical results show that spin-Seebeck currents are induced by the temperature gradient between two leads due to two isolated spin-up and spin-down transport channels above or below the Fermi level. Besides, the embedded four- and eight-membered rings break the mirror symmetry of graphene-like nanoribbons and increase the phonon scattering to lower the lattice conductivity, contributing to the enhancement of the spin figure of merit. Moreover, the increasing width of the nanoribbons can effectively enhance the spin-Seebeck currents and reduce their threshold temperatures to improve the device performances. These systematic investigations not only give us an in-depth understanding into the realistic spin caloritronic device applications of graphene-like nanoribbons, but also help us to choose feasible routes to improve the spin-Seebeck effect with a high spin figure of merit in nanostructures.

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.

Resonant nanodiffraction x-ray imaging reveals role of magnetic domains in complex oxide spin caloritronics.

Spin electronic devices based on crystalline oxide layers with nanoscale thicknesses involve complex structural and magnetic phenomena, including magnetic domains and the coupling of the magnetism to elastic and plastic crystallographic distortion. The magnetism of buried nanoscale layers has a substantial impact on spincaloritronic devices incorporating garnets and other oxides exhibiting the spin Seebeck effect (SSE). Synchrotron hard x-ray nanobeam diffraction techniques combine structural, elemental, and magnetic sensitivity and allow the magnetic domain configuration and structural distortion to be probed in buried layers simultaneously. Resonant scattering at the Gd L2 edge of Gd3Fe5O12 layers yields magnetic contrast with both linear and circular incident x-ray polarization. Domain patterns facet to form low-energy domain wall orientations but also are coupled to elastic features linked to epitaxial growth. Nanobeam magnetic diffraction images reveal diverse magnetic microstructure within emerging SSE materials and a strong coupling of the magnetism to crystallographic distortion.

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.

Spin‐Dependent Thermoelectric Transport in Cobalt‐Based Heusler Alloys

AbstractSitting at the intersection of spintronics and thermoelectricity, research investigating the coupling of thermoelectric, magnetic, and electrical transport properties in materials has recently found that the ferromagnetic Heusler alloys are the ideal testbeds. These materials have attracted a lot of attention due to their useful magnetotransport properties and the possibility of tailoring these properties by modifying their composition or producing heterostructures. With the diverse range of interesting phenomena in the Heusler alloys, ferromagnetic Heusler alloys are also ideal candidates for engineering spin caloritronic devices that can take advantage of the interplay of the physics of heat flow, magnetism, and electric potential. The fundamental physical concepts important to spin‐dependent thermoelectrics research are introduced and recent studies of several ferromagnetic Heusler compounds are reviewed, highlighting some exceptional latest experiments on half‐metallic Co2TiSn and the ferromagnetic Weyl semimetal Co2MnGa. Furthermore, the potential to generate useful magnetothermoelectric voltages in electronic devices based on the anomalous Nernst effect is discussed.

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.

Computational study of spin caloritronics in a pristine and defective antimonene nanoribbon

In this paper, by using first-principle density functional theory (DFT) combined with non-equilibrium Green's function (NEGF), thermally induced spin current in zigzag and armchair Antimonene Nanoribbon (SbNR) is investigated. Also, we obtain higher spin current in Armchair nanoribbon (ANR) than zigzag nanoribbon (ZNR), because the start energy of transmission for ANR is closer to the Fermi level than ZNR. The results show that the device has a perfect spin Seebeck effect under temperature difference without gate voltage or bias voltage. For the ANR configuration, the competition between spin up holes and spin down electrons leads to negative differential behavior of charge current, which is essential for the design of thermal transistors. We also investigate the effect of a single vacancy and double vacancy on the structural and electronic properties of ZNR and ANR. The results show that vacancy in ZNR leads to a decrease in spin currents, whereas in ANR, spin currents increase. Double vacancy in ANR can significantly enhance spin up current up to about 2.5 times the pristine one. The results verify the Armchair Nanoribbon as a promising candidate for spin caloritronics devices, which can be used in future low power consumption technology.

