Theoretical Study On Coupled Magnon-phonon Thermal Transport In Magnetic Nanoscale Systems

Thermal transport in nanoscale systems has been an essential research topic in heat in-sulation,heat dissipation,heat manipulation,and thermoelectric conversion.In recent years,with the rapid progress of spintronics and spin caloritronics,quantum heat transport in mag-netic nanosystems has attracted extensive attention due to showing rich physical phenomena and broad application prospects.Introducing the spin degree of freedom and its interaction with quasi-particles such as phonons enables the thermal transport behavior of nanosystems to be significantly regulated by external fields,which considerably broadens the relevant research prospects in thermal management and energy conversion.However,previous ther-mal transport theories mainly considered the transport of phonons and electrons and rarely involved magnons and their interactions with phonons.The lack of theoretical methods combined with first-principles calculations has seriously hindered the research and applica-tion of quantum heat transport in magnetic nanosystems.At present,theoretical modeling and computational simulation of quantum heat transport in magnetic systems are in their infancy,and there are many challenges and opportunities.This dissertation aims to de-velop several sets of universal ab initio theoretical methods for the quantum heat transport of magnetic nanosystems and systematically study the quantum heat transport properties of several typical magnetic nanosystems so as to provide a theoretical and methodological ba-sis for further investigations of quantum heat transport in magnetic nanosystems.The main research content and innovation points of this dissertation are as follows.（1）In recent years,magnon-phonon scattering（MPS）has attracted extensive attention.The lack of rigorous theoretical approaches for considering MPS has impeded the investiga-tion of quantum thermal transport behavior in magnetic nanostructures with no translational symmetry in the transport direction,such as F/N interfaces.We propose an ab initio theoret-ical method for quantum thermal transport in magnetic nanosystems based on nonequilib-rium Green’s function method and diagrammatic perturbation theory.Applying this method to the F/N interface based on monolayer Cr I₃,we observe the MPS-driven thermal rectifica-tion and negative differential thermal resistance phenomena,consistent with results reported in recent studies.Moreover,this nonlinear transport phenomenon is dominated by the in-elastic transport of the magnons dragged by phonons and can be significantly regulated by an external magnetic field.Based on the above calculation results,the microscopic process of phonon-mediated thermal transport of magnons at the ferromagnetic/nonmagnetic interface is elucidated.The proposed theoretical method paves the way for the first-principles study of magnon-phonon coupled quantum heat transport in magnetic nanosystems.The above results are expected to provide theoretical references for designing new magneto-thermal management and energy conversion devices.（2）An in-depth understanding of magnon-phonon scattering and coupled thermal trans-port behavior in magnetic crystals at the mode level is essential for designing magneto-thermal and spin-functional devices.Based on the Boltzmann transport equation,we es-tablish an ab initio theoretical method for coupled magnon-phonon thermal transport in ferromagnetic crystals.By applying the proposed method,we systematically studied the nonelectronic thermal transport properties of body-centered cubic iron crystals.Reasonable agreement between our results and the available experimental data suggests that phonons dominate the nonelectronic thermal conduction at high temperatures.In contrast,magnons may only contribute to heat conduction at low temperatures.Notably,the abnormal increase in magnon thermal conductivity at high temperatures implies that other magnon-involved scattering events rather than MPS should dominate the thermal transport of magnons.Fur-thermore,analyses of the average scattering rate and heat propagation length indicate that hydrodynamic heat transport behavior may occur at low temperatures.This new method fills the gap in the first-principles evaluation of the thermal transport properties in magnetic crystals.The above results are expected to provide valuable references for further under-standing the interaction between magnons and phonons and broaden the research prospects of thermal management and energy manipulation.（3）The contribution of different scattering mechanisms to thermal transport properties of two-dimensional magnetic van der Waals interfaces has yet to be made clear.The pro-posed nonequilibrium Green’s function method based on the self-consistent Born approx-imation is hard to be applied to two-dimensional magnetic van der Waals interfaces with large atomic numbers due to the enormous computational resource consumption.Therefore,new theories and efficient computational methods need to be developed to clarify the con-tribution of different scattering mechanisms to thermal transport in magnetic van der Waals interfaces.Based on the coupled magnon-phonon nonequilibrium Green’s function method,we develop an effective quantum heat transport theory for the two-dimensional magnetic van der Waals interfaces and give a general ab initio calculation flow combined with first-principles calculations.The developed theoretical methods lay a theoretical and method-ological foundation for the in-depth understanding of the thermal transport mechanism at the two-dimensional magnetic van der Waals interface.The application of this method is expected to provide theoretical references for the efficient screening of two-dimensional magnetic van der Waals thermal functional materials.