Quantitative study of the spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids (1304.6151v1)

Abstract: We experimentally investigate and quantitatively analyze the spin Hall magnetoresistance effect in ferromagnetic insulator/platinum and ferromagnetic insulator/nonferromagnetic metal/platinum hybrid structures. For the ferromagnetic insulator we use either yttrium iron garnet, nickel ferrite or magnetite and for the nonferromagnet copper or gold. The spin Hall magnetoresistance effect is theoretically ascribed to the combined action of spin Hall and inverse spin Hall effect in the platinum metal top layer. It therefore should characteristically depend upon the orientation of the magnetization in the adjacent ferromagnet, and prevail even if an additional, nonferromagnetic metal layer is inserted between Pt and the ferromagnet. Our experimental data corroborate these theoretical conjectures. Using the spin Hall magnetoresistance theory to analyze our data, we extract the spin Hall angle and the spin diffusion length in platinum. For a spin mixing conductance of $4\times10^{14}\;\mathrm{\Omega^{-1}m^{-2}}$ we obtain a spin Hall angle of $0.11\pm0.08$ and a spin diffusion length of $(1.5\pm0.5)\;\mathrm{nm}$ for Pt in our thin film samples.

• The paper presents a quantitative analysis of spin Hall magnetoresistance in FMI/NM hybrids, linking spin currents with magnetization orientation.
• Experimental ADMR measurements distinguish SMR from AMR, yielding a Pt spin Hall angle of ~0.11 and a spin diffusion length of ~1.5 nm.
• A robust theoretical model correlates key parameters with SMR behavior, providing a pathway for enhanced spintronic sensor and memory device development.

Quantitative Study of the Spin Hall Magnetoresistance in Ferromagnetic Insulator/Normal Metal Hybrids

This paper presents a comprehensive experimental and theoretical investigation into the spin Hall magnetoresistance (SMR) effect observed in ferromagnetic insulator (FMI)/normal metal (NM) hybrid structures, specifically focusing on systems such as yttrium iron garnet (YIG)/platinum (Pt), YIG/normal-metal/Pt, as well as nickel ferrite/NM/Pt and magnetite/NM/Pt. The authors leverage the spin Hall effect (SHE) and the inverse spin Hall effect (ISHE) to probe spin current-induced magnetoresistance variations, extracting critical parameters such as the spin Hall angle and spin diffusion length in Pt.

Summary of Findings

• Experimental Approach: The authors employed a series of angle-dependent magnetoresistance (ADMR) measurements to explore the SMR across a variety of FMI/NM architectures. These methodologies allowed for the disambiguation of SMR from conventional anisotropic magnetoresistance (AMR), particularly through out-of-plane magnetization measurements.
• Spin Hall Magnetoresistance Mechanism: SMR manifests through the interaction between spin currents, generated and modulated via the SHE, and the adjacent FMI magnetization. The paper asserts that the orientation of FMI magnetization profoundly influences the absorption/coupling efficiency of these spin currents, thereby modulating the NM resistance.
• Theoretical Model: The authors derived quantitative expressions that describe the SMR effect's dependence on intrinsic parameters such as the spin mixing conductance and spin diffusion length. This theoretical underpinning bridges the spin current interactions with observable resistance changes in the NM layers.
• Numerical Results: The paper reports:
  • A spin Hall angle (α_SH) for Pt of approximately 0.11 with a notable variance across structures and deposition techniques.
  • A spin diffusion length (λ_Pt) around 1.5 nm, consistent but slightly variable dependent on Pt layer thickness.
  • SMR behavior present even when a non-magnetic layer (Cu/Au) is situated between the FMI and Pt, affirming the spin current nature of the SMR rather than being attributed to a static proximity-induced magnetism in Pt.
• Dependence on Layer Thickness and Temperature: The SMR was found to vary with Pt thickness, exhibiting a maximum at about 3 nm Pt thickness, with decreasing impact as the NM layer thins. Temperature modifications also influenced SMR magnitude, though the effect's robustness was maintained even at low temperatures.

Implications and Future Outlook

This investigation illuminates the SMR as a practical tool for remote sensing of FMI magnetization, offering a novel nonspectral avenue to probe spintronic interactions. The extraction of α_SH and λ_Pt and their variation with thickness spotlights the SMR as an indirect characterization method for spin transport parameters in diverse materials.

Future explorations may expand this methodology to a wider range of FMIs and NMs, potentially enhancing magnetic memory devices and spintronic sensors. Moreover, refining theoretical models to include Boltzmann transport corrections or quantum mechanical adjustments may resolve discrepancies at ultra-thin NM layers, further refining technological predictability and application efficacy in emergent spin-based computation and storage paradigms.