toplogo
Sign In

Electron-Magnon Scattering and Spin-Orbit Coupling in Ultrafast Demagnetization: A Non-Equilibrium Microscopic Model


Core Concepts
The interplay of electron-magnon scattering and spin-orbit induced electron-electron scattering is crucial for ultrafast demagnetization in ferromagnets, with magnon emission playing a dominant role over electronic spin polarization changes.
Abstract

Bibliographic Information:

Dusabirane, F., Leckron, K., Rethfeld, B., & Schneider, H. C. (2024). Interplay of Electron-Magnon Scattering and Spin-Orbit Induced Electronic Spin-Flip Scattering in a two-band Stoner model. arXiv preprint arXiv:2304.14978v3.

Research Objective:

This research paper investigates the influence of electron-magnon scattering processes on ultrafast demagnetization in itinerant ferromagnets, focusing on non-equilibrium effects and the interplay between different scattering mechanisms.

Methodology:

The authors develop a microscopic model incorporating electron-magnon interactions and spin-orbit assisted spin-flip electron-electron scattering processes within a two-band Stoner model. They numerically solve dynamical equations for electron and magnon distribution functions, considering electron-phonon and magnon-phonon interactions using relaxation-time approximations.

Key Findings:

  • Electron-magnon scattering primarily involves high-q magnons, leading to their emission and an increase in electronic spin polarization.
  • Elliott-Yafet-type spin-flip scattering, driven by spin-orbit coupling and electron-electron interactions, relaxes the increased spin polarization, facilitating further magnon emission and enhancing demagnetization.
  • The interplay of these scattering mechanisms results in a demagnetization process dominated by magnon emission, with a relatively small contribution from changes in electronic spin polarization.
  • Remagnetization requires a direct coupling between magnons and phonons, highlighting the importance of magnon-phonon interactions.

Main Conclusions:

The study demonstrates the crucial role of non-equilibrium magnons in ultrafast demagnetization, emphasizing the significance of both electron-magnon and spin-orbit induced electron-electron scattering. The findings suggest that magnon emission is the primary driver of demagnetization, while magnon-phonon interactions are essential for remagnetization.

Significance:

This research provides a deeper understanding of the microscopic mechanisms underlying ultrafast demagnetization in ferromagnets, paving the way for developing more accurate models and predicting demagnetization behavior in various materials.

Limitations and Future Research:

The model utilizes a simplified band structure and instantaneous excitation process. Future research could incorporate more realistic band structures, excitation conditions, and a microscopic treatment of magnon-phonon interactions for improved accuracy and predictive capabilities.

edit_icon

Customize Summary

edit_icon

Rewrite with AI

edit_icon

Generate Citations

translate_icon

Translate Source

visual_icon

Generate MindMap

visit_icon

Visit Source

Stats
The magnon stiffness constant (D) is 2 meV nm^2. The exchange coupling constant (I) between itinerant and localized spin densities is 0.9 eV. The Elliott-Yafet spin-flip factor (α) is 0.1. The electron-phonon relaxation time (τe-p) is 1 ps. The magnon-phonon relaxation time (τm-p) is 10 ps. The initial electronic temperature after excitation is 2000 K. The lattice temperature is kept fixed at 300 K.
Quotes

Deeper Inquiries

How can this microscopic model be extended to incorporate more complex magnetic materials and multi-band systems?

Extending this microscopic model to encompass more complex magnetic materials and multi-band systems presents exciting avenues for future research. Here's a breakdown of potential approaches: 1. Incorporating Complex Magnetic Interactions: Beyond the Heisenberg Model: The current model utilizes the Heisenberg model, which assumes a simple exchange interaction between localized spins. More sophisticated models, such as those incorporating Dzyaloshinskii-Moriya interaction (DMI) or anisotropic exchange interactions, can be employed to describe materials exhibiting spin canting, non-collinear magnetism, or complex magnetic textures like skyrmions. Incorporating Orbital Magnetism: The existing model primarily focuses on spin angular momentum. In materials with strong spin-orbit coupling, orbital angular momentum can play a significant role. Extending the model to include orbital degrees of freedom would be crucial for accurately describing such systems. 2. Multi-band Systems: Generalizing the Hamiltonian: The current two-band Stoner model can be generalized to include multiple bands. This would involve incorporating additional electron creation/annihilation operators for each band and considering inter-band electron-magnon and electron-electron scattering processes. Realistic Band Structures: Instead of the simplified tight-binding model, more realistic band structures obtained from first-principles calculations (e.g., density functional theory) can be incorporated. This would allow for a more accurate description of the electronic states and their interactions. 3. Computational Challenges and Approaches: Increased Computational Cost: Incorporating these extensions significantly increases the computational complexity due to the larger number of degrees of freedom and scattering channels. Advanced Numerical Techniques: Efficient numerical techniques, such as Monte Carlo methods, dynamical mean-field theory (DMFT), or tensor network methods, might be necessary to handle the increased computational demands.

