  Two-dimensional (2D) van der Waals (vdW) magnets are crucial for
ultra-compact spintronics. However, so far, no vdW crystal has exhibited
tunable above-room-temperature intrinsic ferromagnetism in the 2D ultrathin
regime. Here, we report the tunable above-room-temperature intrinsic
ferromagnetism in ultrathin vdW crystal Fe$_{3+x}$GaTe$_2$ ($x$ = 0 and 0.3).
By increasing the Fe content, the Curie temperature (TC) and room-temperature
saturation magnetization of bulk Fe$_{3+x}$GaTe$_2$ crystals are enhanced from
354 to 376 K and 43.9 to 50.4 emu/g, respectively. Remarkably, the robust
anomalous Hall effect in 3-nm Fe$_{3.3}$GaTe$_2$ indicate a record-high TC of
340 K and a large room-temperature perpendicular magnetic anisotropy energy of
6.6 * 10^5 J/m$^3$, superior to other ultrathin vdW ferromagnets.
First-principles calculations reveal the asymmetric density of states and an
additional large spin exchange interaction in ultrathin Fe$_{3+x}$GaTe$_2$
responsible for robust intrinsic ferromagnetism and higher Tc. This work opens
a window for above-room-temperature ultrathin 2D magnets in vdW-integrated
spintronics.
