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

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 film of La_0.74Sr_0.26MnO₃ (LSMO) electrically charged with an ionic liquid electrolyte is investigated. Fully reversible, rapid, on/off switching of ferromagnetism in LSMO is demonstrated in combination with a shift in Curie temperature of up to 26 K and a giant ME coupling coefficient of ≈226 Oe V⁻¹. The application of voltages of only ≈2 V results in ultralow energy consumptions of about 90 µJ cm⁻². This work provides a step forward toward low‐power, high‐endurance electrical switching of magnetism for the development of high‐performance ME spintronics.     

Quantitative Determination on Ionic‐Liquid‐Gating Control of Interfacial Magnetism

Ionic‐liquid gating on a functional thin film with a low voltage has drawn a lot of attention due to rich chemical, electronic, and magnetic phenomena at the interface. Here, a key challenge in quantitative determination of voltage‐controlled magnetic anisotropy (VCMA) in Au/[DEME]⁺[TFSI]^?/Co field‐effect transistor heterostructures is addressed. The magnetic anisotropy change as response to the gating voltage is precisely detected by in situ electron spin resonance measurements. A reversible change of magnetic anisotropy up to 219 Oe is achieved with a low gating voltage of 1.5 V at room temperature, corresponding to a record high VCMA coefficient of ≈146 Oe V^?1. Two gating effects, the electrostatic doping and electrochemical reaction, are distinguished at various gating voltage regions, as confirmed by X‐ray photoelectron spectroscopy and atomic force microscopy experiments. This work shows a unique ionic‐liquid‐gating system for strong interfacial magnetoelectric coupling with many practical advantages, paving the way toward ion‐liquid‐gating spintronic/electronic devices.     

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

The control of magnetism by means of low‐power electric fields, rather than dissipative flowing currents, has the potential to revolutionize conventional methods of data storage and processing, sensing, and actuation. A promising strategy relies on the utilization of magnetoelectric composites to finely tune the interplay between electric and magnetic degrees of freedom at the interface of two functional materials. Albeit early works predominantly focused on the magnetoelectric coupling at solid/solid interfaces; however, recently there has been an increased interest related to the opportunities offered by liquid‐gating techniques. Here, a comparative overview on voltage control of magnetism in all‐solid‐state and solid/liquid composites is presented within the context of the principal coupling mediators, i.e., strain, charge carrier doping, and ionic intercalation. Further, an exhaustive and critical discussion is carried out, concerning the suitability of using the common definition of coupling coefficient ${\alpha }_C=\frac{\mathrm{\Delta }M}{\mathrm{\Delta }E}$ to compare the strength of the interaction between electricity and magnetism among different magnetoelectric systems.     

Ionic Gel Modulation of RKKY Interactions in Synthetic Anti‐Ferromagnetic Nanostructures for Low Power Wearable Spintronic Devices

To meet the demand of developing compatible and energy‐efficient flexible spintronics, voltage manipulation of magnetism on soft substrates is in demand. Here, a voltage tunable flexible field‐effect transistor structure by ionic gel (IG) gating in perpendicular synthetic anti‐ferromagnetic nanostructure is demonstrated. As a result, the interlayer Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction can be tuned electrically at room temperature. With a circuit gating voltage, anti‐ferromagnetic (AFM) ordering is enhanced or converted into an AFM–ferromagnetic (FM) intermediate state, accompanying with the dynamic domain switching. This IG gating process can be repeated stably at different curvatures, confirming an excellent mechanical property. The IG‐induced modification of interlayer exchange coupling is related to the change of Fermi level aroused by the disturbance of itinerant electrons. The voltage modulation of RKKY interaction with excellent flexibility proposes an application potential for wearable spintronic devices with energy efficiency and ultralow operation voltage.     

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

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, VO₂, WO₃, 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 NiCo₂O₄, 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 NiCo₂O₄ 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.     

A Fast and Efficient Replacement of CTAB with MUA on the Surface of Gold Nanorods Assisted by a Water‐Immiscible Ionic Liquid

The synthesis and surface modification of gold nanorods (GNRs) is one of the most important and basic issues in nanoscience. Most of the widely investigated GNRs are coated with a cetyltrimethylammonium bromide(CTAB) bilayer. Here, a highly efficient method is proposed to replace CTAB from the surface of GNRs with a bifunctional 11‐mercaptoundecanoic acid in order to decrease the possible toxicity caused by CTAB. This ligand exchange is achieved in a biphasic mixture of an aqueous solution and a water‐immiscible ionic liquid (IL), [BMIM][Tf₂N]. That is, by mixing IL, mercaptoundecanoic acid (MUA)/IL (200 × 10^?3 m ) and a concentrated aqueous solution of GNRs together, followed by vortex stirring for 90 s, CTAB‐capped GNRs with varying aspect ratios can be turned into corresponding MUA‐capped GNRs with the same aspect ratio. Furthermore, the formed MUA‐capped GNRs can be obtained in a large quantity and stored as powders for easy use. The MUA‐capped GNRs with improved biocompatibility and colloidal stability are well suited for further biological functionalization and potential applications. This IL‐assisted ligand exchange can reverse the surface charge, enhance the stability of GNRs, and suppress its cytotoxicity.