Noncollinear nature of the magnetic ground states in an fcc [Fe7|Cu7]n superlattice

Due to its complex magnetic structure, fcc Fe grown on a Cu(100) substrate has been studied for decades. Many experimental measurements have gradually confirmed that fcc $\mathrm{Fe}|\mathrm{Cu}$(100) films have a noncollinear magnetic configuration. In contrast, current theoretical calculations have found that collinear magnetic configurations are always more stable than noncollinear magnetic configurations. To clarify this discrepancy, we set up $\mathrm{Fe}|\mathrm{Cu}$(100) films and ${[{\mathrm{Fe}}_{7}|{\mathrm{Cu}}_{7}]}_{n}$ superlattice and carry out first-principles calculations to study the magnetic ground states therein. We find that the exchange interactions along the normal direction of the thin film exhibit long-range RKKY-like behavior. Using these exchange interactions, we construct an effective Hamiltonian, with which the noncollinear magnetic ground states were found. Moreover, Monte Carlo simulations confirm these noncollinear magnetic ground states to be stable at a finite temperature. Thus, our results catch the physical origin of the noncollinear magnetic ground states in fcc Fe-based films and reveal that even the collinear exchange coupling may induce noncollinear magnetic states.