Dynamical electron correlation in two-dimensional Fe_nGeTe₂ (n=3, 4, 5) magnets

The Fe_nGeTe₂ (n=3, 4, 5) family of two-dimensional magnets has attracted a lot of attention recently as they exhibit high temperature ferromagnetism along with complex temperature dependent magnetization and structural reconstructions, skyrmionic features etc. The complexity is enriched due to the presence of electron correlation and intricate magnetic interactions, where systematic theoretical studies are inadequate to have a consensus. In this work, we have employed first-principles calculations to have a systematic study of this family using (i) standard density functional theory (DFT), (ii) static electron correlation (DFT+U) and (iii) dynamic electron correlation effect (DFT+DMFT) methods. Moreover, complex structural aspects in Fe₅GeTe₂ have been considered in connection to experimental observations. Our results show that DFT+DMFT is the most accurate method to correctly reproduce the magnetic interactions and experimentally observed transition temperatures. Moreover, Monte Carlo simulations reveal peculiar magnetic structures at low temperature. The inaccurate values of structural parameters, magnetic moments and exchange interactions obtained from DFT+U make this method inapplicable for the FGT family.