The Ultrafast Dynamics Research Of Multiferroic Materials

Due to the wide application prospects and the interesting physicalcharacteristics, multiferroic materials have been paid extensive attention in recentyear. Time-resolved pump-probe spectroscopy is widely used to explore the ultrafastdynamics in various materials, which can provide indispensable dynamicalinformation of magnetoelectric coupling in multiferroic materials. In this thesis, byusing time-resolved pump-probe technology, we focuses on the studies of ultrafastdynamics of the multiferroic materials Bi0.8La0.2Fe0.99Nb0.01O3（BLFNO）, hexagoncubic YMnO3film, and α-Fe2O3film. The main contents and conclusions arepresented as follow:(1) By using the femtosecond time resolved reflectance spectroscopy, weinvestigated the coherent longitudinal acoustic phonons in Bi0.8La0.2Fe0.99Nb0.01O3（BLFNO）films. The transient photostriction effect is the generation mechanism ofcoherent longitudinal acoustic phonons. By applying external voltage on thepiezoelectric substrate PbMg1/3Nb2/3-PbTiO3, we demonstrated that the propertiesof the thin film can be modulated by the external field applied on the substrate.(2) By using the time resolved transmission spectroscopy, the carrier dynamicalbehavior of the Mn3+3d of YMnO3thin film was investigated systematically. As thetemperature decreases, the transient transmission signal at the zero-delay time isdecreased. The blue shift of the Mn3+3d energy level in YMnO3film may lead to thetemperature dependent transmission change, which is originated from theshort-range antiferromagnetic order. The relaxation time of the fast process isobserved to increase significantly when the temperature is lower than TN, whichindicates that the strength of electronic-phonon coupling is weakened below TN.(3) By using the time resolved spectroscopy, we investigate longitudinal acousticphonons in high quality hematite thin films (α-Fe2O3) grown on different substrates(BaTiO3, SrTiO3, and LaAlO3). Transient reflection measurements (ΔR/R) indicate the photoexcited electron dynamics. Longitudinal acoustic phonons in α-Fe2O3filmcan be created by ultrafast excitation of the ligand field state. Our experimentalresults reveal that strain modulation and the out-of-plane elastic properties inhematite films can be controlled by using different substrates.