Complex spin structures in frustrated ultrathin films

In this thesis, complex spin structures driven by frustration of magnetic interactions are studied in ultrathin Co and Mn films. Therefore, density functional theory (DFT) calculations using the full potential linearized augmented planewave method are mapped onto an atomistic spin model. On this basis, spindynamics simulations are performed to analyse the properties of non-collinear spin structures such as domain walls (DWs), skyrmions and antiskyrmions. The obtained results are compared to spin-polarized scanning tunnelling microscopy (SP-STM) measurements performed by experimental collaborators. For the prominent Pt/Co/Ir interface, additive effects have been proposed in magnetic multilayer systems to enhance the Dzyaloshinskii-Moriya interaction (DMI); however, here it is shown that this phenomenon does not occur in atomic Pt/Co bilayers (BLs) on Ir(111). Even though an enhancement of the DMI from Co/Ir(111) to Pt/Co/Ir(111) is observed, the interfacial DMI appears more complex and a frustration of DMI is predicted – an effect which has been neglected so far. It results in a dependence of the rotational sense on the period of spin spirals and demonstrates that DMI calculations can give wrong results when unsuitable non-collinear spin structures are considered. Furthermore, the exchange interaction of Co/Ir(111) is tuned with 4d top layers from weakly to strongly ferromagnetic. Replacing Ir(111) by its 4d equivalent Rh(111), the spin-orbit coupling dependent interactions can be controlled. Exceptionally large frustration is found for Rh/Co/Ir(111). This stabilizes atomic-scale non-collinear spin structures such as DWs and skyrmions without an external magnetic field. SP-STM measurements confirm this prediction and reveal a large non-collinear magnetoresistance at the Fermi energy. This effect is explained by a spin mixing of a majority dxz and a minority pz state in the Rh/Co BL and allows for the detection of skyrmions and DWs with non-magnetic tips. Antiferromagnetic (AFM) materials are promising for spintronic applications and the prototypical AFM Mn holds a wide spectrum of magnetic and structural properties. Although a seemingly simple c(2 × 2) AFM ground state in a Mn double layer on W(001) has been suggested by SP-STM measurements, the presented DFT calculations highlight a dead magnetic layer in the Mn subsurface driven by strong frustration effects. This is confirmed by excellent agreement of theory and experiment. In Mn/Re(0001), experimental collaborators have discovered the row-wise antiferromagnetic (RW-AFM) and the 3Q state in STM measurements. Calculations point out strong frustration of exchange and DMI, but also of higher order exchange interactions due to hybridization of Mn and Re(0001). The coupling of the RW-AFM state to the lattice in Mn-fcc is explained by the interplay of dipolar and pseudo-dipolar interactions, whereas a distortion of the 3Q state in Mn-hcp towards the RW-AFM is predicted by DFT.


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