United States

Spin precession and spin wave propagation with ferromagnetic resonance (FMR) rely essentially on the following points: a) the external field Hex is applied in order to set the precession axis of spins in the direction of Hex, b) the generated spin wave propagates in the direction perpendicular to the Hex, and c) the generation of spin waves owes its origin to the ferromagnetic spin exchange interaction acting between neighboring spins. Although in the case of the spin thermoelectric (STE) generation the precession of local 3d-electron spins in Fe3+ is thermally excited and its excitation is irrelevant to the resonance phenomenon,1 the three points noted above on FMR are considered to be important also for generating spin currents thermally in a ferrimagnetic insulator (FMI) due to the temperature gradient ▽T. Ref. 2 noted that the driving force for generating spin currents is due to the nonequilibrium dynamics of magnons in the FMI driven by ▽T.2 Based on our studies on STE generation1,3,4, we consider that the spin exchange interaction plays a major role as the driving force for the generation of spin currents in an FMI. The process in Fig. 1 shows the role of the ferromagnetic spin exchange interaction for generating spin currents in the FMI due to ▽T. We regard the ferromagnetic spin exchange interaction as the cause of the driving force for generating spin currents. Fig. 2 illustrates the generation of spin currents in YIG or Bi-substituted YIG by ▽T in the longitudinal spin Seebeck effect (LSSE). Each spin makes the spin precession of different magnitude due to ▽T, because each position of the spins is different in temperature. The precession of each spin then reaches a steady state, settling down to a uniform-mode-like state at tn from the initial state at t0 due to the exchange force working between spins. This idea well explains the generation of STE voltage for all the Hex and ▽T arrangements that have been reported in experiments so far.&lt;br&gt;&lt;br&gt;
**References**&lt;br&gt; 1M. Imamura, H. Asada and R. Nishimura, IEEE Trans. Magn., Vol. 58, p.4500111 (2022)
2K. Uchida, H. Adachi and T. Kikkawa, Proc. IEEE, Vol. 104, p.1946 (2016)
3M. Imamura, H. Asada and R. Nishimura, AIP Advances, Vol. 11, p.035143 (2021)
4M. Imamura, H. Asada and R. Nishimura, J. Magn. Magn. Mater., Vol. 550, p.169081 (2022)&lt;br&gt;
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MMM 2022

Underlying Mechanism of the Driving Force for Generating Spin Currents Thermally in a Ferrimagnetic Insulator Due to a Temperature Gradient

Spin precession and spin wave propagation with ferromagnetic resonance (FMR) rely essentially on the following points: a) the external field Hex is applied in order to set the precession axis of spins in the direction of Hex, b) the generated spin wave propagates in the direction perpendicular to the Hex, and c) the generation of spin waves owes its origin to the ferromagnetic spin exchange interaction acting between neighboring spins. Although in the case of the spin thermoelectric (STE) generation the precession of local 3d-electron spins in Fe3+ is thermally excited and its excitation is irrelevant to the resonance phenomenon,1 the three points noted above on FMR are considered to be important also for generating spin currents thermally in a ferrimagnetic insulator (FMI) due to the temperature gradient ▽T. Ref. 2 noted that the driving force for generating spin currents is due to the nonequilibrium dynamics of magnons in the FMI driven by ▽T.2 Based on our studies on STE generation1,3,4, we consider that the spin exchange interaction plays a major role as the driving force for the generation of spin currents in an FMI. The process in Fig. 1 shows the role of the ferromagnetic spin exchange interaction for generating spin currents in the FMI due to ▽T. We regard the ferromagnetic spin exchange interaction as the cause of the driving force for generating spin currents. Fig. 2 illustrates the generation of spin currents in YIG or Bi-substituted YIG by ▽T in the longitudinal spin Seebeck effect (LSSE). Each spin makes the spin precession of different magnitude due to ▽T, because each position of the spins is different in temperature. The precession of each spin then reaches a steady state, settling down to a uniform-mode-like state at tn from the initial state at t0 due to the exchange force working between spins. This idea well explains the generation of STE voltage for all the Hex and ▽T arrangements that have been reported in experiments so far. 
**References** 1M. Imamura, H. Asada and R. Nishimura, IEEE Trans. Magn., Vol. 58, p.4500111 (2022)
2K. Uchida, H. Adachi and T. Kikkawa, Proc. IEEE, Vol. 104, p.1946 (2016)
3M. Imamura, H. Asada and R. Nishimura, AIP Advances, Vol. 11, p.035143 (2021)
4M. Imamura, H. Asada and R. Nishimura, J. Magn. Magn. Mater., Vol. 550, p.169081 (2022) 
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technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

