Ab initio investigations of magnetic properties of ultrathin transition ...

5.2 Results of Fe monolayer on different hexagonal substrates from non-collinear cal. 101
in the hexagonal 2DIBZ. If the nearest neighbor interactions are weak, then higher order
interactions will play a crucial role to stabilize the magnetic ground state of the system, and
must not be ignored. This leads to a lifting of the degeneracy in Heisenberg model for linear
combinations of two different spin spirals, Q 3ΓK/4 and Q MΓ/2 . Then we could calculate the
higher order interactions from the total energy difference between Q and a superposition
of ±Q. The superposition of ±Q is mapped to real space by 4 atoms per unit cell will
double-RW-AFM order. Instead of the FM ground state from collinear calculations, we
found that the higher order interactions do stabilize a collinear double-RW-AFM ground
state constructed from Qs along the high symmetry line M-Γ for Fe monolayer on the
Rh(111) substrate. We also studied Fe on Tc by performing non-collinear calculations and
found that Fe monolayer has a 120 ◦ Néel ground state on Tc(0001), which is different from
the RW-AFM ground state that would be predicted by collinear calculations using 2 atoms
per unit cell.
At the end of the chapter we compared our non-collinear results for Fe monolayer on 4d-
TMs. Due to geometrical effects along the high symmetry line M-Γ, the strongly polarized
4d substrate will break the degeneracy of the Q 3ΓK/4 and Q MΓ/2 in Heisenberg model. This
lead us to modify the model Hamiltonian by adding a term which depends on the substrate
Stoner parameter and the induced moments of the substrates.