Browsing by Subject "Cobaltite"

Now showing 1 - 3 of 3
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    Complexity at cobaltite interfaces: the interplay between strain, stoichiometry, magnetism and transport
    (2014-12) Bose, Shameek
    Thin films and heterostructures of the perovskite cobaltites are of great interest, not only from the point of view of fundamental physics and materials science, but also for technological applications such as solid oxide fuel cells and gas membranes. Their properties are, however, severely deteriorated from the bulk, being dominated by the presence of interfacial "dead layers". Working with the prototypical SrTiO3 (001)/La1-xSrxCoO3 (LSCO) system, our group recently discovered that this degradation in the magnetism and electronic transport at the interface is caused by nanoscopic magneto-electronic phase separation. This was shown to occur primarily due to accumulation of oxygen vacancies near the interface, driven by the interplay between the strain state and the ordering of oxygen vacancies. In the present work we show how this understanding allows for engineering of the interfacial magnetic and electronic transport properties via manipulation of this oxygen vacancy superstructure. We first demonstrate a synthesis technique that utilizes a unique high pressure oxygen plasma to sputter LSCO thin films over a wide doping range 0.05  x  0.80. Then, using reciprocal space mapping and transmission electron microscopy, we demonstrate the ability to control, via the vacancy ordering, the critical strain relaxation thickness by changing the sign of the strain (from tensile on SrTiO3 to compressive on LaAlO3) and crystallographic orientation ((001) vs. (110)). We then provide cross sectional electron energy loss spectroscopy data to show that this strain and orientation control preserves both oxygen and hole carrier concentration at the LaAlO3(001)/LSCO and SrTiO3(110)/LSCO interfaces, strikingly different to the severely depleted SrTiO3(001)/LSCO interface. SQUID magnetometry, polarized neutron reflectometry (PNR) and magneto-transport confirm the concomitant mitigation of the interfacial degradation for LSCO films grown on LaAlO3(001) and SrTiO3(110), as compared to films grown on SrTiO3 (001). Finally, we use scanning tunneling microscopy to provide direct real space images of the magneto-electronic phase separation in ultrathin LSCO on SrTiO3(001). Our work thus demonstrates the ability to utilize oxygen vacancy ordering as a tunable control parameter to tailor interfacial electronic and magnetic properties, with profound implications for the myriad other systems that exhibit unique properties due to such ordering.
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    Electrochemical control of oxygen stoichiometry and materials properties in ion-gel-gated cobaltite thin films
    (2023-12) Postiglione, William
    Wide-ranging control of materials properties using applied voltages represents a longstanding goal in physics and technology, particularly for low-power applications. To this end, substantial interest has developed around electric-double-layer transistors (EDLTs) based on functional materials. More recently, electrochemical EDLTs, where ions such as O2-, H+, Li+, etc., are driven into / out of a channel material via voltage, have proven capable of offering unique benefits (including non-volatility) for a variety of novel applications. Cobaltites, such as SrCoO3-δ (SCO) have recently emerged as an archetypal example of electrochemical control of materials properties in electrolyte-gate devices. This is accomplished by voltage-driven redox cycling between two distinct phases: fully oxygenated perovskite (P) (δ ≈ 0) and oxygen-vacancy-ordered brownmillerite (BM) (δ = 0.5). To date, SCO has received the most attention in this regard, despite significant issues with air stability in the P phase, and few alternatives have been considered. Additionally, critical issues of voltage hysteresis and fundamental limits on reversibility and cycling endurance remain unaddressed.To address this, using EDLTs based on epitaxial La1-xSrxCoO3-δ (LSCO) thin films, we first investigate the electrochemical reduction that is known to occur at positive gate voltages (Vg) in such systems, establishing that the P → BM transformation occurs in LSCO over a wide doping range. Importantly, both the P and BM phase of x = 0.5 LSCO are robustly air stable, and the electrochemical reduction behavior was found to be voltage-tunable with both doping and strain. We then leverage this voltage-tuned P → BM transformation to demonstrate large property modulations in electronic transport, magnetism, thermal transport, and optical properties, achieving similar