The Ultrafast Dynamics Of Multiferroics

In the past decade,the development of ultrashort pulse laser provides an new insight into condensed matter physics on critical time scales.The studies on ultrafast optical induced charger order melting,spin-reorientation,and the structural phase transition have not been understood completely yet,more original theoretical and experimental investigations are required.In this thesis,we foucs on the comlex phase transition and the optical manipulation in multiferroic materials and their heterostructures.The main contents are summaried as follows:1)We reported the ultrafast pump-probe study of bismuth ferrite film and its A-site,B-site doped compounds.In the transient reflectivity study of multiferroics,the fast relaxation of photo-excited electrons is attributed to the scattering of electrons with lattice-vibration modes,the slow relaxation process is attributed to the scattering of lattices and the spin system.When the temperature of Bi Fe O₃ film down to 140 K and 200 K,a surface phase transition is determined by the change of these two relaxation processes;the charge transfer transition in Bi Fe_0.5Mn_0.5O₃ is shown to be spin-dependent and element selected.The spin-lattice coupling time shows the element selected feature,which illustrated the two independent magnetic phase transitions to the AFM Mn-O-Mn and the FIM Mn-O-Fe order,respectively;the anomaly of the electron-phonon coupling process and the frequency of coherent longitudinal acoustic phonons in Bix Dy1-x Fe O₃ films take place in a rhombohedral-orthorhombic phase transition induced by increasing the Dy doped ratio.2)We presented the circular-pump time-resolved measurements of the ultrafast optical orientation and optical induced anisotropy in Bi Fe_0.5Mn_0.5O₃ film andSm_0.6Fe_0.4O₃ single crystal.The spin polarization alignment and the followed relaxation in multiferroic manganite Bi Fe_0.5Mn_0.5O₃ film are investigated excited by linearly and circularly polarized femtosecond pulses,respectively.The ultrafast switching of polarization ellipticity?< 150 fs?is arised from a transient coherent spin polarization by optical orientation.The ultrafast spin polarization switching is assigned to the Raman coherence process.In multiferroic single crystal Rare-earth ferrite Sm_0.6Dy_0.4Fe O₃,a circular pump induced spin reversal at the vicinity of its spin reorientation temperature was presented,which is attributed to the optical induced magnetic anisotropy that is related to the electron-phonon coupling in Rare-earth ferrites.3)By means of Terahertz Time Domain Spectroscopy and Optical pump-THz probe technique,multiferroic-semiconductor,graphene-ferroelectric heterostructures were investigated.The transmittance of THz pulse is modulated by the gate voltage on the multiferroic-semiconductor heterostructure Bi Fe O₃/Si.By using optical pump-terahertz?THz?probe spectroscopy,the photoexcited terahertz conductivity dynamics of CVD grown graphene is investigated at different atmosphere environment.It's shown that the Fermi energy of doped graphene is modualted by oxygen adsorption and desorption,which is probed by transient THz conductivity measurement.The graphene-ferroelectric heterostructure graphene-Li Ta O₃ was studied by THz time domain spectroscopy,by varying the temperature,it is found that the pyroelectric effect of Li Ta O₃ helped to modulated the fermi-energy of graphene,which results in the modulation of THz transmittance.