Research On The Unusual Ferromagnetism In Some Strongly Correlated Electronic Systmes

One of the fundamental yet important research fields in the condensed matter physics is to study the magnetic phenomena. Especially, in the strongly correlated electronic systems, the spin degree of freedom is strongly coupled and interplayed with the orbital, lattice, band structure, and electronic transport properties, yielding many rich and peculiar phenomena. Studies on these phenomena can improve our understanding on the complicated interactions involved in the strongly correlated electronic systems. Therefore, in this dissertation we studied in detail on several strongly correlated electronic systems with unusual ferromagnetism and revealed their origins.First, we studied the coupling of spin and orbital orderings as well as the influence of the local structural distortion in the RTiO3 (R = Rare earth) system. With decreasing the size of R3+ ions, the magnetic ordering of Ti3+ (t2g1, S = 1/2) ions changes from antiferromagentic for R = La, Pr, Nd, Sm to ferromagnetic for R = Gd,…, Lu, Y. By measuring the thermal conductivity on a series of high-quality single crystals, we provided the first experimental evidence of the t2g orbital ordering occurring simultaneously at the spin-ordering temperature in this system. Analysis of the critical behaviors based on the magnetic and specific-heat data enabled us to realize the important role of local structural distortion on the changes of spin ordering with decreasing the rare-earth size, and also to construct the patterns of spin and orbital ordering for the ferromagnetic compounds. A special crystal-field environment can give rise to unusual behaviors of spin ordering. One of the most typical examples we found is the A-site 1:3 ordered perovskite CaCu₃Ge₄O₁₂, where the A-site Cu²⁺(t2g6eg3, S = 1/2) ions with an unusual square-planar coordination are ordered ferromagnetically below Tc = 12 K. A detailed analysis on the critical behaviors based on the magnetic and specific-heat data near Tc revealed that CaCu₃Ge₄O₁₂ is a very rare example of three-dimensional short-rang-interaction Ising ferromagnet.Then, we focused on the unusual ferromagnetic phenomena in the 4d and 5d transition-metal perovskite oxides. Among the 4d transition-metal oxides, SrRuO3 is the first known metallic ferromagnet with Tc as high as 165 K. On the other hand, the isostructural CaRuO3 does not show any magnetic ordering down to the lowest temperature. In order to understand such a peculiar magnetic behavior, we have successfully prepared a series of ARuO₃ perovskites under high-pressure and temperature conditions. We not only changed continuously the average A-cation size <rA> from Ca through Sr all the way to Ba, but also introduced the size varianceσ2 while keeping the <rA> constant. A systematic study on the ferromagnetic transition temperature Tc and the critical behavior as a function of <rA>,σ2, hydrostatic versus non-hydrostatic pressures, as well as the synthesis conditions revealed that the ferromagnetism in the perovskite ruthenates is very sensitive to the local structural strain and disorder; the highest Tc occurs only at the composition where the <A-O> and <Ru-O> bond lengths reach equilibrium whereas either compressive or tensile stress reduces Tc. The evolution of the isothermal magnetization curves demonstrated that the nature of the ferromagnetism in the perovskite ARuO₃ is the Heisenberg localized-spin ferromagnetism rather than the Stoner itinerant-electron ferromagnetism. Furthermore, we continued to study the disappearance of ferromagnetism in the perovskite Sr1-zPbzRuO₃ system synthesized under high-pressure and temperature conditions. Measurements of the structural, magnetic, and electronic transport properties under ambient and/or high pressures revealed that the disappearance of ferromagnetism upon Pb doping arises from the bandwidth broadening due to the orbital hybridization between the Pb 6s lone-pair and Ru 4d electrons. Among the 5d transition-metal oxides, 9R BaIrO₃ is the first known ferromagnet with Tc as high as 180 K. In addition, it was found that the ferromagnetic transition is accompanied with the formation of a charge density wave. Aided by the high-pressure and temperature technique, we explored systematically the high-pressure sequences of the 9R BaIrO₃ and discovered three new polytypes, i.e. 5H, 6H, and 3C phases, in which the structure of 5H phase was determined for the first time by an ab initio procedure. A systematic study on their magnetic, electrical transport, and thermodynamic properties showed that with increasing the ratio of corner-shared octahedra along the c axis, or with increasing the bandwidth, the ground state of BaIrO₃ polytypes changes from a ferromagnetic insulator with Tc = 180 K for the 9R phase, through a ferromagnetic metal with Tc = 50 K for the 5H phase, finally to an enhanced Pauli paramagnetic metal approaching a quantum critical point for the 6H phase.Finally, we investigated the influence of quantum critical phase fluctuations on thermopower of the itinerant-electron ferromagnet MnSi, which shows a quantum critical point at Pc≈1.5 GPa when the ferromagnetic transition temperature Tc is suppressed to zero temperature and the resistivity exhibits non-Fermi-liquid behavior. For the first time, we measured the thermopower S(T) under high pressure on a single-crystal MnSi and found that S(T) is enhanced dramatically near Pc and follows exactly the well-known S/T∝-lnT dependence, thus providing an important experimental evidence for the enhanced thermopower owing to the quantum critical phase fluctuations.Besides the above extensive investigations on the anomalous ferromagnetism, we also explored the thermoelectric cobaltites NaCo2O4 and Ca3Co4O9 with layered structure in this dissertation. We have obtained a highly textured NaCo2O4 ceramic with the help of a cold-pressing technique. Compared to the ceramic samples prepared with other methods, our samples exhibit higher degree of texture and density, which reduces the resistivity and increases the thermopower, leading to an enhancement of the thermoelectric performance. For the Ca3Co4O9, we found that the metal-simeconductor transition at TMS≈400 K is of first-order characterized by the thermal hysteresis in the resistivity curves, the endo/exothermic peaks on the differential scanning calorimetry (DSC) curves, and the sharp peak in the thermal expansion coeffcieintα(T). The first-order character of the metal-semiconductor transition can be rationalized from the Virial theorem, i.e. a discontinuous increase of the mean <Co-O> bond length is associated with the transition from itinerant to localized electrons within the [CoO2] planes. This model can also account for the enhancement of the thermoelectric performance above TMS.