Giant Spin Pumping at Polymer/Ferromagnet Interfaces for Hybrid Spintronic Devices

Determination of spin Hall effect and spin diffusion length of Pt from self-consistent fitting of damping enhancement and inverse spin-orbit torque measurements (Conference Presentation)

Materials possessing pronounced spin–orbit coupling (SOC) are fundamental to the development of spin–orbit torque-driven magnetic memory devices, as they facilitate efficient manipulation of spin currents essential for device operation. Transition metal dichalcogenides are promising candidates for such applications because of their inherently high SOC strength. This work investigates the interfacial spin pumping effect between a monolayer of molybdenum disulfide (ML-MoS2) and Ni80Fe20 (Py) thin films, using broadband ferromagnetic resonance (FMR) spectroscopy. FMR measurements reveal a notable enhancement in the effective damping factor for the ML-MoS2/Py (Pt = 0 nm) interface compared to the reference Py thin films, attributed to spin pumping across the ML-MoS2/Py interface. To quantify spin pumping efficiency, we insert a high SOC platinum (Pt) interlayer at the ML-MoS2/Py interface and systematically vary its thickness (1, 2, and 5 nm). This enables the evaluation of key spin transport parameters, including the enhancement of the effective damping parameter and the effective spin mixing conductance, which reflects the transfer of spin angular momentum from Py to the ML-MoS2, thereby allowing us to determine the effective spin current density. The results indicate that ML-MoS2 in combination with Pt layers of 1, 2, and 5 nm thickness is well suited for spintronic applications with promising potential for energy-efficient memory and logic devices.

Spin pumping has been considered a powerful tool to manipulate the spin current in a ferromagnetic/nonmagnetic (FM/NM) system, where the NM part exhibits large spin-orbit coupling (SOC). In this work, the spin pumping in β-W/Interlayer (IL)/Co2FeAl (CFA) heterostructures grown on Si(100) is systematically investigated with different ILs in which SOC strength ranges from weak to strong. We first measure the spin pumping through the enhancement of effective damping in CFA by varying the thickness of β-W. The damping enhancement in the bilayer of β-W/CFA (without IL) is found to be ∼50% larger than the Gilbert damping in a single CFA layer with a spin diffusion length and spin mixing conductance of 2.12 ± 0.27 nm and 13.17 ± 0.34 nm-2, respectively. Further, the ILs of different SOC strengths such as Al, Mg, Mo, and Ta were inserted at the β-W/CFA interface to probe their impact on damping in β-W/ILs/CFA. The effective damping reduced to 8% and 20% for Al and Mg, respectively, whereas it increased to 66% and 75% with ILs of Mo and Ta, respectively, compared to the β-W/CFA heterostructure. Thus, in the presence of ILs with weak SOC, the spin pumping at the β-W/CFA interface is suppressed, while for the high SOC ILs effective damping increased significantly from its original value of β-W/CFA bilayer using a thin IL. This is further confirmed by performing inverse spin Hall effect measurements. In summary, the transfer of spin angular momentum can be significantly enhanced by choosing a proper ultrathin interface layer. Our study provides a tool to increase the spin current production by inserting an appropriate thin interlayer which is useful in modifying the heterostructure for efficient performance in spintronics devices.

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

We systematically investigate the spin-pumping effect in a series of yttrium-iron-garnet (YIG)--platinum (Pt) bilayers, comparing both broadband ferromagnetic resonance and electrically detected spin-pumping experiments. We infer the effective spin-mixing conductance as a function of YIG and Pt layer thickness from ferromagnetic resonance measurements on pure YIG layers as well as on bilayer YIG/Pt samples in a frequency range from 2 to 20 GHz. In bare YIG films, we determine a Gilbert damping constant of approximately $\ensuremath{\alpha}=0.001$. With an additional platinum layer on top of the YIG samples, the damping is significantly enhanced. The effective spin-mixing conductance extracted from the ferromagnetic resonance experiments is in agreement with that determined from independent electrically detected spin-pumping experiments.

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

Spin pumping has significant implications for spintronics, providing a mechanism to manipulate and transport spins for information processing. Understanding and harnessing spin currents through spin pumping is critical for the development of efficient spintronic devices. The use of a magnetic insulator with low damping enhances the signal-to-noise ratio in crucial experiments such as spin-torque ferromagnetic resonance (FMR) and spin pumping. A magnetic insulator coupled with a heavy metal or quantum material offers a more straightforward model system, especially when investigating spin-charge interconversion processes to greater accuracy. This simplicity arises from the absence of unwanted effects caused by conduction electrons unlike in ferromagnetic metals. Here, we investigate the spin pumping in coupled ferrimagnetic (FiM) Y3Fe5O12 (YIG)/Tm3Fe5O12 (TmIG) bilayers combined with heavy-metal (Pt) using the inverse spin Hall effect. It is observed that magnon transmission occurs at both of the FiMs FMR positions. The enhancement of spin pumping voltage (Vsp) in the FiM garnet heterostructures is observed. The plausible reason might be the interfacial exchange coupling between FiMs. The modulation of Vsp is achieved by tuning the bilayer structure. Further, the spin mixing conductance for these coupled systems is found to be ≈1018 m−2. Our findings describe a coupled FiM system for the investigation of magnon coupling providing opportunities for magnonic devices.