Tunable large spin Hall and spin Nernst effects in the Dirac semimetals ZrXY (X=Si,Ge;Y=S,Se,Te)

The ZrSiS-type compounds are Dirac semimetals and have been attracting considerable interest in recent years due to their topological electronic properties and possible applications. In particular, gapped Dirac nodes can possess large spin Berry curvatures and thus give rise to large spin Hall effect (SHE) and spin Nernst effect (SNE), which may be used to generate pure spin current for spintronics and spin caloritronics without applied magnetic field or magnetic material. In this paper we study both SHE and SNE in ZrXY (X = Si, Ge; Y = S, Se, Te) based on \textit{ab initio} relativistic band structure calculations. Our theoretical calculations reveal that some of these compounds exhibit large intrinsic spin Hall conductivity (SHC) and spin Nernst conductivity (SNC). The calculated SHC of ZrSiTe is as large as -755 ($\hbar$/e)(S/cm). Since the electric conductivity of these Dirac semimetals are much smaller than that of platinum which has the largest intrinsic SHC of $\sim$2200 ($\hbar$/e)(S/cm), this indicates that they will have a larger spin Hall angle than that of platinum. Remarkably, we find that both the magnitude and sign of the SHE and SNE in these compounds can be significantly tuned by changing either the electric field direction or spin current direction and may also be optimized by slightly varying the Fermi level via chemical doping. Analysis of the calculated band- and $k$-resolved spin Berry curvatures show that the large SHE and SNE as well as their remarkable tunabilities originate from the presence of many slightly spin-orbit coupling-gapped Dirac nodal lines near the Fermi level in these Dirac semimetals. Our findings suggest that the ZrSiS-type compounds are promising candidates for spintronic and spin caloritronic devices, and will certainly stimulate further experiments on these Dirac semimetals.

Enhancement of temperature change induced by anomalous Ettingshausen effect in thin Ni films on suspended membrane substrates

We demonstrate the effect of substrates on the temperature change induced by the anomalous Ettingshausen effect in ferromagnetic thin films. The experimental results show that, in comparison with conventional experiments using bulk substrates, the temperature modulation due to the anomalous Ettingshausen effect in Ni films can be enhanced by more than an order of magnitude using suspended membrane substrates, which reduce thermal dissipation from the films to the substrates. The temperature modulation can be further enhanced with the help of a simple suspended thermopile configuration consisting of two Ni strips separated by a small distance. These demonstrations will provide a guideline for the thermal design of thin-film-based spin-caloritronic devices.

Distinct Detection of Thermally Induced Spin Voltage in Pt/WS2/Ni81Fe19 by the Inverse Spin Hall Effect.

Conversion of heat into a spin current by means of the spin Seebeck effect (SSE) is one of the exciting topics in spin caloritronics. By use of this technique, the excess heat may be transformed into a valuable electric voltage by coupling SSE with the inverse spin Hall effect (ISHE). In this study, a thermal gradient and an in-plane magnetic field are used as the driving power to mobilize the spin electrons to produce SSE. A spin voltage is detected by ISHE in the Ni81Fe19 heterostructure by means of a WS2/Pt strip. Using WS2 sheets of different thicknesses, we obtained a large spin Seebeck coefficient of 0.72 μV/K, which is 12 times greater than the conventional spin Seebeck coefficient observed in Pt/Ni81Fe19 bilayer devices. We observe the thickness dependence of tungsten disulfide (WS2) flakes and the polarity reversal of pure SSE signals that are measured without influence from the other thermoelectric effects in our Pt/WS2/Ni81Fe19 device-the most intriguing feature of this study. Without the electric charge conduction, the spins are distributed over a longer distance that is greater than the spin diffusion length of the Ni81Fe19 layer. Such features are strongly desired for designing the efficient spin-caloritronics devices that may be used in the thermoelectric spin generators and the temperature sensors such as thermocouples.

Electric-field-induced on–off switching of anomalous Ettingshausen effect in ultrathin Co films

We have investigated the electric field effect on magneto-thermoelectric conversion in ultrathin Co films with a naturally-oxidized surface and a solid-state capacitor structure. By means of the thermoelectric imaging technique based on the lock-in thermography, we demonstrate the reversible on–off switching of heat currents generated by the anomalous Ettingshausen effect in response to the electric-field-induced magnetic phase transition in the Co films. The electric-field-induced switching property is found to be tuned by changing the Co thickness. Our finding will provide a method for reconfigurable and pin-point switching of thermoelectric conversion properties in spin-caloritronic devices.