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MMM 2022 is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a tutorial, and several special sessions. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

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The introduction of magnetism in topological insulators, semiconducting materials that are non-magnetic by nature, has long been sought after in order to realize exotic quantum effects. Most prominent is the quantum anomalous Hall effect (QAHE), where electronic surface states allow the dissipationless propagation of charges through a material [1]. Additionally, these states are 100% spin-polarized and immune to external perturbations, rendering them ideal candidates for spintronic applications as well as quantum computing.

The QAHE is not restricted to a particular temperature range but has so far only been observed at around 1 K in magnetically doped topological insulators [2]. Thus, the task at hand is to find new concepts to magnetize the topological insulator. Here, we investigate a new approach based on proximity induced magnetization in heterostructures, where the topological insulator Bi2Te3 and the topologically trivial, magnetic add-layer MnTe share an interface but are otherwise spatially separated [3]. We employ polarized neutron reflectivity, x-ray spectroscopy and magnetotransport measurements to evaluate the magnetic properties of each individual layer and the magnetic landscape at the MnTe/Bi2Te3 interface. We find the MnTe layer in a ferromagnetic instead of the anticipated antiferromagnetic state and that Mn constituents penetrate into the Bi2Te3 layer. Those effects go often unnoticed and can be misinterpreted as proximity induced magnetization, for which no indication can be found. Hence, we provide a robust, multitechnique approach to gain a complete picture of the magnetic structure and the toolbox to optimize the interface, thereby increasing the temperature range the QAHE can be observed in. Our work thus contributes to the transformation of magnetic topological insulators from fundamental research to practical applications. 
**References** [1] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988) 
[2] C. Z. Chang et al., Science 340, 167 (2013) 
[3] G. Awana et al., Phys. Rev. Mater. 6, 053402 (2022) 

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A toolbox for evaluation and optimization of proximity induced magnetism in MnTe/Bi2Te3 heterostructures

Electrical switching of antiferromagnets is an exciting recent development in spintronics, which promises active antiferromagnetic devices with high speed and low energy cost. In this emerging field, there is an active debate about the mechanisms of the current-driven switching of antiferromagnets. Harmonic characterization is a powerful tool to quantify current-induced spin-orbit torques and spin Seebeck effect in heavy-metal/ferromagnet systems. However, the harmonic measurement of spin-orbit torque technique has never been verified in antiferromagnetic heterostructures. Here, we report for the first time harmonic measurements in Pt/a-Fe2O3 bilayers, which are explained by our modeling of higher-order harmonic voltages. As compared with ferromagnetic heterostructures where all current-induced effects appear in the second harmonic signals, the damping-like torque and thermally-induced magnetoelastic effect contributions in Pt/a-Fe2O3 emerge in the third harmonic voltage. Our results provide a new path to probe the current-induced magnetization dynamics in antiferromagnets, promoting the application of antiferromagnetic spintronic devices. 

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Third Harmonic Characterization of Antiferromagnetic Heterostructures

