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

5.2 Results of Fe monolayer on different hexagonal substrates from non-collinear cal. 93
Figure 5.11: Vortex-like structure, with sixteen atoms per unit cell, each four atoms have
the same spin rotating by π with respect to each other(left). The two spin spiral Q-points
2
used to construct such a coplanar vortex-like structure (right).
atom.
5.2.3 Results for the Fe monolayer on Tc(0001) substrate:
Employing the same procedure as for Fe/Rh(111), we performed also non-collinear calculations
to calculate total energies in terms of spiral vectors for Fe monolayer on technetium
(0001) surface, Tc(0001). The aim of that is to have a complete picture about Fe magnetism
on different hexagonal substrates, especially after the theoretical study performed to study
Fe ground state on Ru(0001) and Re(0001) from non-collinear DFT calculations [57]. Also,
the ground state of Fe monolayer on Ir(111) was experimentally investigated and modeled
by theoretical non-collinear DFT calculations [27, 28].
We used the experimental Tc lattice constants to calculate the spin spirals using the
collinear ground state, RW-AFM, relaxations (Tab. 5.1). All numerical parameters were
the same as for Fe/Rh(111). Results of total energies relative to the FM solution, using
GGA are shown in figure 5.12(a). The moments of Fe, Tc(I) and Tc(I-1) are also presented
in figure 5.12 (b). In addition we performed collinear calculations at the two double-Q,
uudd � MΓ/2 � and uudd � 3ΓK/4 � states, to test the validity of equations (5.8 and 5.14),
since the equality of the difference between the single- and double-Q total energies was
broken for Rh(111) case.
We see, from figure 5.12(a), that also here, we have different energy differences between
the two double-Qs and their single-Q, even though these differences are rather small.
These results indicate that the higher order interactions are very weak and cannot change
the magnetic ground state from what is enforce by the nearest next nearest neighbor
interactions. The global minimum energy is at K, which means that Fe has a Néel ground
state on Tc(0001) with angles of 120 ◦ between adjacent spins, with −180 (−60) meV/Fe
atom below the FM (RW-AFM) solution. The change of Fe ground state from a RW-AFM
to a Néel structure, due to topological frustrations, highlights the importance of taking
non-collinear magnetic structures into account. This can be seen from the RW-AFM