Voltage‐Controlled On/Off Switching of Ferromagnetism in Manganite Supercapacitors

Abstract

liquid devices via surface electrostatic doping [20][21][22][23] (as in electric doublelayer (EDL) capacitors) or via bulk ionic migration [12,[15][16][17][18][19] (akin to electrochemical batteries). Recently, robust ME effect was achieved in magnetic supercapacitors [24,25] via a surface electrochemical mechanism called pseudocapacitance. [26,27] By making use of interfacial redox reactions, pseudocapacitors concurrently benefit of fast, reversible charging/discharging processes (typical of EDL capacitors) and high amounts of stored charge (typical of electrochemical batteries).From a technological perspective, the idea of controlling magnetic phase transitions in a supercapacitor where the magnetic counterpart is composed of a manganese-based oxide is very appealing. Indeed, several manganites [28] exhibit ferromagnetism and high degree of spin polarization intrinsically coupled to each other in proximity of room temperature. Thus, a low-power electric field could enable the simultaneous switching of magnetic and spintronic degrees of freedom without losing reversibility and switching speed.In this work, we selected, as a model system, a magnetic manganite supercapacitor made of an ultrathin film of La 0.74 Sr 0.26 MnO 3 (LSMO) charged with an ionic liquid electrolyte (DEME + -TFSI − ). By exploiting surface capacitive and pseudocapacitive charging mechanisms, we demonstrate the repetitive and complete suppression and recovery of ferromagnetism in LSMO at low-energy cost and remarkable switching speed in the 200-250 K temperature range.Atomically smooth LSMO films (see Figure 1a) with an optimized thickness of ≈3 nm and a surface area of ≈0.4 cm 2 were epitaxially grown on (001)-oriented SrTiO 3 (STO) substrates by magnetron sputtering. For a film thickness of ≈3 nm, a good compromise has been reached to maintain a high surfaceto-volume ratio and to preserve the Curie temperature of LSMO above the freezing point of the ionic liquid.In order to quantitatively track the LSMO magnetization while charging its surface with an ionic liquid, superconducting quantum interference device (SQUID) magnetometry and cyclic voltammetry (CV) measurements were combined and synchronized in situ (see the Experimental Section). The solid/ liquid devices consisted of a ME tuning cell with a configuration similar to an electrolytic capacitor (see sketch in Figure 1b).By application of a positive (negative) external voltage, the TFSI − (DEME + ) ions accumulate onto the surface of LSMO The ever-growing technological demand for more advanced microelectronic and spintronic devices keeps catalyzing the idea of controlling magnetism with an electric field. Although voltage-driven on/off switching of magnetization is already established in some magnetoelectric (ME) systems, often the coupling between magnetic and electric order parameters lacks an adequate reversibility, energy efficiency, working temperature, or switching speed. Here, the ME performance of a manganite supercapacitor composed of a ferromagnetic, spin-polarized ultrathin...

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

“…Subsequently, quantitative studies of ME coupling in epitaxial films of LSMO gated with DEME-TFSI IL revealed that the interfacial charging processes progressively move from electrostatic doping to surface redox pseudocapacitance upon increasing the external voltage. [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). By optimizing the surface to volume ratio of the devices, repeated suppression and recovery of ferromagnetism, with a T C shift of about 26 K, was demonstrated in ultrathin LSMO films (≈3 nm).…”

“…Recently, in IL-gated LSMO films, [42] a ΔM ≈ 54 emu cm −3 was attained by using a potential window ΔV ≈ 3 V, which, owing to the intrinsic small thickness (d ≈ 1 nm) of EDL capacitors, corresponds to an ultrahigh interfacial electric field E ≈ 30 MV cm −1 . Recently, in IL-gated LSMO films, [42] a ΔM ≈ 54 emu cm −3 was attained by using a potential window ΔV ≈ 3 V, which, owing to the intrinsic small thickness (d ≈ 1 nm) of EDL capacitors, corresponds to an ultrahigh interfacial electric field E ≈ 30 MV cm −1 .…”

“…To date, the fastest ME response in solid/liquid ME composites with on/off modulation of magnetism was achieved in ultrathin films of LSMO gated with DEME-TFSI IL, where repetitive charging/discharging cycles were accomplished in about 10 s. [42] The nonvolatility of the ME effect is a prerequisite to realize ME memory storage devices. To date, the fastest ME response in solid/liquid ME composites with on/off modulation of magnetism was achieved in ultrathin films of LSMO gated with DEME-TFSI IL, where repetitive charging/discharging cycles were accomplished in about 10 s. [42] The nonvolatility of the ME effect is a prerequisite to realize ME memory storage devices.…”

“…where α C denotes the converse [23] ME coupling coefficient. To date, a major stream of research focuses on the investigation of electric fields to control a variety of magnetic properties, including magnetic anisotropy, [24][25][26][27][28][29][30][31][32] magnetic transition temperature, [33][34][35][36][37][38][39][40][41][42][43][44] magnetic moment, [45][46][47][48][49][50] spin polarization, [51,52] exchange bias, [53][54][55][56][57] ferromagnetic resonance, [58][59][60][61] magnetic topology, [62,63] and magnetoresistance. [64][65][66][67] Materials that exhibit inherent coupling between magnetic and electric degrees of freedom are classified into two broad categories, [1,68] the single-phase MEs and single-phase ME m...…”

“…By optimizing the surface to volume ratio of the devices, repeated suppression and recovery of ferromagnetism, with a T C shift of about 26 K, was demonstrated in ultrathin LSMO films (≈3 nm). [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). 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 .…”