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Register a new userLong range pure magnon spin diffusion observed in a non-local spin-Seebeck geometry
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The spin diffusion length for thermally excited magnon spins is measured by utilizing a non-local spin-Seebeck effect measurement. In a bulk single crystal of yttrium iron garnet (YIG) a focused laser thermally excites magnon spins. The spins diffuse laterally and are sampled using a Pt inverse spin Hall effect detector. Thermal transport modeling and temperature dependent measurements demonstrate the absence of spurious temperature gradients beneath the Pt detector and confirm the non-local nature of the experimental geometry. Remarkably, we find that thermally excited magnon spins in YIG travel over 120 $mu$m at 23 K, indicating that they are robust against inelastic scattering. The spin diffusion length is found to be at least 47 $mu$m and as high as 73 $mu$m at 23 K in YIG, while at room temperature it drops to less than 10 $mu$m. Based on this long spin diffusion length, we envision the development of thermally powered spintronic devices based on electrically insulating, but spin conducting materials.
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The longitudinal spin Seebeck effect refers to the generation of a spin current when heat flows across a normal metal/magnetic insulator interface. Until recently, most explanations of the spin Seebeck effect use the interfacial temperature difference as the conversion mechanism between heat and spin fluxes. However, recent theoretical and experimental works claim that a magnon spin current is generated in the bulk of a magnetic insulator even in the absence of an interface. This is the so-called intrinsic spin Seebeck effect. Here, by utilizing a non-local spin Seebeck geometry, we provide additional evidence that the total magnon spin current in the ferrimagnetic insulator yttrium iron garnet (YIG) actually contains two distinct terms: one proportional to the gradient in the magnon chemical potential (pure magnon spin diffusion), and a second proportional to the gradient in magnon temperature ($ abla T_m$). We observe two characteristic decay lengths for magnon spin currents in YIG with distinct temperature dependences: a temperature independent decay length of ~ 10 ${mu}$m consistent with earlier measurements of pure ($ abla T_m = 0$) magnon spin diffusion, and a longer decay length ranging from about 20 ${mu}$m around 250 K and exceeding 80 ${mu}$m at 10 K. The coupled spin-heat transport processes are modeled using a finite element method revealing that the longer range magnon spin current is attributable to the intrinsic spin Seebeck effect ($ abla T_m eq 0$), whose length scale increases at lower temperatures in agreement with our experimental data.
Spin current generators are critical components for spintronics-based information processing. In this work, we theoretically and computationally investigate the bulk spin photovoltaic (BSPV) effect for creating DC spin current under light illumination. The only requirement for BPSV is inversion symmetry breaking, thus it applies to a broad range of materials and can be readily integrated with existing semiconductor technologies. The BSPV effect is a cousin of the bulk photovoltaic (BPV) effect, whereby a DC charge current is generated under light. Thanks to the different selection rules on spin and charge currents, a pure spin current can be realized if the system possesses mirror symmetry or inversion-mirror symmetry. The mechanism of BPSV and the role of the electronic relaxation time $tau$ are also elucidated. We apply our theory to several distinct material systems, including transition metal dichalcogenides, anti-ferromagnetic $rm MnBi_2Te_4$, and the surface of topological crystalline insulator cubic $rm SnTe$.
Sharp structures in magnetic field-dependent spin Seebeck effect (SSE) voltages of Pt/Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) at low temperatures are attributed to the magnon-phonon interaction. Experimental results are well reproduced by a Boltzmann theory that includes the magnetoelastic coupling (MEC). The SSE anomalies coincide with magnetic fields tuned to the threshold of magnon-polaron formation. The effect gives insight into the relative quality of the lattice and magnetization dynamics.
We report the observation of longitudinal spin Seebeck effects (LSSE) in an all-oxide bilayer system comprising an IrO$_2$ film and an Y$_3$Fe$_5$O$_{12}$ film. Spin currents generated by a temperature gradient across the IrO$_2$/Y$_3$Fe$_5$O$_{12}$ interface were detected as electric voltage via the inverse spin Hall effect in the conductive IrO$_2$ layer. This electric voltage is proportional to the magnitude of the temperature gradient and its magnetic field dependence is well consistent with the characteristic of the LSSE. This demonstration may lead to the realization of low-cost, stable, and transparent spin-current-driven thermoelectric devices.
The nonlocal transport of thermally generated magnons not only unveils the underlying mechanism of the spin Seebeck effect, but also allows for the extraction of the magnon relaxation length ($lambda_m$) in a magnetic material, the average distance over which thermal magnons can propagate. In this study, we experimentally explore in yttrium iron garnet (YIG)/platinum systems much further ranges compared with previous investigations. We observe that the nonlocal SSE signals at long distances ($d$) clearly deviate from a typical exponential decay. Instead, they can be dominated by the nonlocal generation of magnon accumulation as a result of the temperature gradient present away from the heater, and decay geometrically as $1/d^2$. We emphasize the importance of looking only into the exponential regime (i.e., the intermediate distance regime) to extract $lambda_m$. With this principle, we study $lambda_m$ as a function of temperature in two YIG films which are 2.7 and 50 $mu$m in thickness, respectively. We find $lambda_m$ to be around 15 $mu$m at room temperature and it increases to 40 $mu$m at $T=$ 3.5 K. Finite element modeling results agree with experimental studies qualitatively, showing also a geometrical decay beyond the exponential regime. Based on both experimental and modeling results we put forward a general guideline for extracting $lambda_m$ from the nonlocal spin Seebeck effect.
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