Microscopic Insights for Beyond Room-Temperature Ferromagnetism in Ni doped Two-Dimensional Fe$_5$GeTe$_2$

Enhancement of Curie temperature ($T_\mathrm{C}$) of two-dimensional (2D) magnets is immensely desirable for room temperature spintronic applications. Fe$_{5}$GeTe$_{2}$ is an exceptional van der Waals metallic ferromagnet due to its tunable physical properties and relatively higher $T_\mathrm{C}$ than other 2D magnets. Using density functional theory combined with dynamical mean field theory and Monte Carlo simulations, we show that the $T_\mathrm{C}$ of Fe$_{5}$GeTe$_{2}$ monolayer can increase well-above room temperature by substitutional doping with Ni. It is found that two specific sublattices (Fe1 and Fe4) are the first and second most energetically preferred occupation sites for Ni. $T_\mathrm{C}$ of Fe$_{5-δ}$Ni$_δ$GeTe$_{2}$ increases up to $\sim$ 400 K at $δ\sim$20\%. Exchange interactions between particular Fe5-Fe4 pairs play a dominating role in tuning the transition temperature, influenced by doping-induced structural distortions. Finally, we highlight the effect of dynamical electron correlation in site-specific electronic structure and quasi-particle mass of Fe-$d$ orbitals with varying Ni dopi