Electrostatic versus Electrochemical Doping and Control of Ferromagnetism in Ion-Gel-Gated Ultrathin La_0.5Sr_0.5CoO_3-δ

Recently, electrolyte gating techniques employing ionic liquids/gels in electric double layer transistors have proven remarkably effective in tuning charge carrier density in a variety of materials. The ability to control surface carrier densities at levels above 10¹⁴ cm^-2 has led to widespread use in the study of superconductivity, insulator-metal transitions, etc. In many cases, controversy remains over the doping mechanism, however (i.e., electrostatic vs electrochemical (e.g., redox-based)), and the technique has been less applied to magnetic materials. Here, we discuss ion gel gating of nanoscale 8-unit-cell-thick hole-doped La_0.5Sr_0.5CoO_3-δ (LSCO) films, probing in detail the critical bias windows and doping mechanisms. The LSCO films, which are under compressive stress on LaAlO₃(001) substrates, are metallic and ferromagnetic (Curie temperature, T_C ∼ 170 K), with strong anomalous Hall effect and perpendicular magnetic anisotropy. Transport measurements reveal that negative gate biases lead to reversible hole accumulation (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. The results are thus of general importance, suggesting that hole- and electron-doped oxides may respond very differently to electrolyte gating. Reversible voltage control of electronic/magnetic properties is then demonstrated under hole accumulation, including resistivity, magnetoresistance, and T_C. The sizable anomalous Hall coefficient and perpendicular anisotropy in LSCO provide a particularly powerful probe of magnetism, enabling direct extraction of the voltage-dependent order parameter and T_C shift. The latter amounts to ∼7%, with potential for much stronger modulation at lower Sr doping.