Effective spin injection into the organic semiconductor PTCDA evaluated by a normalization method

The spin Seebeck effect refers to the generation of spin voltage as a result of a temperature gradient in ferromagnetic or ferrimagnetic materials. When a conductor is attached to a magnet under a temperature gradient, the thermally generated spin voltage in the magnet injects a spin current into the conductor, which in turn produces electric voltage owing to the spin-orbit interaction. The spin Seebeck effect is of increasing importance in spintronics, since it enables direct generation of a spin current from heat and appears in a variety of magnets ranging from metals and semiconductors to insulators. Recent studies on the spin Seebeck effect have been conducted mainly in paramagnetic metal/ferrimagnetic insulator junction systems in the longitudinal configuration in which a spin current flowing parallel to the temperature gradient is measured. This ‘longitudinal spin Seebeck effect’ (LSSE) has been observed in various sample systems and exclusively established by separating the spin-current contribution from extrinsic artefacts, such as conventional thermoelectric and magnetic proximity effects. The LSSE in insulators also provides a novel and versatile pathway to thermoelectric generation in combination of the inverse spin-Hall effects. In this paper, we review basic experiments on the LSSE and discuss its potential thermoelectric applications with several demonstrations.

This work investigates the longitudinal spin Seebeck effect (LSSE) in ferromagnetic Fe/Pt bilayer systems, examining the role of Fe layer thickness and substrate type on spin current dynamics and signal characteristics. Thin Fe films (3–20 nm) were sputtered onto Si/SiO2 and Si(B) substrates and analyzed for their structural, magnetic, and spin transport behaviors. The study identifies an optimal Fe thickness below 5 nm for effective spin injection, corresponding to a spin diffusion length of 4.7 nm, a spin Hall angle of 0.094, and a spin injection coefficient of −1.6 V (K·Ω·m)–1. Beyond 15 nm, the LSSE signal reverses due to dominant shunting effects. Substrate interactions significantly affect spin scattering, particularly at the Fe/Si(B) interface, where an insulating layer is necessary to stabilize magnetic and spin properties. These results provide insights into optimizing ferromagnetic metal-based LSSE systems for advanced spintronic applications.

The discovery of magnetic order in atomically-thin van der Waals materials has strengthened the alliance between spintronics and two-dimensional materials. An important use of magnetic two-dimensional materials in spintronic devices, which has not yet been demonstrated, would be for coherent spin injection via the spin-pumping effect. Here, we report spin pumping from Cr2Ge2Te6 into Pt or W and detection of the spin current by inverse spin Hall effect. The magnetization dynamics of the hybrid Cr2Ge2Te6/Pt system are measured, and a magnetic damping constant of ~ 4–10 × 10−4 is obtained for thick Cr2Ge2Te6 flakes, a record low for ferromagnetic van der Waals materials. Moreover, a high interface spin transmission efficiency (a spin mixing conductance of 2.4 × 1019/m2) is directly extracted, which is instrumental in delivering spin-related quantities such as spin angular momentum and spin-orbit torque across an interface of the van der Waals system. The low magnetic damping that promotes efficient spin current generation together with high interfacial spin transmission efficiency suggests promising applications for integrating Cr2Ge2Te6 into low-temperature two-dimensional spintronic devices as the source of coherent spin or magnon current.

The longitudinal spin Seebeck effect (LSSE) has been investigated in ZnFe2O4 (ZFO) thin films on different substrates, such as Si (111), MgO (100), and SrTiO3 (100). The LSSE voltage signal exhibited a linear dependence on the temperature difference between both sides of the samples. The spin Seebeck coefficients were highly sensitive to the thermal conductivities of the magnetic layer and substrate, with values from 3 nV/k to 110 nV/K. Charge currents ({J}_{mathrm{c}}) and spin currents ({J}_{mathrm{s}}) densities were estimated. {J}_{mathrm{s}} values are similar for the samples deposited on MgO and STO. The saturation magnetic field for the LSSE signal was reached with a magnetic field lower than its bulk counterpart. These results were related to the lattice mismatch between the ZFO films and the substrate, the magnetic response, and the anisotropies present in the samples since a twofold in-plane anisotropy was observed for the sample deposited on Si, while the samples deposited on MgO and SrTiO3 showed a fourfold in-plane anisotropy. Moreover, the presence of a notable perpendicular magnetic anisotropy in the thin films may be a consequence of the lattice mismatch and at the same time, it can cause the saturation in the LSSE voltage hysteresis loop measured as a function of the external magnetic be reached with a low magnetic field.

