Sign-tunable exchange bias effect in proton-intercalated Fe3GaTe2 nanoflakes

The exchange-bias (EB) effect, usually arising in ferromagnetic (FM)-antiferromagnetic (AFM) interfaces with uniaxial magnetic anisotropy, holds high potentials in spintronic applications. Here, we report both field-cooling and zero-field cooling EB effects with a maximal EB field $|{H}_{EB}|$ reaches up to 3859 Oe in above-room-temperature van der Waals (vdW) ferromagnet ${\mathrm{Fe}}_{3}\mathrm{Ga}{\mathrm{Te}}_{2}$ nanoflakes at low temperatures. The observed intrinsic EB effects can be largely tuned via the gate-induced proton intercalation. Moreover, we observe an unusual sign-tunable EB effect under different gate voltages after $\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{4pt}{0ex}}\mathrm{T}$ field cooling, leading to a crossover between positive and negative EB effects. Theoretical analysis based on density functional theory indicates that the magnetic coupling at the FM/AFM interface in proton-intercalated ${\mathrm{Fe}}_{3}\mathrm{Ga}{\mathrm{Te}}_{2}$ is highly controllable and can be tuned to be FMI-1 $(\ensuremath{\uparrow}/\ensuremath{\downarrow}\ensuremath{\uparrow}$, positive EB) or FMI-2 ($\ensuremath{\downarrow}/\ensuremath{\downarrow}\ensuremath{\uparrow}$, negative EB) magnetic configurations, depending on different H-absorption sites. Our experiments offer a knob to control the sign of EB effects and further open opportunities for more applicable spintronic devices in high-temperature vdW ferromagnets.