[Paper Review] Anomalous spin orbit torques with large Rashba spin orbit coupling in epitaxial Pt/Co bilayers

Ye Du, Shutaro Karube+5 more|arXiv (Cornell University)|Jul 28, 2018

Magnetic properties of thin films3 citations

TL;DR

This study demonstrates that epitaxial Pt/Co bilayers with strong Rashba spin-orbit coupling exhibit a record inverse Edelstein length (λ_IEE ≈ few nm), enabling a tenfold enhancement in field-like spin-orbit torque efficiency and a twofold increase in damping-like torque efficiency. The results are explained via a novel drift-diffusion model combining Rashba-Edelstein effect and bulk spin Hall effect, enabling precise engineering of spin-orbit torques in spintronic devices.

ABSTRACT

The Rashba-Edelstein effect (REE), which characterizes the generation of a transverse spin polarization at interfaces with a longitudinal external electrical field, provides new opportunities to the efficient spin current generation and the manipulation of spin orbit torques (SOTs) in magnetic hetero-structures. Here in this work, we report an exceptionally large interfacial charge-to-spin conversion efficiency (i.e. the inverse Edelstein length Lamda_IEE of a few nanometers) in epitaxial (epi-) Pt/Co bilayers from a REE-induced magnetoresistance analysis using a new drift-diffusion model that incorporates both REE and bulk spin Hall effect. From the spin torque ferromagnetic resonance analysis, a tenfold enhancement in the field-like (FL) SOT efficiency is demonstrated compared to polycrystalline samples, showing quantitative consistency with the large Lamda_IEE. Additionally, a twofold increase in the damping-like (DL) SOT efficiency is observed in epi-samples, with Pt films having lower resistivity. Our study demonstrates that both DL- and FL-SOTs can be significantly modulated by the Rashba SO coupling with hetero-interface modification, providing new perspectives to the engineering of SOTs in spintronic devices.

Motivation & Objective

• To investigate the role of interfacial Rashba spin-orbit coupling in enhancing spin-orbit torque efficiency in magnetic heterostructures.
• To quantify the interfacial charge-to-spin conversion efficiency (λ_IEE) in epitaxial Pt/Co bilayers using a new drift-diffusion model.
• To correlate the Rashba-Edelstein effect with measurable spin torque ferromagnetic resonance responses.
• To demonstrate engineering potential of spin-orbit torques through hetero-interface modification in epitaxial systems.
• To compare the performance of epitaxial versus polycrystalline Pt/Co samples in terms of field-like and damping-like torque efficiencies.

Proposed method

• Employed a new drift-diffusion model incorporating both Rashba-Edelstein effect (REE) and bulk spin Hall effect to describe spin transport in epitaxial Pt/Co bilayers.
• Conducted magnetoresistance measurements under applied longitudinal electric fields to extract the inverse Edelstein length (λ_IEE).
• Performed spin torque ferromagnetic resonance (ST-FMR) to quantify field-like (FL) and damping-like (DL) spin-orbit torque efficiencies.
• Used epitaxial Pt/Co heterostructures with high structural quality to isolate interfacial spin-orbit effects from bulk and grain boundary contributions.
• Compared results from epi-Pt/Co with polycrystalline samples to isolate the impact of Rashba coupling at the interface.
• Analyzed resistivity and structural properties of Pt films to correlate electronic transport with spin-orbit torque efficiency.

Experimental results

Research questions

• RQ1 What is the magnitude of the interfacial charge-to-spin conversion efficiency (λ_IEE) in epitaxial Pt/Co bilayers with strong Rashba spin-orbit coupling?
• RQ2 How does the Rashba-Edelstein effect contribute to field-like spin-orbit torque efficiency in epitaxial systems compared to polycrystalline ones?
• RQ3 To what extent can both damping-like and field-like spin-orbit torques be enhanced via interfacial engineering and Rashba spin-orbit coupling?
• RQ4 Is there quantitative consistency between the extracted λ_IEE from magnetoresistance and the spin torque efficiency measured via ST-FMR?
• RQ5 How does the resistivity of Pt films influence the observed spin-orbit torque enhancements in epitaxial heterostructures?

Key findings

• An exceptionally large inverse Edelstein length (λ_IEE) of a few nanometers was measured in epitaxial Pt/Co bilayers, indicating highly efficient interfacial charge-to-spin conversion.
• A tenfold enhancement in field-like (FL) spin-orbit torque efficiency was observed in epitaxial samples compared to polycrystalline counterparts, consistent with the large λ_IEE.
• A twofold increase in damping-like (DL) spin-orbit torque efficiency was achieved in epitaxial Pt/Co structures, even with lower Pt resistivity.
• The experimental results show quantitative agreement between the λ_IEE extracted from magnetoresistance and the FL-SOT efficiency derived from ST-FMR, validating the new drift-diffusion model.
• Both field-like and damping-like spin-orbit torques are significantly enhanced by interfacial Rashba spin-orbit coupling, demonstrating its tunability via heterostructure design.
• The study establishes that epitaxial growth enables stronger interfacial spin-orbit coupling, leading to superior spin-orbit torque performance compared to polycrystalline systems.