Date of Award
Doctor of Philosophy in Applied Physics - (Ph.D.)
Onofrio L. Russo
Ken Keunhyuk Ahn
Multiferroics are a class of materials which possess both magnetic and electrical polarization with possible coupling between them. They show promise to enable new sensors and data storage devices with novel features, such as the possibility of writing polarization bits with magnetic fields at low power. The coexisting magnetic and ferroelectric order parameters are usually weakly coupled, preventing practical use. The development and study of new classes of materials with large magnetoelectric couplings is of high importance. Understanding the structure of these materials is key to this effort.
As one class of these systems, the RX3(BO3)4 has been shown to have a giant magnetoelectric (for R=Ho, X=Ai) effect of P = 0.36µC/cm2 in magnetic fields, which is significantly higher than the reported values for other multiferroic compounds. The atomic level origin is still not understood. In this work, macroscopic and atomic level properties of the full class RX3(BO3)4 (R=Ho, Gd, Eu, Sm, Nd, and X=Ai or Fe) are explored by various experimental measurements, complemented by density functional theory calculations. In HoAi3(BO3)4, an anomalous change in the structure is found in the temperature range where large magnetoelectric effects occur. No large structural change or distortion of the HoO6 polyhedra is seen to occur with a magnetic field. The magnetic field dependent structural measurements reveal enhanced structural correlation between neighboring HoO6 polyhedra. A qualitative atomic-level description of the mechanism behind the large electric polarization induced by magnetic fields in the general class of RAi3(BO1)1 systems (R= rare earth) is developed. A detailed structure related mechanism for the general RX3(BO3)4 is developed by high-resolution x-ray diffraction measurements.
Another system under study is the standard class of ferroelectrics: ATiO3 including SrTiO3 and BaTiO3. The A=Sr system is known not to possess a polarization state in bulk form. In this work, pressure dependent structural measurements on monodispersed nanoscale SrTiO3 (STO) samples with average diameters of 10 to ?80 nm are conducted. A structural phase diagram of nanoscale SrTiO3 with reduced dimension is developed. A robust pressure independent polar structure is detected in the 10 nm sample for pressures up to 13 GPa while a size-dependent cubic to tetragonal transition occurs (at P = Pc) for larger particle sizes. The stability and polar characteristics of the A=Ba system are explored, and mechanisms for stabilizing the polar phase in nanoscale SrTiO3 and BaTiO3 are examined.
Zhang, Han, "Structural studies to determine the mechanisms supporting multiferroic and ferroelectric properties of complex oxides" (2018). Dissertations. 1699.