| AVS 56th International Symposium & Exhibition | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM2-TuM |
| Session: | Complex and Multifunctional Oxides |
| Presenter: | G. Lucovsky, North Carolina State University |
| Authors: | G. Lucovsky, North Carolina State University C. Adamo, Penn State University D.L. Schlom, Cornell University K.B. Chung, North Carolina State University |
| Correspondent: | Click to Email |
There is considerable interest in complex oxides comprised of transition metal, TM, and lanthanide rare earth, LRE, atoms with room temperature magnetic properties that can modulated by electrical input. Ferro- and ferri-magnetic properties require strongly correlated spin sub-bands derived from TM(LRE) d(f)-states, and have been reported in elemental oxides: CrO2 and Fe3O4, and EuO2. Strongly correlated bands are created by a double exchange mechanism requiring a transition to a metallic state. The incorporation of tetravalent Ti into a d0 complex oxide with trivalent Gd and Sc, requires that Ti be in a formal trivalent valence state, Ti3+. Alloying with Ti then introduces a d1 occupied state into the complex oxide host in direct proportion to the Ti content.
Gd(Sc1-xTix)O3 alloys > 5 nm thick with x = 0.0, 0.01, 0.05, 0.18 and 0.25 were deposited at room temperature in an UHV system onto (i) LaAlO3 substrates for epitaxial growth, and (ii) superficially oxidized Si(001) to produce nano-grain films. As-deposited nano-grain dimensions are 2-2.5 nm, and are too small for Jahn-Teller distortions and spin ordering of Ti alloy atoms. The correlation exchange energy for spin correlated bands is obtained from room temperature films on Si is approximately equal to the energy difference between the localized Ti impurity state energy and Sc band edge d-state.
A compositionally dependent insulator/metal transition is reported for the first time in Gd(Sc1-xTix)O3 alloys for x > 0.16, and is attributed with a correlated Ti d-state band. This insulator to metal transition is identified by X-ray absorption spectroscopy, XAS, extending into the pre-edge regime for X-ray energies <530 eV. Annealing at 900°C in Ar increases grain size enabling Jahn-Teller distortions, and results in an insulator to metal transition evidenced by opening of a gap between oppositely directed Ti spin states. This insulator to metal transition is also observed in as-deposited epitaxial films as well, and consistent with percolation theory, only if the Ti concentration exceeds a critical concentration of ~16.5 %, as in the 18 and 25 % alloys. Differentiation of XAS spectra indicates ferrimagnetism, with a second partially occupied spin band. As predicted by theory, the separation of these spin bands is the same as the correlation exchange energy obtained from the Ti impurity band energy.