Structural distortion and dynamical electron correlation driven enhanced ferromagnetism in Ni-doped two-dimensional Fe₅GeTe₂ beyond room temperature

Abstract Achieving beyond room-temperature ferromagnetism in two-dimensional (2D) magnets is immensely desirable for spintronic applications. Fe5GeTe2 is an exceptional van der Waals metallic ferromagnet due to its tunable physical properties and relatively higher Curie temperature (TC) than other 2D magnets. Using density functional theory combined with dynamical electron correlation and Monte Carlo simulations, we find the TC of (Fe$_{1-\delta}$Ni$_{\delta})_{5}$GeTe$_{2}$ monolayer can increase up to $\sim$ 400 K at $\delta \sim 0.20$ ($\delta$: fractional occupation). Two specific Fe sublattices are identified to be the most energetically preferred sites to host Ni. Exchange interactions between particular Fe pairs play a dominating role in controlling $T_\mathrm{C}$, influenced by the dopant-induced structural distortions. Dynamical electron correlation induces site- and orbital-specific quasi-particle mass of Fe-$d$ states with varying Ni concentrations. This work provides fundamental insights into 2D magnetism as an interplay of structural and electronic aspects and would guide to tailoring exciting magnetic phenomena in similar systems.