In recent years, flexible electronic devices have become a hot topic of scientific research. These flexible devices are the basis of flexible circuits, flexible batteries, flexible displays and electronic skins. Graphene-based materials are very promising for flexible electronic devices, due to their high mobility, high elasticity, a tunable band gap, quantum electronic transport and high mechanical strength. In this article, we review the recent progress of the fabrication process and the applications of graphene-based electronic devices, including thermal acoustic devices, thermal rectifiers, graphene-based nanogenerators, pressure sensors and graphene-based light-emitting diodes. In summary, although there are still a lot of challenges needing to be solved, graphene-based materials are very promising for various flexible device applications in the future.

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. **

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.

Spin pumping and spin-transfer torques are two reciprocal phenomena widely studied in ferromagnetic materials. However, pumping from antiferromagnets and its relation to current-induced torques have not been explored. By calculating how electrons scatter off a normal metal-antiferromagnetic interface, we derive pumped spin and staggered spin currents in terms of the staggered field, the magnetization, and their rates of change. For both compensated and uncompensated interfaces, spin pumping is of a similar magnitude as in ferromagnets with a direction controlled by the polarization of the driving microwave. The pumped currents are connected to current-induced torques via Onsager reciprocity relations.

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. **

Using Y3Fe5O12 (YIG) thin films grown by our sputtering technique, we study dynamic spin transport in nonmagnetic, ferromagnetic, and antiferromagnetic (AF) materials by ferromagnetic resonance spin pumping. From both inverse spin Hall effect and damping enhancement, we determine the spin mixing conductance and spin Hall angle in many metals. Surprisingly, we observe robust spin conduction in AF insulators excited by an adjacent YIG at resonance. This demonstrates that YIG spin pumping is a powerful and versatile tool for understanding spin Hall physics, spin-orbit coupling, and magnetization dynamics in a broad range of materials.

We present a study of the effective magnetization Meff and the effective damping parameter αeff by means of ferromagnetic resonance spectroscopy on the ferromagnetic (FM) materials Ni81Fe19 (NiFe) and Co40Fe40B20 (CoFeB) in FM/Pt, FM/NM, and FM/NM/Pt systems with the non-magnetic (NM) materials Ru, Cr, Al, and MgO. Moreover, for NiFe layer systems, the influence of interface effects is studied by way of thickness dependent measurements of Meff and αeff. Additionally, spin pumping in NiFe/NM/Pt is investigated by means of inverse spin Hall effect (ISHE) measurements. We observe a large dependence of Meff and αeff of the NiFe films on the adjacent NM layer. While Cr and Al do not induce a large change in the magnetic properties, Ru, Pt, and MgO affect Meff and αeff in different degrees. In particular, NiFe/Ru and NiFe/Ru/Pt systems show a large perpendicular surface anisotropy and a significant enhancement of the damping. In contrast, the magnetic properties of CoFeB films do not have a large influence of the NM adjacent material and only CoFeB/Pt systems present an enhancement of αeff. However, this enhancement is much more pronounced in NiFe/Pt. By the introduction of the NM spacer material, this enhancement is reduced. Furthermore, a difference in symmetry between NiFe/NM/Pt and NiFe/NM systems in the output voltage signal from the ISHE measurements reveals the presence of spin pumping into the Pt layer in all-metallic NiFe/NM/Pt and NiFe/Pt systems.

Studies of spin current injection, transport, and interface control have drawn attention recently for efficient organic spintronic devices. In this study, we apply both spin pumping (SP) and the longitudinal spin Seebeck effect (LSSE) to inject spin currents into a π-conjugated organic semiconductor, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA), and characterize injection and transport by measuring inverse spin Hall voltage VISHE in spin detectors. A normalization factor introduced to SP analysis eliminates a contribution provoked by deviation of spin sources and leads to a more accurate determination of the spin diffusion length in PTCDA. While SP with Permalloy as a spin source is effective in generating detectable VISHE, the LSSE from yttrium iron garnet shows no convincing sign of spin injection. In addition, spin-flip scattering induced by hybrid states undermining electrical spin injection is negligible in SP. These results are attributed to interfaces between spin sources and PTCDA, indicative of the importance of injection methods and material choices.