Magnetoelectric coupling at the La1-xSrxMnO3/ionic liquid interface

One of the major quests in today’s microelectronic era is the development of novel low-power magnetic devices for a variety of applications spanning from memory storage and processing to transduction and sensing. Control of magnetism by means of an electric field, based on the phenomenon of magnetoelectric (ME) effect, may be the key alternative to conventional electronics relying on dissipative electrical currents. In the last years various strategies to interconnect electric and magnetic degrees of freedom have been put to test. A promising approach to straightforwardly and precisely master ME coupling is via charge carrier doping of a magnetic material using an external voltage. This can be realized, akin to the working principle of the field effect transistor, by gating a magnetic electrode with an electrically-polarizable solid (e.g. a dielectric or a ferroelectric) or a liquid electrolyte. This dissertation reports on the investigation of ME coupling at solid/liquid interfaces in a prototypical system consisting of a La1-xSrxMnO3 (LSMO) magnetic electrode electrically charged with an ionic liquid (IL) electrolyte. LSMO - a magnetic perovskite manganite - belongs to the celebrated class of strongly-correlated oxides featuring multiple magnetic states, which directly depend on the oxidation state of the magnetically-coupled manganese ions (Mn3+/4+). Upon voltage-driven charge doping the Mn oxidation state is altered, which, in turn, allows to control the balance between double-exchange and superexchange magnetic interactions in LSMO. Furthermore, LSMO possesses a para/ferromagnetic transition slightly above room temperature, which makes it a promising candidate in the perspective of potential applications. Epitaxial thin (≈ 13 nm) and ultrathin (≈ 3 nm) LSMO films were grown onto single-crystalline SrTiO3 substrates via Large-Distance Magnetron Sputtering (LDMS). This deposition technique demonstrated to be an ideal tool for fabrication of LSMO films with highest quality in terms of crystallinity, surface smoothness and magnetic properties. The interfacial ME coupling was investigated by combining in situ Superconducting Quantum Interference Device (SQUID) magnetometry and Cyclic Voltammetry (CV). This experimental configuration allowed to concurrently extract quantitative information about surface charge density and magnetization as a function of different applied voltages and temperatures. The analysis of the interfacial charging/discharging processes revealed that the accumulation/depletion of charge carriers is not only driven by electrostatic (electric double layer capacitance) but also electrochemical (redox pseudocapacitance) doping. The presence of both charging mechanisms indicated that the LSMO/IL system behaves as an archetypal hybrid supercapacitor. Large values of surface charge density up to ≈ 300 μC/cm2 enabled to robustly and flexibly control the magnetic response of LSMO. In case of LSMO thin films with a thickness of ≈ 13 nm a relative magnetic change ΔM/M of up to 33% was reached, whereas for thinner LSMO films of ≈ 3 nm, featuring an enhanced surface-to-volume ratio, ferromagnetism (FM) could be completely suppressed and restored at will. Together with the significant magnitude of the magnetic tuning effect, IL gating provided an outstanding level of reversibility upon cycling, low energy consumption and remarkable switching speed. Additionally, the magnetic signal could be manipulated in-phase and/or anti-phase with respect to the surface charge modulation by appropriately adjusting the applied voltage. The observed interfacial ME coupling can be qualitatively explained with the major features of the bulk magnetoelectronic phase diagram of LSMO. However, in this study a more precise and consistent microscopic model is proposed on the basis of the quantitative values of the ME coupling coefficient |α| = |ΔM/ΔQ| = 3 μB/h+ and the phenomenon of magnetic phase separation. In such scenario competing FM and non-FM domains expand or shrink at the expense of each other upon voltage-induced charge doping. On the whole, this work intends to elucidate the physico-chemical mechanisms behind the ME effect at solid/liquid interfaces with the aim of fostering further studies in the yet unexplored area of ME supercapacitors.