Recently, incorporation of electrolytes such as ionic liquids into field-effect transistors has been shown to enable electric double layer transistors (EDLTs), which can induce very large (1015 cm-2) charge carrier densities at surfaces. These densities corresponds to significant fractions of an electron or hole per unit cell in most materials, sufficient to electrically control electronic phase transitions. While this has stimulated great interest, challenges remain, including understanding mechanisms (electrostatic vs. electrochemical ), developing operando characterization methods, and assessing the full power and universality of the approach. Here, I review our work applying electrolyte gating using solid ion gels [1-6] to oxides and sulfides, focused on voltage control of magnetism. The latter is important, bearing potential for novel, low-power data storage and processing technologies. Our findings greatly clarify the issue of electrostatic vs. electrochemical mechanisms, showing that electrostatic gating vs. oxygen vacancy creation/annihilation can be understood based on bias polarity, and the enthalpy of formation and diffusivity of oxygen vacancies [1-6]. This understanding was developed via operando probes, such as synchrotron X-ray diffraction  and absorption/dichroism , as well as neutron reflectometry [3,5]. Electrical control of magnetism in La1-xSrxCoO3-d is then demonstrated in both electrochemical  and electrostatic [2,4,5] modes, reversibly modulating Curie temperatures over >200 K windows. Most recently, this has been advanced beyond electrical modulation of ferromagnets, or electrical induction of ferromagnetism from antiferro- or para-magnetic states, demonstrating reversible voltage-induced ferromagnetism in a diamagnet, using FeS2 (Fool’s Gold) as a model system .
 C. Leighton, Nat. Mater. 18, 13 (2019).
 J. Walter, H. Wang, B. Luo and C. Leighton, ACS Nano 10, 7799 (2016).
 J. Walter, G. Yu, B. Yu, A. Grutter, B. Kirby, J. Borchers, Z. Zhang, H. Zhou, T. Birol, M. Greven, and C. Leighton, Phys. Rev. Materials 1, 071403(R) (2017).
 P.P. Orth, R.M. Fernandes, J. Walter, C. Leighton and B.I. Shklovskii, Phys. Rev. Lett. 118, 106801 (2017).
 J. Walter, T. Charlton, H. Ambaye, M. Fitzsimmons, P.Orth, R. Fernandes, B. Shklovskii and C. Leighton, Phys. Rev. Materials 2, 111406(R) (2018).
 B. Yu, G. Yu, J. Walter, V. Chaturvedi, J. Gotchnik, J. Freeland, C. Leighton and M. Greven, Appl. Phys. Lett. 116, 201905 (2020).
 J. Walter, B. Voigt, E. Day-Roberts, T. Birol, R. Fernandes and C. Leighton, Sci. Adv. 6, eabb7721 (2020).