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978-3-8439-1801-5, Reihe Physik
Magnetoelectric Coupling and Thermally Driven Magnetization Dynamics Studied on the Atomic Scale
113 Seiten, Dissertation Universität Hamburg (2014), Softcover, A5
Future magnetic data storage devices require new concepts to increase the data storage capacity and a detailed understanding of the impact of thermal fluctuations on the magnetization of the data carrier. This thesis is concerned with the investigation of magnetoelectric coupling as a method to alter the magnetic properties of nanoscale magnetic systems, and the fundamental processes of thermally driven magnetization dynamics. For this purpose spin-polarized scanning tunneling microscopy (SP-STM) investigations are performed on monolayer iron films on W(110), Mo(110), and Ir(111) single crystalline samples.
In the first part of the thesis, magnetoelectric coupling is studied on the atomic scale. An electric field of up to 6 GV/m is applied to individual Fe/W(110) and Fe/Mo(110) nanomagnets using the tip of the tunneling microscope. Observing the superparamagnetic switching reveals an additional, electric-field-induced magnetic anisotropy that favors in-plane magnetization for E < 0 and out-of-plane magnetization for E > 0. The experiments demonstrate magnetic manipulation on the atomic scale without exploiting spin or charge currents.
To resolve the fast processes during thermal magnetization reversal, an STM combined pump-probe scheme is developed that increases the time resolution of the experimental setup into the nanosecond regime. This technique is used to study Fe/W(110) nanomagnets with switching rates up to 1E7 per second. The experiments show that the magnets switch significantly slower than expected at high temperatures. This is attributed to a process called multi-domain-wall nucleation.
In the last part of the thesis, temperature dependent SP-STM investigations of the nanoskyrion lattice in the monolayer Fe/Ir(111) are presented. In previous studies, a large stabilization energy of 17 meV/atom was found, and it was speculated that the system might be stable up to room temperature. Experimentally, however, a disappearance of the skyrmion lattice is observed at 28K, indicating an unordered phase above this temperature.
The experiments give fascinating insight into previously inaccessible processes in atomic-scale magnets. They demonstrate that the thermally induced dynamics are highly complex and that the internal degrees of freedom play a crucial role. The results of this work thereby enable a better understanding of the underlying physics and open a pathway to a wide range of new experiments on atomic-scale magnetism.