Manipulate the Electronic and Magnetic States in NiCo₂O₄ Films through Electric‐Field‐Induced Protonation at Elevated Temperature

Abstract

it has been demonstrated recently that the residual water would exist ubiquitously within the ionic liquid (IL) when experiments performed in the air. [11][12][13][14] The water molecular would facilitate the electrochemical reaction through electrolysis into H + and O 2ions, which would then be inserted into materials depending on the polarity of the gating bias [11][12][13][14][15][16][17] (as shown in Figure 1a). Clearly, the electrochemical reaction related modulation is a bulk effect, and therefore its associated charge modulation should not be confined by the 2D limit as the conventional ILG, which is about 10 15 cm −2 at the sample surface. [2][3][4][5][6][7][8] Along these studies, the H + (proton) ion, the smallest and lightest ion, emerges as an ideal candidate to incorporate into materials for the manipulation of band filling via charge neutrality induced electron doping with positively charged proton. [13,18] Besides, protonated materials have tremendous application potential in hydrogen storage [18] and fuel cells. [19][20][21][22] So the study of ILG induced protonation is significant for both fundamental physics (realizing electron doping and phase control) and industry application (discovering new protonated functional materials). It is important to note that although the ILG induced protonation Ionic-liquid-gating-(ILG-) induced proton evolution has emerged as a novel strategy to realize electron doping and manipulate the electronic and magnetic ground states in complex oxides. While the study of a wide range of systems (e.g., SrCoO 2.5 , VO 2 , WO 3 , etc.) has demonstrated important opportunities to incorporate protons through ILG, protonation remains a big challenge for many others. Furthermore, the mechanism of proton intercalation from the ionic liquid/solid interface to whole film has not yet been revealed. Here, with a model system of inverse spinel NiCo 2 O 4 , an increase in system temperature during ILG forms a single but effective method to efficiently achieve protonation. Moreover, the ILG induces a novel phase transformation in NiCo 2 O 4 from ferrimagnetic metallic into antiferromagnetic insulating with protonation at elevated temperatures. This study shows that environmental temperature is an efficient tuning knob to manipulate ILG-induced ionic evolution. Electron Doping

Reversible manipulation of the magnetic state in SrRuO3 through electric-field controlled proton evolution

“…Notably, the structural transformation possesses a threshold gating voltage, as well as a clear voltage dependence with higher voltage corresponding to shorter transition time ( Supplementary Fig. 2), which is consistent with the diffusion model suggested by previous studies [21][22][23] .…”

Emergent electric field control of phase transformation in oxide superlattices

“…We find that the phase B sample after thermal annealing in 18 O2 shows a dramatic increase in 18 Supplementary Fig. 4c).…”

“…To fully exploit this mechanism, materials that can be electrically switched from one crystalline phase to another, with distinct physical properties, are highly desirable. Although ion transfer has been reported in many singlephase TMOs [8][9][10][11][12][13][14][15][16][17][18] , reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature (RT) has been limited to a few systems, including binary oxides VO2 8 and WO3 12,13 , and perovskite oxides SrCoO3-δ 14 and SrFeO3-δ 17 that exhibit topotactic transformations. Among these systems, coupled electronic, magnetic and optical phase transitions have only been demonstrated in SrCoO3-δ thus far 14 .…”

A selective control of volatile and non-volatile superconductivity in an insulating copper oxide via ionic liquid gating

“…Several possible effects, such as electrostatic charge modulation [15,16], oxygen removal [28,35], and H + ions injection [23,[36][37][38] may arise during the PGV process in the oxide system. Considering the fact that the thickness of the PCO thin film,~60 nm, is much larger than the Thomas-Fermi screening length (several nanometers), pure electrostatic field effect can only modulate the electronic property of the surface layers, and therefore cannot extend to the whole film and suppress the superconductivity of the bulk.…”