| Cr-based compounds A Cr2X4 (X= O, S, Se) with spinel structure hold many fascinating physical properties such as magnetic frustration, magnetoelectric coupling, magnetostriction, and also exhibit ferromagnetic, ferromagnetic, spin ice, spin liquid, which are important strongly correlated materials having both fundamental research and application values. The magneto-optical spectroscopy study under high magnetic field is one of basic ways to understand the complicated physical properties of the correlation and coupling between charge, oribital, lattice and spin, which is also an essential fundament to develop spintronics device. With the application of external static magnetic field, the magnon excitation energy corresponding to the ferromagnetic resonance, exchange resonance and electromagnon in such materials may beyond the measurement region of conventional microwave electronic spin resonance, and consequently move into terahertz (THz) spectrum region. Magneto-optic terahertz time domain spectroscopy based on femtosecond ultrafast optics can provide the information of both temporal profiles with femtosecond precision and frequency-domain spectrum with Gigahertz precision, and also can accurately detect the evolution of electromagnetic wave amplitude, phase and polarization arising from the interaction between high magnetic field and materials, which offer a special method to measure high-frequency spin dynamics. The research in this dissertation focus on three kinds of chromium spinels materials and aim to understand the basic spin dynamics including magnenon excitation, spin precession mode, magnetic frustration induced novel spin state, dynamic magnetoelectric coupling, etc. The magneto-optic THz spectroscopy techniques were developed and employed to systemically investigate the interaction between high-magnetic-filed THz and this material system. The main results are summarized as follows:(1) We developed the Terahertz time domain spectroscopy system (THz-TDS) under high magnetic field up to 10 T. This system can realize the spectroscopy measurement of THz absorbtion and Faraday rotation in the frequency region of 0.1-2.0 THz and the temperature range of 4-300 K by using GaAs photoconductive antenna terahertz source and free space electro-optic detection method. Meanwhile, we conducted the basic characterization of La0.66Ca0.33MnO3 films and related materials by using this system, and observed THz photoconductivity anisotropy behavior of Mn-based oxides under magnetic field.(2) We have investigated the THz transmission through ZnCr2Se4 single crystal with THz time-domain spectroscopy at low temperatures and under the application of magnetic fields up to 10 T. The magnetic resonance related to screw spin structure is moved into the THz spectra range and subsequently shifted further toward the high frequency side of the spectrum as the external magnetic field is increased above 5 T. A splitting of the resonance absorption can be distinguished when the applied magnetic field is above 8 T for temperature below 20 K, which also exhibit the absorption selection rules associated with the direction of magnetic field and crystal. The measured Faraday rotation spectra as the function of THz frequency and temperature reveal the antiferromagnetic magnetic phase transition around 20 K, which indicates that THz-TDS may provide a effective detection approach to resolve magnetic phase transitions.(3) We have investigated the THz transmission through MnCr2O4 polycrystalline sample with THz-TDS at different temperatures and under the magnetic fields up to 9 T. The magnetic resonance absorption can be observed in THz frequency range with the application of external magnetic field above 4 Tesla and subsequently shifted further toward the high frequency side with increasing magnetic field. The measured linear co-H relation agrees well with the Larmor precession frequency, which reveals that the magnon excitation in MnCr2O4 is dominated by ferromagnetic resonance in high magnetic field.(4) We have investigated the THz transmission through CoCr2O4 single crystal with THz-TDS at different temperatures and under the magnetic fields up to 8 T. The THz absorptions originating from the sublattice exchange magnon resonance in CoCr2O4 ferrimagnet are observed. We also found the circular absorption dichroism in THz spectra range by THz-TDS. When the applied magnetic field is along THz wave vector, the left-handed circularly polarized (LCP) absorption is active and right-handed circularly polarized absorption (RCP) is suppressed. The pronouncedly Faraday ellipticity can be found both in time and frequency domain. The complex indices of refraction for LCP and RCP THz are obtained under different magnetic field. The dispersions show remarkable chirality determined by the exchange magnon resonance. |
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