or greater ranges of control than in SCO. Next, to explore the reversibility of the transformation, we performed detailed analysis of Vg hysteresis loops, revealing a wealth of new mechanistic findings, including asymmetric transformations due to differing oxygen diffusivities in the P vs. the BM phase, non-monotonic transformation rates due to the first-order nature of the P-BM transformation, and limits on reversibility due to first-cycle structural degradation. Additionally, using minor hysteresis loops, we demonstrate the first rational design of an optimal Vg cycle, leading to state-of-the-art cycling of electronic and magnetic properties, encompassing >105 transport ON/OFF ratios at room temperature, reversible and non-volatile metal-insulator-metal and ferromagnet-nonferromagnet-ferromagnet cycling, all at ultrathin 10-unit-cell thickness. Finally, to further investigate the magnetic properties of the BM nonferromagnet “OFF” state, we performed neutron diffraction experiments, finding the first direct evidence of antiferromagnetic order in BM-SCO films and identifying weak ferromagnetism in x = 0.5 BM-LSCO. These findings thus significantly advance the understanding of voltage-induced P ↔ BM transformations in cobaltite films and pave the way for future work establishing the ultimate cycling frequency and endurance in such electrolyte-gated devices.
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    Ion gel gating of perovskite cobaltite thin films: Understanding mechanisms and control of magnetism
    (2018-05) Walter, Jeff
    Recently, electrolyte gating techniques using ionic liquids and gels have proven highly effective in tuning large carrier densities at material surfaces. These electrolytes enable electric double layer transistor operation, the large capacitances (10’s of µF/cm2) generating electron/hole densities up to 1015 cm-2, i.e., significant fractions of an electron per unit cell in most materials. Uncertainties remain, however, including the true doping mechanism (i.e., electrostatic vs. electrochemical), the challenge of in operando characterization, and the need to understand the full potential and universality. In regard to universality, superconductivity and insulator-metal transitions have been extensively studied with electrolyte gating, but this promising technique has been less applied to controlling magnetism. Electrical control of magnetism is a long-standing goal in physics and technology, electrolyte gating techniques providing a promising route to realization. Employing electric double layer transistors based on ultrathin epitaxial La1-xSrxCoO3 as a model system, our findings first address the true doping mechanism, clarifying charge carrier vs. oxygen defect creation. Transport measurements reveal dramatic asymmetry with respect to bias polarity. Negative gate biases lead to reversible behavior (i.e., predominantly electrostatic operation) up to some threshold, whereas positive bias immediately induces irreversibility. Experiments in inert/O2 atmospheres directly implicate oxygen vacancies in this irreversibility, supported by atomic force microscopy and X-ray photoelectron spectroscopy. We then demonstrate the use of synchrotron hard X-ray diffraction and polarized neutron reflectometry as in operando probes to further investigate the gating mechanism. An asymmetric gate bias response is confirmed to derive from electrostatic hole accumulation at negative bias vs. oxygen vacancy formation at positive bias. The latter is detected via a large gate-induced lattice expansion (up to 1 %), complementary bulk measurements and density functional calculations enabling quantification of the bias-dependent oxygen vacancy density. Remarkably, the gate-induced oxygen vacancies proliferate through the entire thickness of 30-40-unit-cell-thick films, quantitatively accounting for changes in the magnetization depth profile and demonstrating electrochemical control of magnetism. This is interpreted in a simple picture where electrostatic vs. electrochemical response is dictated by the low formation enthalpy and high diffusivity of oxygen vacancies in La1-xSrxCoO3. These results, therefore, directly elucidate the issue of electrostatic vs. redox-based response in electrolyte-gated oxides, also demonstrating powerful new approaches to their in operando investigation. Control of ferromagnetism is then demonstrated in electrostatic mode by working at negative bias. Guided by theory, we demonstrate reversible electrical control of Curie temperature over a 150 K window. This is achieved via gate-induced cluster percolation, leading to optimized control of ferromagnetism, directly verified by magnetoresistance, anomalous Hall effect, and PNR measurements.