| Multiferroics, a new multifuncitional material, possess magnetism and ferroelectricity simultaneously. More importantly, the coupling between magnetism and ferroelectricity induces novel physical phenomena and offers possibilities for application usage. Recently, the frustrated materials, in which the transition of the magnetic ordering can induce the presence and evolvement of ferroelectricity, has received a lot of research interests. However, because of the complexity of magnetic properties in the frustrated materials, the magnetic structure and the phase diagram of these frustrated materials are not very clear.To solve the above problem, the heat transport properties of multiferroic RMnO3(R=Y, Lu, Tm, Ho) and CuFeO2single crystals are studied carefully, and their magnetic properties and magnetic structures are discussed. The dissertation consists of three chapters and the main contents of each chapter are as following.In chapter One, the research progress of multiferroics are reviewed. First, some possible physical mechanisms about the coexistence and the coupling between magnetism and ferroelectricity are introduced in detail. Second, the elementary excitations in multiferroics are discussed. Third, research progress of the magnetic structures and the phase diagrams in multiferroics RMnO3and CuFeO2are introduced. Finally the basic information of heat transport is briefly mentioned.Chapter Two reports the heat transport study of multiferroics RMnO2(R=Y, Lu, Tm, Ho) single crystals. Thermal conductivity results provide a transparent demonstration of the strong spin-phonon coupling in RMnO3. For Y(Lu)MnO3, in which the Y3+(Lu3+) ions are nonmagnetic, the low-T heat transport behaves like usual insulating crystals. In TmMnO3and HoMnO3, in which the Tm3+ions and Ho3+ions are magnetic, because of the spin-phonon coupling at low temperatures, the thermal conductivities are strongly suppressed, but the magnetic field can significantly enhance the thermal conductivities. For TmMnO3, the Tm3+moments do not form a long-range ordered state but are antiferromagnetically correlated, the thermal conductivity is lower than that of YMnO3because of the magnetic scattering on phonons; additionaly, a sharp dip-like feature in κ(H) isotherms for H‖c is observed. For HoMnO3, the Ho3+ions present antiferromagnetic order at4K, the number of magnetic excitations is most in RMnO3; therefore, the thermal conductivity is the smallest. In particular, one and three dip-like anomalies are also observed in the low-T κ(H) isotherms for H along the a and the c axes, respectively. The evolution of the magnetic structures of TmMnO3and HoMnO3at low temperatures can be deduced from the κ(H). The comparison of transport properties among these materials confirms that the magnetism of rare-earth ions is the key to determine the H-T phase diagram and the strength of spin-phonon coupling.Chapter Three shows the heat transport study of multiferroics CuFeO2single crystals. The phonon thermal conductivity of CuFeO2in zero field is strongly suppressed and a dip-like feature is observed at10K due to the partically disordered phase to the4-sublattice phase transition. This dip vanishes with substitution of3.5%Ga for Fe, indicating weakened critical scattering across the transiton. For CuFeO2, The κ(H) isotherms show two irreversible transitions which coincide with the two first-order transitions, and a clear hysteresis for the whole field regime at low temperatures. Because the gap in magnon spectrum can be changed with the magnetic field in different phase, the κ(H) isotherms exhibit complex behavior. These results indicate a strong spin-phonon coupling in this material. |
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