Synthesis And Magnetoelectric Properties Of New Single Phase Multiferroic Materials

High-temperature superconductor and colossal magnetoresistance materials have promoted enormous interests around the world since their discovery. While recently multiferroic materials, one branch of condenced matter and spintroics. become one of the hottest researches. Multiferroic materials are those who simultaneously have ferromagnetic/antiferromagnetic (FM/AFM) and ferroelectric (FE) properties. The main characteristic of the multiferroics is the presence of magnetoelectric coupling. The polarization can be switched under the applied magnetic field, and magnetization can be switched under the applied electric field. Multiferroics have been extensively studied for their appealing applications in memory storage and functional sensors. Practically, only the coexistence of both FM and FE properties above room temperature can be used. However, multiferroic materials with all the aspects are rare and have not been reported. The motivation of this dissertation is to prepare new single phase multiferroic materials and improve their room-temperature (RT) FM property and other physical properties. The main systems include LaFe1-xCrxO3 microcrystal and Bi1-xGdxFeO3 nanoparticles.1. LaFe1-xCrxO3 (0≤x≤1) has been synthesized by mild hydrothermal method. Highly crystallized cubic were obtained. The alkalinity in initial reaction mixtures plays a critical role in controlling the designed stoichiometry of final compositions. Their magnetic properties are strongly dependent on the compositions. A symmetric trend was formed with a maximum magnetization for the sample at x=0.5. It can be attributed to that the substitution of Cr ions affects the relative proportion of Fe-O-Cr FM superexchange interaction. Weak FM interaction was observed for the samples from x=0 to 0.9, but it is very weak due to the random distribution of Fe and Cr ions in the B sites, confirmed by the linear variation of lattice parameters. Nevertheless, compared to that prepared by solid state method, samples prepared by hydrothermal route shows much higher FM ordering. The evolution of magnetic ordering transition temperatures has a close relationship with substituent ratios, for the competition of AFM and FM interactions.2. We further study the magnetic and electric (or dielectric) properties of LaFe0.5Cr0.5O3 prepared by hydrothermal method. It shows a frustrated magnetic system with simultaneous FM and AFM interactions. However, it shows a much higher FM ordering compared to that prepared by solid state method, even at room temperature. LaFe0.5Cr0.5O3 shows a semiconductor behavior. Temperature-dependence of dielectric constant reveals that there are two phase dispersions. The dispersion at high temperature (>300 K) implies a FE relaxor transition, confirmed by the room-temperature FE hysteresis. The anomaly at low temperature (<300 K) attributes to the coupling of phonon and spins, as the short-range FM transition temperature is around 300 K. The field-dependence of dielectric data also implies the magnetocapacitance effect near the magnetic transition temperature.3. Bi1-xGdxFeO3 (x= 0,0.1,0.2,0.3) nanoparticles were synthesized by polyol-mediated method with particle size of 21-47 nm. Rietveld refinements show a phase transition from a rhombohedral phase (R3c) for x= 0 to an orthorhombic phase (Pn21a) for x= 0.2 and 0.3, and x= 0.1 is a mixture of both phases. Improved FE hysteresis loops are observed at room temperature for all samples. The substitution of Gd ions for Bi enhances the FM properties of this system. Compared to bulk materials, a much larger magnetization (Ms= 0.11μB/f.u.) is obtained for x= 0.2 at 300 K. Temperature- and field-dependence of magnetization curves reveal the frustrated magnetic behavior of this system. It arises from the coexistence of canted AFM interaction of Fe-O-Fe and paramagnetic properties of Gd3+ ions. The variation of magnetic behaviors at different formation temperatures is also related to the competition between canted AFM Fe sublattice and paramagnetic Gd sublattice.