Fast domain wall motion induced by antiferromagnetic spin dynamics at the angular momentum compensation temperature of ferrimagnets (1703.07515v1)

Abstract: Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices. A central motivation toward this direction is that antiferromagnetic spin dynamics is expected to be much faster than ferromagnetic counterpart because antiferromagnets have higher resonance frequencies than ferromagnets. Recent theories indeed predicted faster dynamics of antiferromagnetic domain walls (DWs) than ferromagnetic DWs. However, experimental investigations of antiferromagnetic spin dynamics have remained unexplored mainly because of the immunity of antiferromagnets to magnetic fields. Furthermore, this immunity makes field-driven antiferromagnetic DW motion impossible despite rich physics of field-driven DW dynamics as proven in ferromagnetic DW studies. Here we show that fast field-driven antiferromagnetic spin dynamics is realized in ferrimagnets at the angular momentum compensation point TA. Using rare-earth 3d-transition metal ferrimagnetic compounds where net magnetic moment is nonzero at TA, the field-driven DW mobility remarkably enhances up to 20 km/sT. The collective coordinate approach generalized for ferrimagnets and atomistic spin model simulations show that this remarkable enhancement is a consequence of antiferromagnetic spin dynamics at TA. Our finding allows us to investigate the physics of antiferromagnetic spin dynamics and highlights the importance of tuning of the angular momentum compensation point of ferrimagnets, which could be a key towards ferrimagnetic spintronics.

• The paper demonstrates that ferrimagnetic GdFeCo transitions to antiferromagnetic-like spin dynamics at the angular momentum compensation temperature, achieving domain wall speeds up to 20 km/s.
• Experimental techniques—including thin film deposition and Hall-effect measurements backed by atomistic spin simulations—validate the decoupling of translational and angular domain wall dynamics.
• The findings indicate that controlling domain wall dynamics at T_A could lead to energy-efficient, high-speed spintronic devices, highlighting the potential of ferrimagnets in advanced memory applications.

Fast Domain Wall Motion in Ferrimagnets

The paper examines the domain wall (DW) dynamics in ferrimagnetic materials, with particular emphasis on antiferromagnetic spintronics. The research underscores the importance of the angular momentum compensation temperature ($T_A$) in rare-earth (RE)–transition metal (TM) compounds, specifically considering GdFeCo-based structures. This paper identifies the potential of such materials to transition from ferromagnetic-like dynamics to antiferromagnetic-like dynamics at $T_A$, enabling fast, field-driven DW motion under specific conditions.

Key Findings and Methodology

The paper demonstrates that at the angular momentum compensation point ($T_A$), ferrimagnets such as GdFeCo can exhibit antiferromagnetic spin dynamics. By tuning the temperature or composition, the authors achieved remarkably high DW mobility in ferrimagnetic GdFeCo microwires, reaching speeds up to 20 km/s at $T_A$. This was achieved experimentally through deposition and structuring of thin films followed by observations using Hall-effect measurement techniques.

The theory underpinning these results is grounded in a collective coordinate approach, which allows for the characterization of ferrimagnetic DWs. According to the model, the dynamics of the DW at $T_A$ mirrors that of antiferromagnetic DWs, free from the Walker breakdown, an effect severely hampering speed in ferromagnets due to gyrotropic coupling effects. The theory suggests that at $T_A$, the translational and angular dynamics become decoupled, facilitating higher DW speeds under magnetic fields.

The authors used atomistic spin model simulations to validate their experimental observations and theoretical models. In these simulations, parameters such as the exchange interaction, anisotropy, and gyromagnetic ratio were varied to mimic experimental conditions. The outcomes from these simulations showed excellent agreement with both theory and experiments, particularly illustrating a rapid increase in DW speed as the system approached $T_A$.

Implications and Future Directions

This research opens several avenues for developing advanced spintronic devices. The ultra-high-speed domain wall motion observed near room temperature suggests potential applications in racetrack memory and other data storage technologies that require rapid information encoding. The ability to achieve fast speeds and potentially low-energy operations compared to ferromagnetic systems highlights the attractiveness of ferrimagnets in future device architectures.

Moreover, the control over DW dynamics through external magnetic fields due to the nonzero net magnetic moments at $T_A$ presents an opportunity to explore field-driven antiferromagnetic dynamics more thoroughly. This could lead to the development of novel spintronic devices that leverage the fast dynamics and stability benefits intrinsic to antiferromagnetic systems.

On the theoretical front, further exploration is needed to understand the broader implications of angular momentum compensation in complex ferrimagnetic systems. Specifically, unraveling the precise mechanisms that contribute to the breakdown of conventional gyrotropic coupling relationships is essential for tailoring properties of ferrimagnets for specific applications.

In conclusion, the research provides critical insights into the high-speed domain wall dynamics achievable in ferrimagnets, facilitated by the angular momentum compensation point. By coupling experimental evidence with robust theoretical backing, the findings suggest that ferrimagnets can outperform their ferromagnetic counterparts in specific domains, particularly in field-driven spintronics. Continued exploration may yield even more efficient and versatile spintronic devices, positioning ferrimagnets as pivotal materials in the future of magnetic technology.