Theoretical study on spintronic and spin caloritronic applications for equiatomic quaternary Heusler alloy NiCoMnAl

Equiatomic quaternary Heusler alloys (EQHAs) have received more and more attention in spintronic applications due to the half-metallicity and the long spin diffusion length. Recently, nearly half-metallicity (nearly 100% electronic spin polarization) and high Curie temperature (above 583 K) have been found in the equiatomic quaternary Heusler alloy of NiCoMnAl. In order to explore the potential spintronic applications, in this work, we design the magnetic tunnel junction (MTJ) of NiCoMnAl/InP/NiCoMnAl(0 0 1), and theoretically investigate the spin transport properties. It is shown that the MTJ exhibits a spin filtering effect and a high tunnel magnetoresistance ratio (up to 1.4 × 107%) when a bias voltage is applied to the electrodes of NiCoMnAl. Interestingly, without a bias voltage, a temperature difference can also drive the spin filtering effect. These versatile transport properties are discussed in terms of the half-metallicity of electrode, the spin-polarized transmission spectrum, and the difference of Fermi–Dirac distribution. The present work indicates promising applications for NiCoMnAl in spintronic devices and spin caloritronic devices.

First Principle Study of Temperature-Dependent Magnetoresistance and Spin Filtration Effect in WS2 Nanoribbon.

An applicable use of density functional theory (DFT) along with nonequilibrium Green's function (NEGF) is done for exploring the temperature-dependent spin electron transport nature in a ferromagnetic tungsten disulfide (WS2) nanoribbon. To demonstrate the effect of temperature on spin filtration and spin Seebeck effect, we evaluated vital parameters such as spin-polarized current and spin filtration efficiency. Spin filtration efficiency of around ∼95% is obtained in the high-temperature difference range. The high temperature (TL) of the left electrode in comparison to the high temperature (TR) of the right electrode results in higher and lower spin filtration efficiency in parallel magnetization (PM) and antiparallel magnetization (APM), respectively. Transmission spectrum plots at equilibrium are also calculated in PM and APM to justify the temperature-dependent spin transport behavior in the WS2 nanoribbon. Giant thermal magnetoresistance around 1.934 × 103% is achieved. The temperature-dependent negative differential resistance behavior of the current plot has been observed. Huge value of thermal magnetoresistance (MR) and excellent spin filtration obtained for WS2 nanoribbon suggests the potential application of this material in spin caloritronic devices.

Thermoelectric transport properties of magnetic carbon-based organic chains

We propose a perfect spin-caloritronics device based on a carbon-based organic chain. A spin-semiconducting property is achieved, which originates from the edge localized states. The appearance of spin-dependent transport gaps results in a large spin Seebeck coefficient. Moveover, the dimensionless spin thermoelectric figure of merit (FOM) at room temperature can be enhanced to as high as 35. Furthermore, the pure spin current or single-spin current can be produced at some chemical potentials under a temperature difference, and their transport directions can also be tuned by the chemical potential. Therefore, the carbon-based organic chain is well suited for designing the multifunctional spin-caloritronics devices.

Molecular Boolean logic gates based on spin caloritronic transport properties of planar four-coordinate Fe complex-based molecular devices

Based on density functional theory and non-equilibrium Green’s function technique, we design planar four-coordinate Fe complexes-based molecular spin caloritronic devices with carbon nanotube bridges and electrodes, and investigate their spin caloritronic transport properties. The results show the thermally-driven spin currents are strongly dependent on the external magnetic field modulation. The underlying mechanisms are elucidated by Fermi-Dirac distribution function, spin-resolved transmission spectra, current spectra, spatial distributions of molecular projected self-consistent Hamiltonian orbitals. On the basis of these interesting spin caloritronic transport properties, we further propose the thermal spin molecular Boolean logic gates, such as AND, NOT, XOR and OR.

Exploration of Spin-Dependent Thermoelectricity in the Chiral Double-Stranded DNA Molecule Coupled to Ferromagnetic Leads

Spin selectivity in double-stranded DNA (dsDNA) molecules is unambiguously linked to their intrinsic chirality. Nevertheless, how the chiral-induced spin selectivity (CISS) effect in dsDNA influences spin-dependent thermoelectricity needs to be understood well, to comprehend the relationship between spin and heat transport. To this end, the authors explore the spin Seebeck effect of a chiral dsDNA-based magnetic junction, and elucidate the underlying physics relevant to the CISS effect by means of linear-response theory and nonequilibrium Green's functions. Their results provide valuable guidelines for engineering innovative spin-caloritronic devices based on chiral organic molecules.