Could other mechanisms, such as superdiffusive spin transport, contribute significantly to the observed demagnetization dynamics in conjunction with the proposed mechanism?

Yes, superdiffusive spin transport could indeed contribute significantly to the observed demagnetization dynamics, working in conjunction with the electron-magnon scattering mechanism described in the paper. Here's how: Superdiffusion: A Quick Primer: Superdiffusion refers to a transport regime where the mean squared displacement of particles increases faster than linearly with time. In magnetic systems, this can lead to a rapid spatial spreading of spin excitations. Synergistic Effect: While the paper focuses on local electron-magnon scattering as the primary driver of demagnetization, superdiffusive spin transport could enhance the spatial extent of demagnetization. Imagine this: Electron-magnon scattering initially reduces the magnetization locally. The resulting spin imbalance can then propagate rapidly through the material via superdiffusion, effectively transporting the demagnetization away from the initial excitation region. Experimental Evidence: Experimental observations often show that demagnetization can propagate over distances much larger than typical spin diffusion lengths, suggesting the involvement of faster transport mechanisms like superdiffusion. Theoretical Incorporation: Incorporating superdiffusive spin transport into the model would require going beyond the local scattering picture. One approach could involve using spin-dependent diffusion equations with modified diffusion coefficients that capture the superdiffusive behavior.

How can the understanding of ultrafast demagnetization dynamics be leveraged to develop faster and more energy-efficient magnetic storage and memory devices?

The insights gained from understanding ultrafast demagnetization dynamics hold immense potential for revolutionizing magnetic storage and memory devices, paving the way for faster and more energy-efficient technologies: 1. Speeding Up Magnetic Writing: Ultrafast Switching: By harnessing ultrafast laser pulses to manipulate magnetization on femtosecond timescales, we can potentially achieve significantly faster magnetic writing speeds compared to conventional magnetic field-based writing. This could lead to orders-of-magnitude improvements in data writing rates. Exploring New Materials: A deeper understanding of electron-magnon interactions and spin transport mechanisms can guide the search for new magnetic materials with desirable properties for ultrafast switching, such as low damping and high spin polarization. 2. Enhancing Energy Efficiency: Lower Switching Energies: Ultrafast demagnetization often occurs with minimal energy dissipation into heat, making it an inherently energy-efficient process. By optimizing materials and excitation conditions, we can potentially achieve magnetic switching with significantly lower energy requirements compared to traditional methods. Heat-Assisted Magnetic Recording (HAMR): Understanding the interplay between heat, spin, and magnetism is crucial for technologies like HAMR, where laser heating is used to temporarily reduce the coercivity of magnetic media, enabling higher storage densities. 3. Novel Device Concepts: All-Optical Switching: The ability to control magnetization with light opens up possibilities for all-optical magnetic switching devices, potentially leading to faster and more compact magnetic memory and logic elements. Spintronics and Beyond: Insights into ultrafast spin dynamics can contribute to the development of advanced spintronic devices, where information is processed and stored using the spin of electrons rather than their charge. 4. Key Challenges and Future Directions: Material Optimization: Identifying and engineering materials that exhibit ultrafast demagnetization with desirable properties (e.g., high Curie temperature, low switching energy) remains a key challenge. Device Integration: Integrating ultrafast laser-based switching mechanisms into practical device architectures while maintaining reliability and scalability is crucial. Understanding Complex Systems: Extending our understanding to more complex magnetic materials and multi-layered structures will be essential for realizing the full potential of ultrafast magnetism in technological applications.
0
star