Low-Voltage-Manipulating Spin Dynamics of Flexible Fe₃O₄ Films through Ionic Gel Gating for Wearable Devices

Mechanical flexible electronic/spintronic devices have shown enormous application potential to impact our daily life. Here, an in situ low-voltage-controlled flexible field-effect transistor structure was exploited, which consists of a support layer (mica), functional layer (Fe₃O₄), and control layer (ionic gel). By applying a low voltage (1.5 V) on the ionic gel, the spin-dynamic properties of the function layer were manipulated and a reversible, nonvolatile 345 Oe ferromagnetic resonance field ( H_r) shift was achieved, which corresponds to a large magnetoelectric (ME) coefficient of 230 Oe/V. In addition, a reversible 126 Oe H_r shift (84 Oe/V) was obtained when the layers were bent at curvature radius r = 15 mm. The ME tunability could be attributed to the E-field induced ionic transformation between Fe²⁺ and Fe³⁺ at the interface via electrostatic induction. This sandwich structure shows an excellent and effective ionic gel gating system and paves the way for low-voltage-tunable, nonvolatile, and flexible spintronic devices such as memory devices, sensors, and logical devices.