Thermal excitation of magnons and phonons in the magnetic insulator Thulium Iron Garnet

Abstract

Magnons, phonons and their interactions in the magnetic materials are attracting an interest of spintronic researchers not only for the related physical phenomena but also for the technical applications. In this dissertation, we study about the thermal excitation of magnons and phonons in the thulium iron garnet (TmIG) thin films. Firstly, we introduce a method for simultaneously but independently observing a thermal excitation of magnons and phonons by optical reflectometry. The change of reflectance and Kerr rotation are related to the excitation of phonons and magnons, respectively. Secondly, we study the non-equilibrium between magnons and phonons in thulium iron garnets. By showing that the thermal excitation of magnons and that of phonons have different spatial profiles, we prove that there is a non-equilibrium between magnons and phonons, experimentally. We also show that the degree of non-equilibrium increases at the low external magnetic field and large electric current injection. Finally, we provide a control of thermal conductivity of phonons in TmIG by magnons. It is possible to tune the thermal excitation of magnons by the external magnetic field, and the amount of thermal excitation of phonons is followed. Including the micro-second scale time-resolved measurement with numerical solutions of system of heat equations, we show that the external magnetic field dependent thermal excitation of phonons are related to the change of thermal conductivity of phonons in TmIG film. All these results are understood by introducing the nonlinear interactions among magnons or between magnons and phonons. We believe that our study not only extends our fundamental understanding about magnons, phonons and their interactions, but also can be applied to brand-new spintronic devices with new functionalities.