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Register a new userControlling the magnetism of oxygen surface vacancies in strontium titanate $mathrm{(SrTiO_3!)}$ through charging
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We discuss, based on first principles calculations, the possibility to tune the magnetism of oxygen vacancies at the (001) surface of strontium titanate $(mathrm{SrTiO_3}!)$. The magnetic moment of single and clustered vacancies stemming from Ti-O broken bonds can be both quenched and stabilized controllably by chemical potential adjustment associated with doping the system with electrons or holes. We discuss to what extent this route to magnetization state control is robust against other external influences like chemical doping, mechanical action and electric field. Such control of vacancy state and magnetization can conceivably be achieved experimentally by using local probe tips.
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Strontium titanate (SrTiO$_3$) is a foundational material in the emerging field of complex oxide electronics. While its electronic and optical properties have been studied for decades, SrTiO$_3$ has recently become a renewed materials research focus catalyzed in part by the discovery of magnetism and superconductivity at interfaces between SrTiO$_3$ and other oxides. The formation and distribution of oxygen vacancies may play an essential but as-yet-incompletely understood role in these effects. Moreover, recent signatures of magnetization in gated SrTiO$_3$ have further galvanized interest in the emergent properties of this nominally nonmagnetic material. Here we observe an optically induced and persistent magnetization in oxygen-deficient SrTiO$_{3-delta}$ using magnetic circular dichroism (MCD) spectroscopy and SQUID magnetometry. This zero-field magnetization appears below ~18K, persists for hours below 10K, and is tunable via the polarization and wavelength of sub-bandgap (400-500nm) light. These effects occur only in oxygen-deficient samples, revealing the detailed interplay between magnetism, lattice defects, and light in an archetypal oxide material.
We show that oxygen vacancies at titanate interfaces induce a complex multiorbital reconstruction which involves a lowering of the local symmetry and an inversion of t2g and eg orbitals resulting in the occupation of the eg orbitals of Ti atoms neighboring the O vacancy. The orbital reconstruction depends strongly on the clustering of O vacancies and can be accompanied by a magnetic splitting between the local eg orbitals with lobes directed towards the vacancy and interface dxy orbitals. The reconstruction generates a two-dimensional interface magnetic state not observed in bulk SrTiO3. Using generalized gradient approximation (LSDA) with intra-atomic Coulomb repulsion (GGA+U), we find that this magnetic state is common for titanate surfaces and interfaces.
The ability to manipulate oxygen anion defects rather than metal cations in complex oxides can facilitate creating new functionalities critical for emerging energy and device technologies. However, the difficulty in activating oxygen at reduced temperatures hinders the deliberate control of important defects, oxygen vacancies. Here, strontium cobaltite (SrCoOx) is used to demonstrate that epitaxial strain is a powerful tool for manipulating the oxygen vacancy concentration even under highly oxidizing environments and at annealing temperatures as low as 300 C. By applying a small biaxial tensile strain (2%), the oxygen activation energy barrier decreases by ~30%, resulting in a tunable oxygen deficient steady-state under conditions that would normally fully oxidize unstrained cobaltite. These strain-induced changes in oxygen stoichiometry drive the cobaltite from a ferromagnetic metal towards an antiferromagnetic insulator. The ability to decouple the oxygen vacancy concentration from its typical dependence on the operational environment is useful for effectively designing oxides materials with a specific oxygen stoichiometry.
The temperature dependent Hall mobility data from La-doped SrTiO3 thin films has been analyzed and modeled considering various electron scattering mechanisms. We find that a ~6 meV transverse optical phonon (TO) deformation potential scattering mechanism is necessary to explain the dependence of transport on temperature between 10-200 K. Also, we find that the low temperature electron mobility in intrinsic (nominally undoped) SrTiO3 is limited by acoustic phonon scattering. Adding the above two scattering mechanisms to longitudinal optical phonon (LO) and ionized impurity scattering mechanisms, excellent quantitative agreement between mobility measurement and model is achieved in the whole temperature range (2-300K) and carrier concentrations ranging over a few orders of magnitude (8x1017 cm-3 - 2x1020 cm-3).
The surface composition of polycrystalline niobium-doped strontium titanate (SrTiO3 : Nb) is studied using X-ray photoelectron emission microscopy (XPEEM) for many grain orientations in order to characterise the surface chemistry with high spatial resolution. The surface sensitivity is maximised by the use of soft X-ray synchrotron radiation (SR). The grain orientation is determined by electron backscattering diffraction (EBSD). Stereographic plots are used to show the correlation between surface composition and orientation for several grains. Predominant surface terminations are assigned to major orientations.
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