Generation of coherent spin-wave modes in Yttrium Iron Garnet microdiscs by spin-orbit torque

Abstract: Spin-orbit effects [1-4] have the potential of radically changing the field of spintronics by allowing transfer of spin angular momentum to a whole new class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed that by depositing Platinum (Pt, a normal metal) on top of a 1.3 $\mu$m thick Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer spin angular momentum through the interface between these two different materials. The outstanding feature was the detection of auto-oscillation of the YIG when enough dc current was passed in the Pt. This finding has created a great excitement in the community for two reasons: first, one could control electronically the damping of insulators, which can offer improved properties compared to metals, and here YIG has the lowest damping known in nature; second, the damping compensation could be achieved on very large objects, a particularly relevant point for the field of magnonics [6,7] whose aim is to use spin-waves as carriers of information. However, the degree of coherence of the observed auto-oscillations has not been addressed in ref. [5]. In this work, we emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current. This requires to reduce both the thickness and lateral size in order to reach full damping compensation [8] , and we show clear evidence of coherent spin-orbit torque induced auto-oscillation in micron-sized YIG discs of thickness 20 nm.

• The paper demonstrates that spin-orbit torque induces coherent spin-wave auto-oscillations in 20 nm thick YIG microdiscs integrated with Pt.
• It reveals that minimizing YIG thickness and lateral size effectively lowers the threshold current by reducing the impact of quasi-degenerate modes.
• The study quantifies interface properties such as spin-mixing conductance and transparency, providing key insights into efficient spin current transfer.

Coherent Spin-Wave Modes in Yttrium Iron Garnet Microdiscs Driven by Spin-Orbit Torque

The research presented in this paper focuses on the coherent generation of spin-wave modes in Yttrium Iron Garnet (YIG) microdiscs through the application of spin-orbit torque (SOT). The paper builds upon previous findings on the transfer of spin angular momentum across interfaces from a normal metal, such as Platinum (Pt), to a magnetic insulator like YIG. This transfer is pivotal because it extends spin-transfer effects to insulating materials, offering new avenues for electronics, notably in magnonics—a field aiming to use spin-waves for information processing.

Key Outcomes

1. Spin-Orbit Torque in YIG: By integrating YIG, known for its exceptionally low damping properties, with a thin Pt layer, this study demonstrates that SOT can induce auto-oscillations in 20 nm thick YIG microdiscs. This is a significant achievement as it shows coherent spin-wave generation in insulating materials, previously limited to conductors.
2. Optimizing SOT Efficiency: The investigation underlines the importance of reducing both the thickness and the lateral size of YIG materials to decrease the threshold current necessary for full damping compensation. The research demonstrates clear evidence of coherent spin-torque-induced auto-oscillation in micron-sized YIG discs, highlighting an advancement in spintronics applications for very thin films.
3. Interface Characteristics and Spin Current: The interfacial nature of the SOT effect affects the transfer of spin currents and is sensitive to interface quality. The research measures parameters such as spin-mixing conductance and transparency of the interface, providing a quantitative framework for evaluating spin current transfer efficacy.
4. Impact of Mode Degeneracy: A crucial discovery of this study is the influence of quasi-degenerate spin-wave modes on the threshold current. The presence of these modes increases the threshold conditions for auto-oscillations, which necessitates device miniaturization to avoid their detrimental effects through confinement and mode separation.

Implications and Future Prospects

The experimental observations provide critical insights into the mechanisms of spin-wave auto-oscillations in YIG|Pt hybrids and their dependence on device geometry and material properties. The findings have several important implications:

• Technological Advancement in Magnonics: The successful generation of coherent spin-waves using insulating materials like YIG could enable the development of more efficient spintronic devices, potentially reducing energy consumption and increasing the operational frequency of next-generation electronic components.
• Scalability and Miniaturization: Future research may explore the fabrication of even smaller YIG structures to effectively manage and eliminate mode degeneracy, enhancing coherence and reducing threshold currents further. This could greatly benefit the scalability of magnonics-based computing technologies.
• Enhanced Understanding of SOT Dynamics: The detailed characterization of the interface properties and their role in spin current transmission enriches the underlying theoretical models, offering a better understanding of SOT dynamics, crucial for optimizing the materials and geometries used in spin-wave-based applications.

The work presented contributes significantly to the understanding and practical realization of SOT applications in insulating magnets, with promising implications for the future of computing and information technology grounded on spin-wave logic and signal processing.