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.
University of Minnesota Ph.D. dissertation. May 2018. Major: Material Science and Engineering. Advisors: Chris Leighton, C. Daniel Frisbie. 1 computer file (PDF); xxi, 147 pages.
Ion gel gating of perovskite cobaltite thin films: Understanding mechanisms and control of magnetism.
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