Spintronics utilizes the spin orbit coupling (SOC) of a heavy metal layer to generate spin currents by the spin Hall effect (SHE), which can then induce spin orbit torques (SOT) to alter the magnetization of an adjacent ferromagnet. SOT can be both fast and energy-efficient and enable technological applications in spintronic devices such as spin Hall nano-oscillators (SHNOs) [1-3], SOT based magnetic random access memory(SOT-MRAM), and spin logic devices [4,5]. However, spin currents generated by SOC also exist in magnetic materials. Recent studies evidenced very large emission of spin currents and strong SOT for magnetization switching in ferrimagnets near compensation [6,7].
In the present study nearly compensated GdFeCo, and Py, are used as spin Hall layer and ferromagnetic layer, respectively. We studied the tunable behaviour of both damping-like(DL) and field-like(FL) SOT efficiencies (θDL and θFL) with respect to a Cu insertion layer in Gd26.7(FeCo)73.3 (10 nm)/Cu(1-5 nm)/Py(3 nm) stack. We performed spin transfer torque ferromagnetic resonance (ST-FMR) measurements on microbars (6 ×12 μm2) to determine the magneto dynamical properties such as current induced DL and FL SOT efficiencies. Figure 1 (a) and (b) show the ST-FMR spectra in the frequency range of 10-15 GHz, and the corresponding linewidth vs. frequency plot of Gd26.7(FeCo)73.3 (10 nm)/Cu(4 nm)/Py (3 nm); (c) and (d) show the Cu thickness dependent SOT efficiencies at f=8 GHz. The DL-SOT efficiency is found to be maximum, θDL~ 20%, at Cu (2 nm) followed by a decreasing trend at higher Cu thicknesses. However, FL-SOT efficiency shows a minimum, θFL~ 10%(-7%), up to Cu (2 nm) followed by ~-53%(48%) at Cu (5 nm) for +ve(-ve) direction of applied magnetic field. The larger value of θFL at higher Cu thickness will be useful for ultrafast current induced magnetization switching as seen in other ferrimagnetic systems. 
**References**
1. H. Mazraati et al., Low operational current spin Hall nanooscillators based on NiFe/W bilayers, Appl. Phys. Lett. 109, 242402 (2016).
2. H. Fulara et al., Spin-orbit torque–driven propagating spin waves, Sci. Adv. 5, eaax8467 (2019).
3. M. Zahedinejad et al., Two-dimensional mutually synchronized spin Hall nano-oscillator arrays for neuromorphic computing. Nat. Nanotechnol. 15, 47–52 (2020).
4. K. Garello et al., Ultrafast magnetization switching by spin-orbit torques, Appl. Phys. Lett. 105, 212402 (2014).
5. M. Cubukcu et al., Spin-orbit torque magnetization switching of a three-terminal perpendicular magnetic tunnel junction, Appl. Phys. Lett. 104, 042406 (2014).
6. D. Céspedes-Berrocal et al., Current-induced spin torques on single GdFeCo magnetic layers, Adv. Mater., 33, 2007047(2021).
7. R. Mishra et al., Anomalous Current-Induced Spin Torques in Ferrimagnets near Compensation, Phys. Rev. Lett. 118, 167201 (2017). 
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Tunable spin orbit torques in Ferrimagnet/Ferromagnetic heterostructures

Magnetic-tunnel-junction-based hard disk drive read-heads have been highly successful. They face pressing current challenges, however, due to high resistance-area product, which leads to impedance-mismatch-driven signal-to-noise issues as scaling continues [1]. Alternative all-metal spintronic devices are thus of high interest, including spin accumulation sensors based on non-local spin valves (NLSVs) [2]. Due to low output signals in such devices, and also for fundamental research, it is highly desired to improve the spin signal in metallic NLSVs. As this spin signal is essentially proportional to the square of the current spin polarization, α [3], we have explored integration of high-spin-polarization Co1-xFex alloy ferromagnetic injectors/detectors into NLSVs. Co1-xFex alloys are known to display a peak in α at x ≈ 0.25 and have been employed in other spintronic devices [4,5]. Otherwise identical Al-based NLSVs were fabricated with either conventional Co or Co75Fe25 ferromagnetic injectors/detectors via single-shot ultrahigh vacuum evaporation into electron-beam-lithographic masks (Fig 1, inset). X-ray and spectroscopic characterization indicates site-disordered BCC Co75Fe25 films. Comparing Co75Fe25 and Co, the non-local spin signal (ΔRNL) increases by factors of 3 and 4, respectively, at 275 and 5 K (Fig. 1), confirming significant enhancement. Full analysis of the temperature and injector/detector separation dependence reveal α values of ~60%, with a strikingly weak temperature dependence (Fig. 2). We thus establish a facile route to ~2-fold enhancement of spin polarization over conventional ferromagnets used in NLSVs (Fig. 2), thereby inducing ~4-fold enhancement in spin signal, of interest for both applied and fundamental purposes. This work is supported by the NSF and the Advanced Storage Research Consortium. 
**References** [1] T. Nakatani, Z. Gao, and K. Hono, MRS Bull., Vol. 43, p. 106 (2018) [2] H. Takagishi, K. Yamada, and H. Iwasaki, IEEE Trans. Mag., Vol. 46, p. 2086 (2010) [3] S. Takahashi, and S. Maekawa, PRB, Vol. 67(5), p. 052409 (2003) [4] D. J. Monsma, and S. S. P. Parkin. Appl. Phys. Lett., Vol. 77(5), p. 720 (2000) [5] S. V. Karthik, T. M. Nakatani, A. Rajanikanth, J. Appl. Phys., Vol. 105(7), p. 07C916 (2009) 
 