The Y3Fe5O12 (YIG) ferrimagnetic insulator is recognized as a model system for spintronic and magnonic phenomena, including spin pumping, spin Seebeck, proximity effects, and spin wave propagation. In these materials, the spin current is produced by spin-waves or magnons. Magnons can be electrically excited and sensed via the Spin Hall Effect (SHE) and Anomalous Spin Hall Effect (ASHE) or they can be thermally excited through the Spin Seebeck Effect (SSE). To detect the spin current, a thin non-magnetic layer such as Pt is deposited on top of the material of interest. This transforms the spin current into an observable thermoelectric voltage (VSSE) via the Inverse Spin Hall Effect (ISHE). Pt/YIG systems allow efficient spin-charge conversion and pure detection of spin-current effects, respectively, due to their unique magneto-optical and spin dynamic properties [1]. These properties allow a clean detection of the pure spin current thanks to the spin-orbit effects in the Pt/YIG microstructures [2]. Additionally, Rare Earth (RE) garnet films are magnetic and magnetoelastic, and their properties can be manipulated through choice of composition and substrate. When a proportion of bismuth (Bi) is added, the magneto-optical properties of YIG increase according to the concentration of bismuth (Bi) at the yttrium site [3]. But there are few studies on how the spin dynamic properties are modified due to this substitution. In this work we study different parameters in the manufacture of ferrimagnetic/non-magnetic bilayers to raise the thermoelectric spin voltage VSSE.Samples with different thicknesses of pure Y3Fe5O12 (YIG) films and a 5% Bi substitution, that is, thin films of BiY2Fe5O12 (BiYIG), were grown on Gd3Ga5O12 (111) and Gd3Sc2Ga3O12 (111) substrates by pulsed laser deposition (PLD). XRD, HR-TEM / STEM-HAADF and GPA measurements revealed high quality crystalline films as it can be seen in Figure 1. Ferromagnetic resonance (FMR) measurements were performed at room temperature for all heterostructures. A resonance field (Hres) and a resonance linewidth (FWHM) were extracted and are shown in Figure 2 as a function of frequency. These values allow the calculation of a Gilbert damping coefficient that is directly correlated with the SSE according to Chang et. Al [4]. In our case, the Gilbert damping coefficient is (5-14)x10-4 for the YIG//GGG samples and (18-19) x10-4 for the YIG//SGGG samples. The linewidth of BiYIG samples is much wider. After growth of an 8 nm-Pt coating layer by sputtering, the damping in all the samples increases due to the contribution due to spin pumping from the YIG or BiYIG to the metal film. The difference between the Heff result and the bulk value of 4πMS reported (1750 G [5]) demonstrates the existence of a perpendicular anisotropy.With a maximum 20K thermal difference across the entire structure, we measure longitudinal SSE voltages in both YIG and BiYIG samples with sputtered Pt. The YIG//SGGG combination reported to have the best voltage signal from the Inverse spin-Hall effect (VISHE) and Spin Seebeck Effect (VSSE), along with the co-occurrence of a positive perpendicular anisotropy. In the case of the BiYIG samples, the voltages induced in the BiYIG//SGGG bilayers were lower with respect to YIG//SGGG but higher with respect to YIG//GGG. The samples with the lowest perpendicular magnetic anisotropy showed the highest VSSE in both cases, YIG and BiYIG.This work is funded thanks to the SPICOLOST (GA 734187) project framed within “Horizon 2020 Funding” of the European Union with the MARIE SKLODOWSKA-CURIE - RISE Actions (Research and Innovation Staff Exchanges) program coordinated by the University of Zaragoza. **

A thermal gradient as the driving force for spin currents plays a key role in spin caloritronics. In this field the spin Seebeck effect (SSE) is of major interest and was investigated in terms of in-plane thermal gradients inducing perpendicular spin currents (transverse SSE) and out-of-plane thermal gradients generating parallel spin currents (longitudinal SSE). Up to now all spincaloric experiments employ a spatially fixed thermal gradient. Thus, anisotropic measurements with respect to well defined crystallographic directions were not possible. Here we introduce a new experiment that allows not only the in-plane rotation of the external magnetic field, but also the rotation of an in-plane thermal gradient controlled by optical temperature detection. As a consequence, the anisotropic magnetothermopower and the planar Nernst effect in a permalloy thin film can be measured simultaneously. Thus, the angular dependence of the magnetothermopower with respect to the magnetization direction reveals a phase shift, that allows the quantitative separation of the thermopower, the anisotropic magnetothermopower and the planar Nernst effect.

