Quantitative analysis of vectorial torques in thin 3d Co ferromagnet using orbital-spin conversion (2501.09864v1)

Abstract: Recent findings in orbitronics pointed out large current-induced torques originating, in the current understanding, from incident orbital currents. These are generated by orbital Rashba-Edelstein effect (OREE) produced at the interface between some light metal and oxides films e.g. by naturally oxidized copper layer (Cu*). In the present work, by using second harmonic Hall techniques, we determine the ratio of orbital vs spin currents exerting torques on thin transition metals Co ferromagnet in systems using an orbit-to-spin Pt converter as interlayer with Cu*. Our results quantifying damping like torques show that both orbital and spin currents are enhanced in these systems. Moreover, the experimental determination of the decoherence length in a sample series with varying Co thickness clearly demonstrates the interfacial generation of the orbital currents in Cu* by Orbital Rashba-Edelstein effects (REE) leading to subsequent magnetic torque in Co over a typical lengthscale of several nanometers

• The paper demonstrates that orbital currents in Co|Pt|Cu* structures extend the spin decoherence length from below 0.5 nm to 1.78 nm, emphasizing the impact of orbital rather than spin mechanisms.
• The paper employs second harmonic Hall techniques to precisely measure damping-like and field-like torques, delineating the distinct roles of orbital and spin contributions.
• The paper reveals that while orbital currents significantly enhance damping-like torques in optimized layer structures, field-like torques remain largely determined by interfacial Co|Pt interactions.

Analysis of Orbital-to-Spin Conversion in Ultrathin Ferromagnets

The paper entitled "Quantitative analysis of damping-like and field-like torques using orbital-to-spin conversion in ultrathin ferromagnets" presents a detailed quantitative investigation into the interaction of spin and orbital currents within ultrathin ferromagnets, specifically those incorporating naturally oxidized copper (noted as Cu*) in combination with cobalt (Co) and platinum (Pt) layers. The paper addresses the contemporary interest in orbitronics, a field considering the role of orbital angular momentum alongside conventional spin dynamics in spintronics. The focus is on differentiating and quantifying the effects of orbital Rashba-Edelstein effect (OREE) and spin-orbit coupling in such hybrid layered systems.

The experimental methodology revolves around utilizing second harmonic Hall technique to measure the torques induced in different ferromagnetic systems. Specifically, the systems under consideration are Co|Pt, Co|Pt|Cu*, and Co only with naturally oxidized Cu. By measuring the decoherence lengths, identified as the scale over which spin and orbital currents lose their precession coherence, the paper reveals insights into the interplay between orbital and spin currents, challenging traditional notions of spin-dominated torque scenarios.

Numerical Results and Observations

Several key results emerge from this analysis:

1. Decoherence Lengths: The typical spin decoherence length was reaffirmed as short (<0.5 nm), particularly in Co|Pt systems, coherent with prior understanding. However, the introduction of a Pt layer with a thickness of 4 nm in Co|Pt|Cu* systems extended this decoherence length dramatically to 1.78 nm, indicating a predominance of orbital torques over spin-induced torques.
2. Damping-like Torques: In Co|Pt|Cu* configurations, where the Pt thickness is optimized for orbit-to-spin conversion, orbital currents generate pronounced damping-like torques, overshadowing conventional spin contributions observable in simpler Co|Pt contexts.
3. Field-like Torques: The paper meticulously notes the lack of orbital enhancement on field-like torques across all configurations examined, elucidating that these contributions are primarily interfacial and linked to Co|Pt interactions independent of the orbital contexts.
4. Cu* Interfacial Nature: By varying the Cu thickness in both Co|Cu* and Co|Pt|Cu* systems, the findings confirm the interfacial characteristic of orbital current generation at the Cu|oxidized Cu interface. This is deduced from the plateau in torque values with increased Cu thickness in the presence of a Pt layer, suggesting limited modeling of bulk effects like the orbital Hall effect in Cu.

Implications and Future Directions

The findings pose several important implications for the domain of spintronics, particularly orbitronics:

• The demonstrated dominance of orbital torque mechanisms underlines the potential for enhanced device efficiencies, especially in magnetic memory and spintronic applications where energy-dissipation is a key consideration.
• The work advocates a more nuanced appreciation of orbital contributions, which could usher in new paradigms for charge-to-spin conversion mechanisms, potentially optimizing material choices in device engineering.

Looking ahead, the paper suggests future work to refine understanding of the orbital-to-spin conversion phenomenon—specifically, whether it can be consistently achieved across different thicknesses and materials, and how these dynamics might be further exploited to maximize torque-induced efficiencies in devices. The exploration of alternative ferromagnetic substrates with distinct spin-orbit characteristics, such as Ni, could yield additional insights. Such studies are pivotal to advancing theoretical models beyond simple conversion schema, accommodating the complex synergy between spin and orbital currents that this research highlights. The methods and results could serve as a template to dissect other potential orbitronic systems, enhancing both theoretical comprehension and practical applications in next-gen electronics.