Tuning the hysteresis loop for the anomalous Hall effect in Pt ultrathin films on $\rm{CoFe_2O_4}$ by electrolyte gating
Pt ultrathin films on ferromagnetic insulators have been widely studied for spintronics applications, and magnetic moments of interface Pt atoms were considered to be ferromagnetically ordered due to a magnetic proximity effect (MPE). An anomalous Hall effect (AHE) is usually used to examine an out-of-plane magnetic moments of the Pt layer. To tune ferromagnetic properties of an Pt ultrathin film, we fabricated electric double layer transistors on Pt thin films with thicknesses of 5.9 nm and 7.0 nm on a $\rm{CoFe_2O_4}$ (CFO) ferrimagnetic insulator. For the Pt (7.0 nm)/CFO sample, a hysteresis loop was observed in the anomalous Hall resistivity without the gate bias, and the coercive field was tuned by applying the gate bias. For the Pt (5.9 nm)/CFO sample, a hysteresis loop was not observed without a gate bias, but was opened by applying a gate bias ($V\rm{_G} =$ $\pm$3 V). This indicated that the long-range ferromagnetic ordering of magnetic moments in the Pt film was switched on and off by the electric field effect. The hysteresis loop was observed up to 19.5 K for a $V\rm{_G}$ of +3 V, while the AHE was observed up to approximately room temperature.