Injection of spin currents into solids is crucial for exploring spin physics and spintronics. There has been significant progress in recent years in spin injection into high-resistivity materials, for example, semiconductors and organic materials, which uses tunnel barriers to circumvent the impedance mismatch problem; the impedance mismatch between ferromagnetic metals and high-resistivity materials drastically limits the spin-injection efficiency. However, because of this problem, there is no route for spin injection into these materials through low-resistivity interfaces, that is, Ohmic contacts, even though this promises an easy and versatile pathway for spin injection without the need for growing high-quality tunnel barriers. Here we show experimental evidence that spin pumping enables spin injection free from this condition; room-temperature spin injection into GaAs from Ni(81)Fe(19) through an Ohmic contact is demonstrated through dynamical spin exchange. Furthermore, we demonstrate that this exchange can be controlled electrically by applying a bias voltage across a Ni(81)Fe(19)/GaAs interface, enabling electric tuning of the spin-pumping efficiency.

Using a minimum model consisting of a magnetic quantum dot and an electron lead, we investigate spin pumping by its precessing magnetization. Focusing on the "adiabaticity", which is quantified using a comparison between the frequency of precession and the relaxation rate of the relevant system, we investigate the role of nonadiabaticity in spin pumping by obtaining the dependence of the spin current generated on the frequency of precession using full counting statistics. This evaluation shows that the steady-state population of the quantum dot remains unchanged by the precession owing to the rotational symmetry about the axis of precession. This implies that in the adiabatic limit the spin current is absent and that spin pumping is entirely a nonadiabatic effect. We also find that the nonadiabatic spin current depends linearly on the frequency in the low-frequency regime and exhibits an oscillation in the high-frequency regime. The oscillation points to an enhancement of spin pumping by tuning the frequency of precession.

Spin currents, which are excited in indium tin oxide (ITO)/yttrium iron garnet (YIG) by the methods of spin pumping and spin Seebeck effect, are investigated through the inverse spin Hall effect (ISHE). It is demonstrated that the ISHE voltage can be generated in ITO by spin pumping under both in-plane and out-of-plane magnetization configurations. Moreover, it is observed that the enhancement of spin Hall angle and interfacial spin mixing conductance can be achieved by an appropriate annealing process. However, the ISHE voltage is hardly seen in the presence of a longitudinal temperature gradient. The absence of the longitudinal spin Seebeck effect can be ascribed to the almost equal thermal conductivity of ITO and YIG and specific interface structure, or to the large negative temperature dependent spin mixing conductance.

Analogous to the spin-Hall effect (SHE), abinitio electronic structure calculations reveal that acoustic phonons can induce charge (spin) current flowing along (normal to) its propagation direction. Using the Floquet approach we have calculated the elastodynamically induced charge and spin pumping in bulk Pt and demonstrate that (i)the longitudinal charge pumping originates from the Berry curvature, while the transverse pumped spin current is an odd function of the electronic relaxation time and diverges in the clean limit. (ii)The longitudinal charge current is of nonrelativstic origin, while the transverse spin current is a relativistic effect that to lowest order scales linearly with the spin-orbit coupling strength. (iii)Both charge and spin pumped currents have parabolic dependence on the amplitude of the elastic wave.

