Excitation And Probing Of Spin Dynamics In SmDyFeO₃ And LuFe₂O₄ Single Crystals With Terahertz Time-Domain Spectroscopy

Since the electron-electron and electron-lattice interactions in the quantum material can generate a large number of ordered states,ultrafast spectroscopy can selectively pump the collective excitation mode or single particle excitation mode of these ordered states.The manipulation of electron spins in the ultra-fast time scale has become an important research fields.The magnetic field of terahertz?THz?pulse has been proved to be an effective and powerful method to excite and manipulate electron spins in magnetic ordering materials without thermal effect.In this thesis,we focus on the magnetic resonance excitation mode and spin reorientation transition in the SmDyFeO₃ single crystal,and the low-energy collective excitation mode in the spin-and charge-ordered multiferroic material LuFe₂O₄,with the temperature dependent THz time domain spectroscopy system?THz-TDS?.The specific content includes the following two aspects:?1?By using terahertz time-domain spectroscopy?THz-TDS?,we investigate the temperature range of the SRT,and the resonant frequency and relaxation time of the THz spin waves in the doped Sm_xDy_1-xFeO₃ single crystal.We show that the resonant frequency of the FM mode?measured at 40 K?increases linearly with the Sm dopant concentration within the range from x=0.5 to 0.7.The temperature-and dopant-induced changes of the magnetic anisotropy of Fe³⁺ions are accessed by the resonant frequency shifts.Upon cooling,the lifetime of oscillations in the AFM mode increases exponentially and can be subtly tuned by varying the Sm dopant.These results lead to an improved understanding of dopant-tuned spin wave dynamics and magnetoanisotropy parameter in rare-earth orthoferrites.?2?A low-energy collective excitation mode in charge-ordered multiferroic LuFe₂O₄ is reported via terahertz time-domain spectroscopy.Upon cooling from 300 K to 40 K,the central resonance frequency showed a pronounced hardening from 0.85THz to 1.15 THz.In analogy to the well-known low-energy optical properties of LuFe₂O₄,this emerging resonance was attributed to the charge–density–wave?CDW?collective excitations.By using the Drude–Lorentz model fitting,the CDW collective mode becomes increasingly damped with the increasing temperature.Furthermore,the kinks of the CDW collective mode at the magnetic transition temperature are analyzed,which indicate the coupling of spin order with electric polarization.