  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.