The effect of graphene interlayer at Pt/YIG interface on spin pumping

In the framework of the longitudinal spin Seebeck effect (LSSE), we developed an experimental setup for the characterization of LSSE devices. This class of device consists in a layered structure formed by a substrate, a ferrimagnetic insulator (YIG) where the spin current is thermally generated, and a paramagnetic metal (Pt) for the detection of the spin current via the inverse spin-Hall effect. In this kind of experiments, the evaluation of a thermal gradient through the thin YIG layer is a crucial point. In this work, we perform an indirect determination of the thermal gradient through the measurement of the heat flux. We developed an experimental setup using Peltier cells that allow us to measure the heat flux through a given sample. In order to test the technique, a standard LSSE device produced at Tohoku University was measured. We find a spin Seebeck SSSE coefficient of 2.8×10−7 V K−1.

All-optical investigation of anisotropic spin pumping in W/CoFeB/W heterostructure

The spin Hall effect (SHE) is closely related to the spin-orbit coupling. Therefore, heavy metals are usually considered as materials of choice to obtain large spin Hall angles. Among these materials, some 5d transition metals such as Pt, Ta or W offer the largest spin-charge interconversion efficiency for spin current detections through the inverse spin Hall effect or to obtain efficient current induced magnetization switching in SOT-MRAM devices [1].Despite a weaker spin orbit coupling, spin Hall angles comparable to that of the best 5d materials has also been obtained in 3d ferromagnetic metals such as Co [2] or Ni [3]. More recently nonmagnetic alloys containing Nickel atoms such as NiCu [4] and CoNiB [5] attracted great interest for their large spin Hall angles and their potential for spintronic applications. Using the spin pumping by ferromagnetic resonance technique we could compare the spin to charge current conversion in Ni60Cu40 to the one of Pt. As can be seen in figure 1 a) the normalized spin pumping signal obtained in CFB/NiCu is comparable to that of CFB/Pt. We performed a thickness dependence shown in figure 1 b) to extract the spin Hall angle (θSHE) and spin diffusion length (λs) and obtained results comparable to the one of 5d metals using similar method with λs = 2.4 ± 0.3 nm and θSHE = 4.1 ± 0.5 % [6].Keller and coworkers [4] noticed similarities in the bandstructure of paramagnetic Nickel and Platinum possibly explaining the origin of the effect. The calculated band structure of ferromagnetic Nickel does not resemble that of Platinum and the origin of the large spin Hall effect observed in ferromagnetic Ni [3] could thus be different. The interplay between the spin-orbit coupling and magnetism is known to be at the origin of specific transport properties such as the anomalous magnetoresistance and the anomalous Hall effect. However, the link between the SHE and the magnetic order is complex and still unexplored [7].In order to study the effect of the phase transition we performed spin pumping as a function of temperature in Ni60Cu40 and Ni50Cu50 alloys above and below the Curie temperature. To avoid direct contact between the ferromagnetic spin injector CFB and the magnetic NiCu alloy that would modify CFB resonance properties due to the direct coupling [8] we used a 5 nm thick Cu interlayer. The spin pumping measurements performed in CFB\Cu\ Ni60Cu40 and CFB\Cu\ Ni50Cu50 shown in figure 2 indicate that in NiCu the SHE is insensitive to the magnetic order [6].A large variety of alloys composed of Ni, Co or Fe in the ferromagnetic or paramagnetic phase could be explored as spin current generators and detectors, extending the number of possible light metal systems with a large SHE. In particular, the large spin Hall effect in NiCu shows that it is a promising alternative to obtain large spin orbit torque without heavy metals. **

Various spintronic phenomena originate from the exchange of angular momentum between the spin of electrons and other degrees of freedom in crystalline materials. Many degrees of freedom, such as magnetization1 and mechanical motion2, have already been united into this exchange framework. However, the nuclear spin—a key angular momentum—has yet to be incorporated. Here we observe spin pumping from nuclear magnetic resonance (NMR), in which nuclear spin dynamics emits a spin current, a flow of spin angular momentum of electrons. By using the canted antiferromagnet MnCO3, in which typical nuclear spin-wave formation is established due to the reinforced hyperfine coupling, we find that a spin current is generated from an NMR. Nuclear spins are indispensable for quantum information technology3 and are also frequently used in various sensors, such as in magnetic resonance imaging4. The observed NMR spin pumping allows spin-current generation from nuclei and will enable spintronic detection of nuclear spin states. Spin current is generated by pumping from nuclear spin waves. The nuclear magnetic resonance is used to transfer angular momentum from the nuclei of an antiferromagnet to a propagating spin current that is subsequently collected in a distant electrode.