Analysis of the Intrinsic Spin Seebeck Effect in Au/YIG Systems
The research conducted by Qu et al. focuses on the intrinsic spin Seebeck effect (SSE) in gold (Au) when interfaced with yttrium iron garnet (YIG), a well-known ferromagnetic insulator. The primary objective is to investigate the potential of Au as a detector for pure spin current generation in spin caloritronic systems, by addressing the limitations presented by platinum (Pt), notably, its magnetic proximity effects (MPE) that confound the isolation of the intrinsic SSE.
Key Findings and Methodology
A central premise of this paper is the evaluation of Au as a substitute for Pt, which typically suffers from complications such as the anomalous Hall effect (AHE) and magnetoresistance (MR) when used in conjunction with YIG. Adding to these issues, the entanglement of the anomalous Nernst effect (ANE) with SSE has rendered them difficult to decouple in Pt-based systems, issuing a clarion call for alternative materials.
In order to circumvent these issues, the paper employs Au in a longitudinal SSE configuration with an unequivocal out-of-plane temperature gradient in order to measure the intrinsic SSE. The experimental arrangement includes magnetron-sputtered thin Au films, with varying thicknesses atop polished, polycrystalline YIG substrates. Critical measurements such as thermal voltage and MR are performed using both 4-probe and 2-probe methodologies under a ΔT of approximately 10 K.
The paper reveals several key results:
- Resistivity and AHE in Au/YIG: The resistivity ρ of Au increases as film thickness decreases, accurately described by the Fuchs-Sondheimer model. Hall measurements confirm the absence of AHE in Au/YIG, indicating a negligible impact from MPE, contrary to Pt/YIG systems.
- SSE and Thermal Voltage: Au/YIG demonstrates a non-monotonic thickness-dependent thermal voltage, reaching a maximum of 1.3 µV at 8 nm, attributed primarily to the SSE, with no significant contributions from ANE—indicating a negligible MPE.
- MR Behavior: Although exhibiting very minimal MR different in angular dependence from conventional anisotropic MR, Au/YIG's MR signature further corroborates the lack of strong MPE, in stark contrast to Pt/YIG results.
- Spin-Polarized DFT Calculations: Theoretical calculations using spin-polarized density functional theory highlight negligible induced magnetic moments in Au when interfaced with YIG. This aligns with the empirical findings of no observable AHE and weak inverse MR behavior, supporting the intrinsic nature of the measured SSE in Au/YIG.
Theoretical and Practical Implications
These findings contribute significantly to the understanding of pure spin currents in spin caloritronics, providing a tangible alternative to Pt by using Au, which evades the typical complications of magnetic proximity interference. The notable factor here is the establishment of an upper limit of 0.1 µV/K for the intrinsic SSE in Au/YIG, with the potential to improve spin mixing conductance through surface modification, ensuring further optimization of the system's performance.
Future research should focus on extending this characterization to different temperature regimes and probing deeper into the interface phenomena through advanced characterization techniques, such as x-ray magnetic circular dichroism, on a broader variety of ferromagnetic-insulator interfaces. This paper forms the basis for potential novel applications in thermoelectric generation and spintronic devices, offering higher efficiency by minimizing parasitic charge currents and maximizing angular momentum transfer.