New forms of polar and spin ordering in Pb2(Mn,Co)WO6 double perovskites: symmetry analysis of ferroic properties

In recent years, the scientific community focused on the study of new materials, in which the simultaneous presence of different functionalities in the same compound represents the basis for the fabrication of multifunctional device. In this framework the multiferroics materials, in particular the magneto-electric ones, have a special place. In this type of systems the symmetry constrains, dictated from the magnetic point group, play an essential role in regulating all the physical properties and in particular the magneto-electrical coupling in the system. This Ph.D. thesis presents the study of a double perovskite system, Pb2(Mn,Co)WO6, that reveals a complex and quite interesting magneto-electric multiferroic character. The work is based on the accurate analysis of the magnetic symmetry, exploiting the use of the coloured groups and its generalization to the superspace formalism, which joined with a comprehensive physical characterization, lead to define in detail the complicated picture of the system properties. The study is mainly focused on the two end members of the solid solution and on the intermediate composition containing Mn and Co in the B site in a 50/50 ratio. The manganese end member of the solid solution displays at room temperature (RT) an unusual ferrielectric state characterized by the presence of two independent Pb sublattices. By decreasing the temperature, the system exhibits two magnetic phase transitions and a magnetic ground state characterized by an incommensurate to commensurate (IC/C) magnetic phase transition at 8 K. The magnetic structure is analysed with the use of the superspace formalism that allows an easy description of the IC/C transition. The mixed compound Pb2Mn0.5Co0.5WO6 shows an antiferroelectric character and presents a first magnetic transition at 190 K, characterized by short-range order, whereas a second magnetic transition leading to long-range order is observed at 9 K. The long-range ordered phase is an antiferromagnetic collinear structure presenting an acentric magnetic space groups, suggesting the presence of a spontaneous electrical polarization confirmed by the electrical characterization. The combined use with the magnetic symmetry suggests the symmetric exchange striction mechanism to be at the basis of the observed phenomena. The other end member presents at RT an incommensurate modulated structure that undergoes at 230 K a structural phase transition to an orthorhombic centrosymmetric structure. The magnetic structure analysis indicate a transition to an incommensurate spin structure below 12 K and, as in the previous case, the symmetry analysis suggests the presence of a spontaneous electrical polarization below the magnetic transition. Finally, joining these results with the preliminary ones obtained on other compositions, the multiferroic phase diagram of the Pb2(Mn,Co)WO6 system is sketch, pointing out, with the increase of the Co content, a transition from proper ferrielectricity to improper ferroelectricity, driven by the breaking of the centrosymmetric character of the nuclear structure operated by the magnetism.