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978-3-8439-0222-9, Reihe Physik
Non-collinear magnetic ground states observed in iron nanostructures on iridium surfaces
161 Seiten, Dissertation Universität Hamburg (2011), Softcover, A5
In this thesis, investigations of Fe nanostructures on the reconstructed (001) and on the (111) surface of an Ir single crystal are presented. Both sample systems exhibit complex, non-collinear magnetic ground states which are studied by means of spin-polarized scanning tunneling microscopy (SP-STM).
Fe atoms deposited on the (5×1)-reconstructed Ir(001) surface grow in the trenches of the reconstruction, thereby forming chains with a width of only two atoms. SP-STM measurements in an external magnetic field reveal a modulation with a periodicity of three atomic distances along the chain axes, which can be attributed to a spin spiral ground state. Without an external magnetic field, the spin spiral fluctuates as a macrospin at the measurement temperature due to thermal excitations, which leads to a vanishing contrast in SP-STM measurements. Density functional theory (DFT) calculations reveal a combination of an almost quenched Heisenberg exchange and the antisymmetric Dzyaloshinskii-Moriya (DM) interaction as the microscopic origin of this non-collinear ground state.
The first atomic layer of Fe grows pseudomorphically on the Ir(111) surface which leads to a hexagonal arrangement of the Fe atoms. In SP-STM measurements, the Fe layer exhibits an almost square magnetic superstructure with a (1×1) nm² unit cell. Measurements of four different magnetization components are superimposed and reveal a lattice of skyrmions as the ground state of this sample system, which can be detected in spin-averaged STM measurements due to its non-collinearity. Atomically resolved STM images show that the skyrmion lattice is incommensurate to the atomic lattice. DFT calculations reveal that the Heisenberg exchange in this sample system is extremely weak, similar to the Fe chains on the Ir(001) surface. Therefore, the DM interaction and the often neglected four-spin interaction play crucial roles, and drive the Fe layer into the skyrmion lattice ground state.