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Enhanced Spin Signal in Metallic Non Local Spin Valves with Highly

Competition between the symmetric and antisymmetric exchange interactions can give rise to non-trivial magnetic spin textures that are both physically interesting and
technologically promising. Here, I present results from a recent experiment conducted at LCLS that probed the magnetic fluctuations of an amorphous FeGe film. In this experiment, the
magnetic phase transition between a helical and a paramagnetic state was investigated by means of X-ray photon fluctuation spectroscopy. By analyzing the statistical distribution of photon
arrival events, sub-nanosecond timescales could be probed. This work provides both insight into the stabilization of magnetic spin textures as well as a template for future sub-nanosecond
experiments that are now available due to X-ray Free Electron Lasers.

Investigation of Sub Nanosecond Fluctuations on Amorphous FeGe Near a Magnetic Phase Transition

To obtain the magnetic properties of silicon steel in different directions, it is necessary to establish a magnetostrictive model related to the anisotropy of silicon steel. Studies on the magnetostrictive model based on the magnetic domain deflection can simulate the hysteresis magnetization of silicon steel in different magnetization directions [1, 2]. But it did not consider the effect of stress. This paper proposed a hysteresis anisotropy-dependent magnetostriction model based on the free energy of silicon steel, which can consider stress-induced and magnetocrystalline anisotropy. 
II Method and Discussion 
Firstly, the two-dimensional free energy model of silicon steel, which includes magnetic field energy, magnetic crystal anisotropy energy and stress anisotropic energy, is incorporated into the anhysteretic magnetization parameter Man, which is coupled with the Jiles-Atherton model, as shown in Fig.1. Based on the two-dimensional free energy model and thermodynamic principle, the hysteresis anisotropy-dependent magnetostriction model of silicon steel is proposed. Then, to obtain the magnetic field and deformation parameters related to stress-induced and magnetocrystalline anisotropy, the magnetostrictive deformation curves of silicon steel at different magnetization directions are measured. Finally, based on the measured parameters of the proposed model, magnetostrictive loops with the magnetic field under varying magnetization directions are simulated. By comparing the measurement results with the simulation results, the effectiveness and accuracy of the proposed model are verified, as shown in Fig.2. 
III Conclusion 
In this paper, a hysteresis anisotropy-dependent magnetostriction model of silicon steel is proposed. The measured and simulated results show that the magnetostrictive strain decreases with the stress increases in a certain range. The magnetostrictive strain will be larger and anisotropy will be more obvious with the increase of magnetization direction under the same stress. 
**References:** [1] A.P.S. Baghel, B. Sai Ram, K. Chwastek, et al. Hysteresis modelling of GO laminations for arbitrary in-plane directions taking into account the dynamics of orthogonal domain walls[J]. Journal of Magnetism and Magnetic Materials, 2016, 418:14-20. 
[2] S. Mbengue, N. Buiron, and V. Lanfranchi. Macroscopic modeling of anisotropic magnetostriction and magnetization in soft ferromagnetic materials[J]. Journal of Magnetism and Magnetic Materials, 2016, 404:74-78. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/18846a1231a80077d6117c1919bda102.png) 
Fig.1 Flow chart of modeling magnetostriction property considering anisotropy 
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Fig.2 The hysteresis loops and magnetostrictive loops under different stress when the magnetization directions are 0°, 30° and 90°

