Comparative Measurements of Inverse Spin Hall and Magnetoresistance in YIG|Pt and YIG|Ta (1302.4416v1)

Abstract: We report on a comparative study of spin Hall related effects and magnetoresistance in YIG|Pt and YIG|Ta bilayers. These combined measurements allow to estimate the characteristic transport parameters of both Pt and Ta layers juxtaposed to YIG: the spin mixing conductance $G_{\uparrow \downarrow}$ at the YIG$|$normal metal interface, the spin Hall angle $\Theta_{SH}$, and the spin diffusion length $\lambda_{sd}$ in the normal metal. The inverse spin Hall voltages generated in Pt and Ta by the pure spin current pumped from YIG excited at resonance confirm the opposite signs of spin Hall angles in these two materials. Moreover, from the dependence of the inverse spin Hall voltage on the Ta thickness, we extract the spin diffusion length in Ta, found to be $\lambda_{sd}^\text{Ta}=1.8\pm0.7$ nm. Both the YIG|Pt and YIG|Ta systems display a similar variation of resistance upon magnetic field orientation, which can be explained in the recently developed framework of spin Hall magnetoresistance.

• The paper confirms that YIG|Pt and YIG|Ta bilayers generate ISHE voltages with opposite spin Hall angles, validating theoretical predictions.
• It employs ferromagnetic resonance to quantify key spin transport parameters, including a spin diffusion length of 1.8±0.7 nm in Ta.
• It demonstrates the consistency between magnetoresistance and ISHE measurements, reinforcing spin mixing conductance models for advanced spintronic devices.

Analyzing the Dynamics and Magnetoresistance in YIG|Pt and YIG|Ta Bilayers

The paper undertaken in the paper focuses on the comparative analysis of inverse spin Hall effects (ISHE) and magnetoresistance in bilayers of Yttrium Iron Garnet (YIG) with Platinum (Pt) and Tantalum (Ta). By exploring these systems, the paper evaluates crucial transport parameters, including spin mixing conductance $G_{\uparrow\downarrow}$, spin Hall angle $\Theta_{SH}$, and spin diffusion length $\lambda_{sd}$, which are fundamental to leveraging spintronics for innovative applications.

Key Findings

The experimental investigation described in the article confirms distinct differences in the spin Hall angle signs between Pt and Ta. Through the inverse spin Hall voltage measurements, the paper shows that Pt and Ta generate voltages of opposite signs when subjected to a spin current from a precessing YIG magnetization. This corroborates the theoretical predictions about the opposite spin Hall angles in these materials.

A noteworthy outcome is the determination of the spin diffusion length in Ta, gained through a systematic paper of the thickness-dependent behavior of the ISHE. This was found to be $\lambda_{sd}^\text{Ta}=1.8\pm0.7$ nm, a value somewhat shorter than those extracted from other methodologies such as non-local spin-valve measurements.

Detailed Analysis of the Systems

1. Inverse Spin Hall Voltage (V$_\text{ISH}$):
   • The research uses ferromagnetic resonance (FMR) to generate a spin current through YIG layers into adjacent Pt or Ta layers. This spin current is converted into a charge current by ISHE, resulting in measurable voltages.
   • Addressing both YIG|Pt and YIG|Ta, the paper reveals ISHE's dependency on material properties, with significant voltage amplitudes noted, particularly in the YIG|Ta configuration due to its low conductivity.
2. Spin Hall Magnetoresistance (SMR):
   • Observations in YIG|NM bilayers show that magnetoresistance derived from the spin Hall effect is consistent with theories and is linked to the dynamic spin accumulation at interfaces. The data align with predictions from recently formulated spin Hall magnetoresistance frameworks, underscoring the universality of SMR across different bilayer compositions.
3. Consistency Across Measurements:
   • By comparing the behavior of magnetoresistance and ISHE, the paper identifies consistency in SMR across YIG|Pt and YIG|Ta bilayers despite differing electronic structures and spin diffusion lengths. This strengthens the validity of the spin Hall angle calculations and spin mixing conductance approximations.

Practical Implications and Future Directions

The implications of these findings are significant for advancing spintronic device technologies. Understanding the parameters $G_{\uparrow \downarrow}$, $\Theta_{SH}$, and $\lambda_{sd}$ for different material combinations opens new pathways for efficient spin current manipulation and utilization in electronic devices. The ability to generate larger ISHE voltages with materials like Ta highlights opportunities for optimizing active layers in spintronic devices.

Future research may focus on exploring broader material systems with varied resistive and conductive properties to refine the understanding of interfacial effects and spin transport dynamics further. Moreover, experiments at varying temperature conditions could provide deeper insights into thermal effects and their influence on spin currents.

Conclusion

This comprehensive examination of YIG|Pt and YIG|Ta bilayers enhances the understanding of spin and charge dynamics crucial for spintronic applications. By reconciling experimental data with theoretical models, the paper provides a substantial contribution toward realizing efficient spin-to-charge conversion systems and optimizing magnetoresistive properties in multilayered materials, paving the way for future research and technological advances in the field of spintronics.