FMR and Thermal Spin Pumping in Garnets|Pt Bilayers

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.

Spin Seebeck effect refers to the creation of spin currents due to a temperature gradient in the magnetic materials or across magnet-normal metal interfaces, which can be electrically detected through the inverse spin Hall effect when in contact with heavy metals. It offers fundamental insights into the magnetic properties of materials, including the magnetic phase transition, static magnetic order, and magnon excitations. The behavior of the spin Seebeck effect across the spin-flop transition has been extensively studied, whereas the spin Seebeck effect across the spin-flip transition remains poorly understood. Here, we demonstrate the spin Seebeck effect in a weakly exchange-coupled van der Waals antiferromagnet CrPS4. The spin Seebeck effect increases as the magnetic field increases before the spin-flip transition due to the enhancement of the thermal spin current as a function of the applied field. A peak of spin Seebeck effect is observed at the spin-flip field, which is related to the magnon mode edges across the spin-flip field. Our results extend spin Seebeck effect research to van der Waals antiferromagnets and demonstrate an enhancement of spin Seebeck effect at the spin-flip transition.

In ferromagnetic metals, the interconversion of spin and charge currents via the spin Hall effect and its inverse can depend on the angle between the ferromagnets magnetization and the spin current polarization direction. Here, such a spin-dependent (inverse) spin Hall effect is found in the ferromagnetic alloy Co$_{60}$Fe$_{20}$B$_{20}$. In a nonlocal magnon transport experiment, Co$_{60}$Fe$_{20}$B$_{20}$ is used to both excite and detect magnonic spin currents flowing in the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$. We find that the signal amplitude is significantly modulated by tuning the direction of the Co$_{60}$Fe$_{20}$B$_{20}$ magnetization. We design a sample structure that completely prevents direct magnonic coupling between the ferromagnets. Thus, we can identify unambiguously an intrinsic electronic origin of the observed effect.

An insulator‐based spin‐thermoelectric (STE) generator is composed of a thin paramagnetic metal (PM: Pt) layer for generating the STE voltage VSTE via the inverse spin Hall effect and a ferrimagnetic insulator (FMI) film or slab used for producing spin‐wave spin currents due to a temperature gradient . Yttrium iron garnet (YIG) is a key material used for the FMI of STE generators. We examined in detail the ferromagnetic resonance (FMR) properties of YIG (Y3Fe5O12), Bi‐substituted YIG (Y3‐xBixFe5O12) and Bi‐ and Al‐substituted YIG (Y3‐xBixFe5‐yAlyO12) films grown by liquid phase epitaxy (LPE). Bi‐substituted YIG films exhibited the large FMR damping and high STE voltage when the films were incorporated in the STE generator. The LPE‐grown Y2.35Bi0.65Fe5O12 film exhibited the FMR linewidth ΔH of 30 mT and the VSTE of 70 μV for the ∇T of . This paper comprehensively presents the origin of the STE generation in the insulator‐based generators on the basis of the results of FMR and STE measurements. To clarify the origin of STE generation in the generators fabricated from single crystal YIG (YIG, Bi‐substituted YIG and Bi‐ and Al‐substituted YIG) films grown by LPE, three principal factors are explained: (a) thermal energy transfer from the phonon system to the spin system, which strengthens the heat excitation of spin precession, through the spin–orbit coupling enhanced by the growth‐induced magnetic anisotropy of LPE‐grown YIG films, (b) in the YIG films incorporated in the STE generators, the generation of spin‐wave spin currents owing its origin to the ferromagnetic spin exchange interaction acting between neighboring spins due to , and (c) how the spin currents pumped into the Pt layer from the YIG film at the Pt/YIG bilayer interface are affected by the magnetization processes of single crystal YIG films. It is concluded that the STE generation of insulator‐based STE generators can be explained from a viewpoint of the spin dynamics under the effect of in the Pt/YIG bilayer structure. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimulating a growing interest in spin caloritronics, a field that exploits the interaction between spin and heat currents 1,2 . Amongst the most intriguing phenomena is the spin Seebeck effect 3-5 , in which a thermal gradient gives rise to spin currents that are detected through the inverse spin Hall effect 6-8 . Non-magnetic materials such as graphene are also relevant for spin caloritronics, thanks to efficient spin transport 9-11 , energy-dependent carrier mobility and unique density of states 12,13 . Here, we propose and demonstrate that a carrier thermal gradient in a graphene lateral spin valve can lead to a large increase of the spin voltage near to the graphene charge neutrality point. Such an increase results from a thermoelectric spin voltage, which is analogous to the voltage in a thermocouple and that can be enhanced by the presence of hot carriers generated by an applied current 14-17 . These results could prove crucial to drive graphene spintronic devices and, in particular, to sustain pure spin signals with thermal gradients and to tune the remote spin accumulation by varying the spin-injection bias.

We report the observation of magnetic-field-induced suppression of the spin Peltier effect (SPE) in a junction of a paramagnetic metal Pt and a ferrimagnetic insulator ${\rm Y_{3}Fe_{5}O_{12}}$ (YIG) at room temperature. For driving the SPE, spin currents are generated via the spin Hall effect from applied charge currents in the Pt layer, and injected into the adjacent thick YIG film. The resultant temperature modulation is detected by a commonly-used thermocouple attached to the Pt/YIG junction. The output of the thermocouple shows sign reversal when the magnetization is reversed and linearly increases with the applied current, demonstrating the detection of the SPE signal. We found that the SPE signal decreases with the magnetic field. The observed suppression rate was found to be comparable to that of the spin Seebeck effect (SSE), suggesting the dominant and similar contribution of the low-energy magnons in the SPE as in the SSE.

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. **

Spin pumping in ferromagnets provides a source of pure spin currents. Via the\ninverse spin Hall effect a spin current is converted into a charge current and\na corresponding detectable DC-voltage. The ratio of injected spin current to\nresulting charge current is given by the spin Hall angle. However, the number\nof experiments more or less equals the number of different values for spin Hall\nangles, even for the most studied normal metal platinum. This publication\nprovides a full study of inverse spin Hall effect and anisotropic\nmagnetoresistance for different NiFe(Py) / normal metal bilayers using a\ncoplanar waveguide structure. Angle and frequency dependent measurements\nstrongly suggest that spin pumping and inverse spin Hall effect can be used to\nquantify spin Hall angles only if certain conditions are met. Ruling out the\nanisotropic magnetoresistance as a parasitic voltage generating effect\nmeasurements of the inverse spin Hall effect in Py/Pt and Py/Au yield spin Hall\nangles of 0.09 and 0.008 respectively. Furthermore, DC-voltages at\nferromagnetic resonance for Py/Pt are studied as a function of temperature and\nthe results are compared to theoretical models.\n

Spin Hall effect and its reciprocal, the inverse spin Hall effect (ISHE), provide the means for conversion between spin and charge currents [1, 2]. In particular, the ISHE transforms a pure spin current into a transverse charge current due to spin-orbit coupling. Recently, the decisive role of the ISHE effect on extending the field of spintronics into the terahertz (THz) regime was revealed [3, 4]. Here, we demonstrate the generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses of Fe (2 nm) / Pt (3 nm) were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer [5]. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals. Furthermore, we correlate the interface structural properties with the THz-E-field amplitude and the bandwidth of the THz radiation. By allowing the Pt layers to grow along different crystallographic directions on top of Fe we modify the THz emission characteristics of the optimized emitters. We present a theoretical model which correlates the loss of energy of the hot electrons and the electron-phonon/defect scattering lifetime at the Fe/Pt interface with the ISHE current that causes the THz emission. By taking into account the response function of the THz detector we describe the influence of the crystal structure of Pt onto the THz signal shape and spectrum. The demonstration of the role of the individual layer thicknesses and of the interface quality in the THz emission spectra paves the way for more efficient spintronic THz emitters.

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 spin Seebeck effect refers to the conversion of a heat current into a spin voltage in a ferromagnet/paramagnet junction. Since the spin Seebeck effect appears not only in metals and semiconductors but also in magnetic insulators, it enables the construction of "insulator-based thermoelectric generators" in combination with the inverse spin Hall effect, which would be impossible if only conventional thermoelectric technology were used. In this paper, we briefly report on the experimental observation of the spin Seebeck effect in magnetic insulator/metal junctions and discuss the potential for novel thermoelectric technologies based on spin currents.

Spintronics is showing promising results in the search for new materials and effects to reduce energy consumption in information technology. Among these materials, ferrimagnets are of special interest because they can produce large spin currents that trigger the magnetization dynamics of adjacent layers or even their own magnetization. Herein, a study of the generation of spin current by GdFeCo in a GdFeCo/Cu/NiFe trilayer where the FeCo sublattice magnetization is dominant at room temperature is presented. Magnetic properties such as the saturation magnetization are deduced from magnetometry measurements while damping constant is estimated from spin‐torque ferromagnetic resonance (ST‐FMR). It is shown that the overall damping‐like (DL) and field‐like (FL) effective fields as well as the associated spin Hall angles can be reliably obtained by performing the dependence of ST‐FMR by an added dc current. The sum of the spin Hall angles for both the spin Hall effect (SHE) and the spin anomalous Hall effect (SAHE) symmetries is: and . From the symmetry of ST‐FMR signals, it is found that is positive and dominated by the negative . The present study paves the way for tuning the different symmetries in spin conversion in highly efficient ferrimagnetic systems.

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.

The Inverse Spin-Hall Effect (ISHE), Conversion of Spin Currents into Charge Currents, Has Recently Been Observed in Paramagnetic Metal/Ferrimagnetic Insulator Bilayer Films by Means of the Spin Pumping. Here we Investigate the ISHE Induced by the Spin Pumping on Pt/Ti/Y3Fe5O12 and Pt/Ti/BiY2Fe5O12 Films, where the Y3Fe5O12 (BiY2Fe5O12) and Pt Layers Are Separated by Thin Ti Layer. In these Systems, we Measured Electric Voltage due to the ISHE in Pt/Ti/Y3Fe5O12 (BiY2Fe5O12) Systems at Room Temperature. The Experimental Results Show that the ISHE Signal Disappears in the Pt/Ti/Y3Fe5O12 (BiY2Fe5O12) Systems. These Results Indicate that the Spin Pumping between Y3Fe5O12 (BiY2Fe5O12) and Ti Layer Is Suppressed. This Method Enables us to Suppress the Spin Currents without Non-Magnetic Insulators.

We experimentally investigate spin-orbit torques and spin pumping in NiFe/Pt\nbilayers with direct and interrupted interfaces. The damping-like and\nfield-like torques are simultaneously measured with spin-torque ferromagnetic\nresonance tuned by a dc bias current, whereas spin pumping is measured\nelectrically through the inverse spin Hall effect using a microwave cavity.\nInsertion of an atomically thin Cu dusting layer at the interface reduces the\ndamping-like torque, field-like torque, and spin pumping by nearly the same\nfactor of ~1.4. This finding confirms that the observed spin-orbit torques\npredominantly arise from diffusive transport of spin current generated by the\nspin Hall effect. We also find that spin-current scattering at the NiFe/Pt\ninterface contributes to additional enhancement in magnetization damping that\nis distinct from spin pumping.\n