Structure And Multiferroicity Of LuFe₂O₄and LuFeO₃-Based Ceramics

Recently, multiferroic materials that combine at least two ferroic parameters have attracted remarkable interest due to the rich physical contents and the great potential applications in smart and multifunctional devices. In the present thesis, the crystal structure, dielectric, ferroelectric and magnetic properties of LuFe2O4and LuFeO3-based ceramics were investigated systematically. Moreover, the possible multiferroic characteristics in these ceramics were discussed. The following primary conclusions were obtained.Single-phase LuFe2O4could be obtained under vacuum environment and the crystal structure was identified to be rhombohedral in space group R3m. An obvious dielectric relaxation with activation energy of0.29eV was observed between175K and275K, which was ascribed to the electron hopping between Fe2+and Fe3+ions. The Neel temperature of the present ceramics was-250K, and a re-entrant spin glass transition was indicated at-216K. The nonlinear M-H hysteresis loop indicated weak ferromagnetic characteristics at room temperature. Meanwhile, a remarkable decrease appeared in the dielectric constant of the as-magnetized sample, implying the magnetodielectric effect in the LuFe2O4ceramics. No convincing ferroelectric hysteresis could be obtained in LuFe2O4ceramics at room temperatrure, which might be caused by high leakage.LuFeMgO4, which was isomorphous to LuFe2O4, was indentified to be antiferromagnetic with a Neel temperature of27K. The slim hysteresis loop measured at10K demonstrated the weak ferromagnetic or ferrimagnetism ordering in LuFeMgO4ceramics, which might be attributed to the canting angle in Fe moment. An obvious Debye-type dielectric relaxation with an activation energy of0.35eV was observed between240and420K. The replacement of Mg2+caused the suppression of electron transfer, resulting a larger activation energy than that of LuFe2O4. The dielectric relaxation could be strongly suppressed by oxygen annealing, indicating that oxygen vacancies played an essential role in the dielectric response of LuFeMgO4ceramics.The crystal structure of LuFeCuO4was similar with those of LuFe2O4and LuFeMgO4. An obvious dielectric relaxation with activation energy0.37eV was observed between250and415K, which could be ascribed to the electron hopping between the ions on Fe sites. With the decrease of spin moments of the divalent ions, the values of TN became smaller in the order of LuFe2O4, LuFeCuO4(TN～57K) and LuFeMgO4. A P-E hysteresis loop was observed in LuFeCuO4ceramics at150K, and the replacement of Cu2+resulted in the weaker ferroelectricity than LuFe2O4. At room temperature, remarkable magnetodielectric response (～7%) was observed in LuFeCuO4ceramics.LuFeO3was identified to be an orthorhombically distorted perovskite structure with space group Pbnm. Two Debye-type dielectric relaxations were observed in LuFeO3ceramics. The low temperature relaxation was originated from the electron hopping between Fe2+and Fe3+ions, while the high temperature relaxation was related with oxygen vacancies. The Neel temperature for LuFeO3was628K, and the weak ferromagnetic characteristics owing to the canting angle of magnetic spins were detected at room temperature. Obvious ferroelectric loops were observed at room temperature, and the values of electric polarization increased with an applied external magnetic field. Meanwhile, room-temperature magnetodielectric effect was detected, and LuFeO3was expected as a promising candidate for new multiferroic materials.In LuFeO3, partially substituting Mn for Fe did not change the crystal structure. The low temperature dielectric relaxation was suppressed and only one dielectric anomaly around360-530K was observed in LuFe0.95Mn0.05O3ceramics. The smaller polarization compared with LuFeO3suggested the important role of electron hopping between Fe2-and Fe3+ions in ferroelectricity. The results of thermal analysis indicated that the Neel temperature decreased with Mn-substitution, which could be ascribed to the weaker Fe3+-O-Mn3+and Mn3+-O-Mn3+exchange interactions compared with Fe3+-O-Fe3+interaction. Due to the strong anisotropy of Mn3+ions, a spin reorientation transition could be induced in LuFeo.95Mno.05O3.