Magnetostriction Property Modeling of Silicon Steel Considering Stress induced and Magnetocrystalline Anisotropy

The crystallographic orientation plays the foremost significant role in the modern magnetic heterostructure devices, especially in the arena of spintronics device. In terms of magnetic property, domain orientation is solely dependent on the structural homogeneity. This research work deals with the development of epitaxial thin films of Ni/NiO on various single-crystalline substrates including sapphire (0006) and LiNbO3 (104). Pulsed laser deposition (PLD) technique was used at 650°C in constant oxygen pressure of 0.01 mbar to deposit films on reported substrates. Phase mixture of Ni (ferromagnetic)/NiO (antiferromagnetic) in the thin films was achieved via hydrogen reduction annealing at 600°C for 30 minutes which induced point defects by incorporating oxygen vacancies into the as-deposited pure NiO crystal structure. Two major structural characterizations— X-ray diffraction and Scanning electron microscopy were done to find out the desired phases and surface morphology. X-ray diffraction shows preferential growth of NiO on both sapphire and lithium niobate along the (111) plane (XRD peak at 37.25° on sapphire, 37.28° on LiNbO3), sharp peaks of Ni at 44.4° and 50.3° on sapphire and lithium niobate substrates indicate that a handful amount of NiO converted to Ni phase. Additionally, Rietveld refinement using Diffrac plus Topas shows 99.7% and 99.6% Ni phase presence after doing the reduction annealing of NiO thin films on sapphire and lithium niobate substrates accordingly. Moreover, single crystalline domains with two different crystallographic orientations (due to having twin phase) were noticed at 60° apart via phi scan and pole figure techniques. Scanning electron microscopy images with Energy dispersive spectroscopy shows the surface homogeneity with atomic percentages of Ni in both annealed thin films. Temperature and field dependent magnetization data collected using SQUID magnetometer show ferromagnetism and antiferromagnetism in pure Ni and NiO films, respectively. Spin canting has been observed in the phase mixture, holding a potential wide range magnetic exchange coupled phenomena. 
**References:** 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/f3cd904bc40e975e3862c691cf220686.png) 
Fig .1: Pole figure of NiO (111) film grown on Sapphire (0006) 
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Fig.2 : Phi Scan of Ni (111) film grown on Sapphire (0006)

In depth structural and magnetic study of Ni/NiO epitaxial thin films on various single crystalline substrates grown using pulsed laser deposition

In this paper, we mainly focus on the magnetic and magnetocaloric properties of YbVO3 single crystals. As shown in Fig.1 (a, b) the grown crystals unveil several phase transitions close to 104 K, 65 K and 20 K. The observed 104 K feature originates from the occurrence of a C-type antiferromagnetic ordering of V3+ magnetic moments while that close to 65 K corresponds to the transformation of the established C-type spin ordering (SO) into a G-type antiferromagnetic ordering which is associated with a change in the orbital ordering (OO) from G-type to C-type. The phase transition taking place close to 20 K is more probably attributed to the ordering of Yb3+ magnetic moments. On the other hand, the below 65 K ordering state constituted of G-type SO/C-type OO remains very stable even under sufficiently high magnetic fields as reported in Fig.1-b. This underlines the weakness of f-d exchange couplings in the YbVO3 vanadate being in contrast with other RVO3 orthovanadates containing smaller rare earth elements such as DyVO3 compound [1] . Additionally, we particularly demonstrate that the complex electronic and magnetic structures of YbVO3 result in interesting levels of thermal effects over a wide temperature range including a giant negative magnetocaloric effect close to 60 K. In fact, the maximum entropy change is found (Fig. 1-c) to be almost 12 J/kg K in the magnetic field change of 7 T applied along the c axis and the ab-plane at 64.25 K and 65.5 K, respectively. More interestingly, the strong magnetic anisotropy shown by the V3+ sublattice (Fig. 1-a) results in a large rotating magnetocaloric effect (RMCE) at relatively high temperatures (Fig.1-d). This would open the avenue for the design of efficient magnetic refrigerators with simplified designs [2]. Our findings are discussed in the frame of DFT calculations.

