Disorder and Topological Defects in Magnetically Frustrated Single Crystals and Thin-Film Metamaterials

The "heavy" 4d and Sd transition elements (TE) have more extended d-orbitals compared to 3d electron materials. Stronger p-d hybridization, spinwarbit (SO) and electron-lattice couplings, and reduced intra-atomic Coulomb U and crystalline electric field (CEF) interactions generate competitions between metallic and insulating states, or paramagnetism and magnetic order. Small variations of composition, pressure or applied fields can induce drastic changes in the varied ground states (ferroelectric, orbital or magnetic order, superconductivity, density waves) exhibited by TE oxides, as well as control technologically important phenomena such as colossal magnetoresistance (CMR) and giant magneto electric effects (GME). We are investigating perovskite variallts ABOj "8 (A=Ca,Sr,Ba; B=MIl,Fe,Ru) alld R-type ferrites (Ba,Sr)M1:b:Ru4:b:On (M=Fe,Co,MII, Ti). Fe-bearing examples of both phases exhibit a rare coexistence of long-range ferromagnetic (FM) order accompanied by narrow-gap semiconducting properties at temperatures above 400 K. Our research addresses the followil'g qllestions: 1) The rarity ofroom-temperatllre, FM semiconductors remains an obstacle to development of spinpolarized semiconductor devices for spintronics; We are identifYing fundamental physical and chemical factors that govern the occurrence ofFM order above room temperature in ferrite and perovskite phases. 2) Some perovskite materials appear to exhibit a rare coexistence ojmagl1etic alld electric polarizatiolls 01' tI,e same B lattice sites, and various R-type ferrites show evidence for coexisting magnetic and electric polarizations, whose interplay can be controlled by modest applied fields or electric currents. We are verifYing these coexistent phenomena and exploring their ramifications for potential devices. 3) Both ferrites and perovskites exhibit highly anisotropic physical properties that can be sensitively controlled by varying the relative concentration of 3d versus 4d or 5d elements, which we intend to show adjusts the strengths of the CEF and SO interactions that compete with magnetic frustration and other fundamental interactions to detennine the wide-ranging ground states exhibited by these materials. We take an illtegrated, i1lterdisciplillary approach to the discovery and characterization of novel TE oxides whose physical properties reflect competing interactions: We synthesize and identifY novel materials, grow bulk single crystals to comprehensively study physical properties relevant to fundamental theories, as well as fabricate and study thin films and heterostructures relevant to device applications. Our broad expertise anti techllical assets pel'll';t compre/,ellsive illvestigatio"s of electrical transport, magnetic, dielectric and thernlOdynamic properties over a wide range of temperatures 0.05 < T < 1000 K and magnetic fields 0 < !loH < 14 T, and high-pressure electrical resistivity and magnetic moment measurements to 10 GPa. We are using National Laboratory facilities andlor external collaborators to conduct EXAFS, and magnetic soft X-ray and neutron scattering experiments to characterize small single crystals and thin films that are not easily studied via conventional electrical transport, magnetic or optical techniques.