Spin Hall magnetoresistance in metallic bilayers (1503.08903v2)

Abstract: Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find nearly a ten-fold increase of SMR in W/CoFeB compared to previously studied HM/ferromagnetic insulator (FI) systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity with temperature. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively describes the HM layer thickness dependence of SMR, allowing accurate estimation of the spin Hall angle and the spin diffusion length of the HM layer, but also can account for the temperature dependence of SMR by assuming a temperature dependent spin polarization of the FM layer. These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the HM/FM interface.

• The paper reveals a ten-fold increase in SMR in W/CoFeB bilayers over traditional HM/FI systems, enhancing spintronic performance.
• The study employs detailed temperature-dependent measurements and a robust spin transport model to quantify SMR behavior in metallic bilayers.
• The findings underscore the critical role of longitudinal spin current absorption in ferromagnetic layers, guiding future spintronic device design.

Spin Hall Magnetoresistance in Metallic Bilayers: An In-Depth Analysis

The paper of spin Hall magnetoresistance (SMR) presents significant implications for the field of spintronics, particularly in the optimization of spin information transmission. In the paper "Spin Hall Magnetoresistance in Metallic Bilayers," the authors investigate SMR in metallic bilayers consisting of a heavy metal (HM) layer and a ferromagnetic metal (FM) layer. The discovery of a substantial ten-fold increase of SMR in W/CoFeB bilayers as compared to previously studied HM/ferromagnetic insulator (FI) systems is a noteworthy result emerging from this investigation.

Key Findings

1. Significant Increase in SMR: The SMR in W/CoFeB is notably increased, reaching comparable levels to the anisotropic magnetoresistance (AMR) found in Ni-based alloys. This increase implies enhanced spintronic performance in systems using metallic ferromagnets due to the intrinsic properties of these materials that foster efficient spin transmission.
2. Temperature Dependence: The increase of SMR with decreasing temperature is noteworthy, occurring despite the negligible changes in the resistivity of the W layer. This behavior contrasts with traditional HM/FI systems where SMR typically decreases with lower temperatures.
3. Spin Transport Model: The development and implementation of a spin transport model was pivotal in this paper, providing quantitative descriptions of both the thickness dependence and the temperature dependence of SMR in these HM/FM bilayers.
4. Absorption of Longitudinal Spin Current: The paper highlights the significant role played by the absorption of longitudinal spin currents into the FM layer. Spin polarization in the FM layer notably impacts SMR, as demonstrated by the temperature-dependent model results, which suggests that the spin absorption characteristics are different in highly polarizable ferromagnetic environments.

Methodological Approaches

The authors employed a robust methodology involving magnetron sputtering for sample preparation, followed by detailed measurement techniques using Hall bars to discern longitudinal and transverse resistances under various states (as-deposited and annealed). The employment of both a conventional saturation method and novel field- and angle-sweep techniques provided thorough evaluations of magnetoresistance changes in different film configurations.

Implications and Future Directions

The findings of this work have substantial implications for the field of spintronics, especially in systems where metallic bilayers are preferred. The high magnitude SMR realized in metallic W/CoFeB bilayers makes these materials promising candidates for future device applications where efficient spin transmission is critical. Furthermore, the elucidation of mechanisms involving spin polarization and longitudinal spin current absorption contributes to a better theoretical understanding of spin transport across HM/FM interfaces.

The understanding of temperature-dependent polarizations and resistivity alterations also opens avenues for exploring low-temperature operation environments, which could further optimize device efficiency.

Future research could explore varying the material compositions, particularly exploring other HM/FM combinations to enhance SMR further. Detailed exploration into the phase transitions of materials such as W and their effects on resistivity and SMR is another promising domain that could reveal new aspects of spintronic interface behavior.

Overall, this paper represents a comprehensive examination of spin Hall magnetoresistance enhancement in metallic bilayers, solidifying the practical and theoretical basis for their application in advanced spintronic technologies.