Strong spin orbital coupling

The characterization and analysis of magnetization dynamics and magnetic damping of antiferromagnetic magnons is important in the field of antiferromagnetic
spintronics. This is primarily because antiferromagnets have magnetic resonances in the THz range as compared to ferromagnetic resonance which tends to occur in the GHz range. In this
work, we employ frequency-domain THz spectroscopy to explore the antiferromagnetic resonance frequency associated with monocrystalline NiO [1]. Specifically, a continuous-wave THz
spectroscopy system is used to measure the antiferromagnetic resonance of three NiO cuts (111), (110), and (100). To find the absorption peak, a reference measurement is taken in addition
to the NiO measurement. Raw data from the reference and NiO can be visually compared to see two effects: the THz system instrument response and Fabry-Perot interference pattern.
During analysis, the instrument response and Fabry-Perot pattern are subtracted so that a clear absorption peak at the resonant frequency of NiO can be observed. An antiferromagnetic
resonance, near 1 THz, is seen in each cut of NiO. Our results demonstrate the utility of working in the frequency domain to characterize antiferromagnetic materials that will be used
eventually for ultrafast antiferromagnetic spintronic device technologies. 

**References:** 

[1] Moriyama, Takahiro, et al., Physical Review Materials 3.5, 051402 (2019)

Exploring antiferromagnetic resonances of NiO with frequency domain THz spectroscopy

Fe5-xGeTe2 is a layered and exfoliable van der Waals (vdW) ferromagnet that displays Curie temperatures Tc ranging between 270 and 330 K, which are within a useful range for spintronic applications. Nevertheless, little is known about the interplay between topological spin textures (e.g., merons, skyrmions) and their transport properties (e.g., the topological Hall effect (THE)) which might influence the response of devices and be harvested for applications. Here, we show through a Lorentz transmission electron microscopy study in Fe5-xGeTe2 the coexistence of merons and anti-meron pairs with Néel skyrmions and antiskyrmions over a broad range of temperatures upon approaching a magneto-structural transition. We measured the THE associated to both types of spin textures finding that it is affected by their coexistence being measureable at room temperature and beyond [1]. 
Remarkably, we also observe a sizeable unconventional THE for currents and magnetic fields aligned along a planar direction, a configuration that is insensitive to the Lorentz force [2]. 
We attribute this to carrier interaction with magnetic field-induced chiral spin textures. Given that the Curie temperature of Fe5-xGeTe2 can be increased via either Ni [3] or Co [4] doping, we conclude that 2D centro-symmetric vdW magnets are ripe for the exploration of their possible applications in skyrmionics/meronics [5] and also of new hybrid spin quasiparticles as well as their possible manipulation via external stimuli such as current and light. Our results suggest that unconventional topological spin textures (that is, those distinct from merons or skyrmions) might exist in atomically thin vdW layers whose properties have yet to be unveiled.

**References:**

[1] Brian W. Casas, Yue Li, Alex Moon et al., Room temperature unconventional topological Hall response in a van der Waals ferromagnet driven by meron-antimeron pairs (unpublished) 
[2] Juan Macy, Danilo Ratkovski, Purnima P. Balakrishnan et al., Magnetic field-induced non-trivial electronic topology in Fe3−xGeTe2, Appl. Phys. Rev. 8, 041401 (2021). 
[3] Xiang Chen, Yu-Tsun Shao, Rui Chen et al., Pervasive beyond Room-Temperature Ferromagnetism in a Doped van der Waals Magnet, Phys. Rev. Lett. 128, 217203 (2022). 
[4] Hongrui Zhang, David Raftrey, Ying-Ting Chan et al., Room-temperature skyrmion lattice in a layered magnet (Fe0.5Co0.5)5GeTe2, Sci. Adv.8, eabm7103 (2022). 
[5] Hamed Vakili, Jun-Wen Xu, Wei Zhou et al., Skyrmionics—Computing and memory technologies based on topological excitations in magnets, J. Appl. Phys. 130, 070908 (2021). 

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Underlying Mechanism of the Driving Force for Generating Spin Currents Thermally in a Ferrimagnetic Insulator Due to a Temperature Gradient

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