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Authors: Pfeiffer, Alexander
Title: Pure spin current transport and magnetic state manipulation in lateral spin valves
Online publication date: 22-Jan-2020
Language: english
Abstract: Lateral spin valves are model systems that are well-suited for the study of pure spin current transport across interfaces. Furthermore, these systems allow for an efficient manipulation of magnetic states, e.g. a magnetic vortex, due to the intrinsic reduction of accompanying Oersted fields and Joule heating at the position where the magnetization is manipulated. This makes lateral spin valves attractive also for future device concepts such as data storage devices. In the first part of this thesis, the field and pure spin current induced switching of a magnetic disc, acting as a detector electrode, is studied and discussed. By a judicious choice of the applied field direction and the amplitude and polarity of the applied current pulses in the injector, the switching behaviour of the magnetic disc can be measured as a function of the resulting absorbed spin current. Although nominally round and therefore without expected in-plane shape anisotropy, it is observed that small variations in the shape and randomly distributed imperfections can strongly affect the device behaviour. Suitable field sweep directions are identified, which allow for a study of the switching from a magnetic mono-domain state to a magnetic vortex state and from the vortex state to the opposite mono-domain state. Although most of the changes in the magnetic switching occur due to Joule heating, the spin transfer torque from the pure spin current also contributes to the switching, which is detectable thanks to our optimized device geometry which allows for synchronous current pulses from two injector electrodes. The second part of the thesis tackles the fundamental properties of pure spin current transport across metallic interfaces. In a first work, Pt-Py-Cu multi terminal spin valves have been fabricated, which allow to compare the temperature dependence of the spin current generated by two different spin current generation and detection methods. As the first method, the conventional electric spin injection is exploited where current injection and detection are based on two parallel ferromagnetic wires, bridged by a Cu conduit. As a second method, a spin to charge interconversion is used and either spin currents are generated by the spin Hall effect and detected by conventional non-local detection or spin currents are generated by conventional spin current injection and detected by the inverse spin Hall effect. Differences in the temperature dependences of these two detection methods are found, which means that the temperature dependence of the signals is not only governed by the spin transport and spin injection as previously claimed. In a second work and motivated by previous results, again Pt-Py-Cu lateral spin valves, yet with an optimized device geometry, are studied which allow for a comparison of the conventional non-local signal, the spin absorption signal and the inverse spin Hall effect signal for varying temperatures. Depending on the fabricated induced quality of the relevant interfaces for the different samples, significant differences in the signals, i.e. either a very large spin absorption and a low spin Hall effect signal or no significant absorption but a very large spin Hall effect signal are determined. These large differences cannot be attributed to structural variations visible with conventional imaging and are also not reflected in the very similar electrical charge transport properties of the devices. