| Transition-metal oxides characterized with anisotropic d-orbital electrons are subject to intense discussion in strongly correlated electron systems, ranging from colossal magnetoresistance (CMR) to high temperature superconductivity (HTSC). The orbital degree of freedom often underpins complex physical properties and a variety of extraordinary phenomena while coupling with charge, spin and lattice. In this thesis, we demonstrate a fascinating example of orbital-related physical properties in Mn-based spinels. The strong octahedral preference of Jahn-Teller ions Mn3+ results in simultaneous chemical and structural phase separation into Mn-poor (cubic) and Mn-rich (tetragonal) regions, forming two types of rectangular nanorods with cross section checkerboard-like (CB). Because of the pioneering discovery of checkerboards in the nonmagnetic ZnMnxGa1-xO4, we chose to study two magnetic spinel systems: (1) Mg(MnxFe1-x)O4, where unfortunately only poorly-ordered magnetic nano CBs were observed; and (2) Mn-doped CoFe 2O4, the nano CBs with distinct magnetic properties and tunable sizes achieved here are highly ordered, exhibiting a nearly ideal architecture for the patterned perpendicular recording medium. The evolution of such compositional separation and topological nanoscale ordering is reasonably understood based on a three dimensional (3D) phase-field microelasticity (PFM) model. |
