Crystal Growth And Low-temperature Physical Properties Of Magnetically Frustrated Mutiferroics CuFe_1-xGa_xO₂ And ErMnO₃

Multiferroic materials have been a hot topic in condensend matter physics out of their potential applications in magnetoelectronics,spintronics and magnetic memory technology.In broad terms,multiferroicity refers to a kind of coupling effect between magnetic and electric orders in solids.Magnetically frustrated materials usually have very complex magnetic-structure transitions,in which the occurrence of some special magnetic orders may break the space-inversion symmetry resulting in the variation of electric order.Triangular-lattice CuFeO2 and hexagonal ErMnO3 have been confirmed to be two typical cases of single-phase multiferroics with frustrasted magnetism,and it is very important to carry out deep study on the complex magnetic properties for the clear understanding of the multiferroic mechanism in them.In this dissertation,high-quality CuFe1-xGaxo2(x = 0,0.035,0.08,012)and ErMno3 single crystals are obtained,and the magnetic properties of as-grown CuFe1-xGaxo2 and ErMno3 single crystals are studied through the detail characterization of magnetic susceptibility,specific heat and thermal conductivity.The whole dissertation consists there chapters as following:In chapter one,some multiferroic mechanism based on spin-orbit and spin-lattice coupling are described at first.Then the research progress of magnetic properties and multiferroics of CuFeo2,CuFe1-xGaxo2 and ErMno3 are introduced in detail,respectively.Chapter two reports the growth of high-quality CuFe1-xGaxo2(x = 0,0.035,0.08,012)single crystals by optical floating-zone method and the low-temperature physical properties.Pure-phase CuFe1-xGaxo2 single crystals with good crystallinity are obtained through continually lowering the heat power at the initial stage of the growth,which can be an effective way to preventing the reduction of Fe2o3 in the growth.Then it is found that the magnetic anisotropy of CuFe1-xGaxo2 is gradually suppressed with the increasing of Ga-content from the results of magnetic susceptibilities.The low-temperature specific heats of Ga-doped samples are obviously larger than those of x = 0 sample,which reveals more stronger magnetic excitation in the ground state of Ga-doped samples.In contrast,the low-temperature thermal conductivities of Ga-doped samples are strongly suppressed compared with those of x = 0 sample,which should be caused by the stronger scattering of phonons by magnetic excitation.Moreover,zero-field ?(T)of x = 0 and 0.035 samples show clear anomalies at the magnetic-transition temperatures because of drastic scattering of phonons by spin fluctuation near magnetic transition.Applying c-aixs magnetic fields,the ?(H)isotherms of x = 0 sample show step-like change at the transition field,while in the case of x=0.035 and 0.08 samples,a clear minimum is observed in ?(H)at the transition field.Besides,the ?(H)isotherms of x = 0,0.035 and 0.08 samples obtained in the field-up and field-down process show clear irreversibility in a wide field-range,and the origin of the irreversibility is discussed.Because of the strong spin-phonon coupling in CuFe1-xGaxO2,the behavior of thermal conductivity can well reflect the variation of magnetic properties.Chapter three reports the study on the magnetism and magnetic transitions of ErMnO3 single crystal through magnetization,specific heat and heat transport measurements.The magnetization M((H)show that ErMnO3 undergoes three magnetic transitions at 0.8,12 and 28 T in the c-axis magnetic field.The specific heat C(T)appear a peak-like feature at 2.2 K,which should be due to the magnetic transition of Er3+ moments.For the low-temperature thermal conductivity(?),a clear dip-like feature appears in ?(H)isotherm for H//ab at 1-1.25 T.While in the case of H//c,a step-like increase is observed at 0.5-0.8 T.The transition fields in(H)are in good agreement with those obtained from magnetization,and the anomaly of ? can be well undersood by a spin-phonon scattering scenario.Based on above results,the natures of magnetic structures and corresponding field-induced transtions at low temperatures are discussed.