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

Abstract

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 10(14) 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 La0.5Sr0.5CoO3-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, TC ∼ 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 TC. 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 TC shift. The latter amounts to ∼7%, with potential for much stronger modulation at lower Sr doping.

Interface-induced phenomena in magnetism

“…Similarly, ferroelectric-based modulation of T C via electrostatic manipulation of Mn valence has been demonstrated at La 1- x Sr x MnO 3 /PbZr 1- x Ti x O 3 interfaces [Vaz et al ., 2010] and linked to doping (screening) homogeneity and local structural distortions [Spurgeon et al ., 2014]. When using ionic liquids and ion gels, care must be taken to distinguish between electrostatic and electrochemical processes within the oxide layer [Jeong et al ., 2013]; recent progress has been made with this issue in specific oxides (Walter et al ., 2016), but more work is required.…”

“…Substantial further progress is anticipated in the understanding of electrostatic vs . electrochemical operation (Walter et al ., 2016), relevant length scales, associated disorder, etc . Using such methods to manipulate and harness electronic correlations is a related frontier with potential.…”

Voltage‐Control of Magnetism in All‐Solid‐State and Solid/Liquid Magnetoelectric Composites

“…[49] In case of 13 nm films, the attained large values of surface charge up to 250 µC cm −2 enabled a maximum reversible modulation in magnetization of about 30% at room temperature. Several other comprehensive studies contributed to disentangle the complex relationship between charge carrier doping and magnetism in electrolyte-gated magnetic oxides, including investigations on LCMO, [80,91] LSMO, [43,156] LaMnO 3 , [67] Pr 1−x (Ca 1−y Sr y ) x MnO 3 , [157,158] LSCO, [82] Fe 3 O 4 , [159] and γ-Fe 2 O 3 . [42] In addition, the magnetic response was flexibly modulated in-phase or antiphase with respect to the induced surface charge by judiciously adjusting the applied bias voltage (see Figure 4b).…”

“…Charge doping is a surface or near-surface effect, ionic migration can extend deeper into the bulk, whereas strain intervenes on a broader macroscopic scale. [49,[79][80][81][82] In this respect, the physicochemical nature of the ME interface plays a key role in determining the response of a magnetic material upon application of an external electrical stimulus. In particular, distinguishing between electrostatic and electrochemical effects is a hotly debated topic in the literature.…”

“…In particular, distinguishing between electrostatic and electrochemical effects is a hotly debated topic in the literature. [49,[79][80][81][82] In this respect, the physicochemical nature of the ME interface plays a key role in determining the response of a magnetic material upon application of an external electrical stimulus. Since the renaissance [83] of the studies on ME effect, the vast majority of research on artificial composites has focused on voltage-control of magnetism in all-solid-state ME systems.…”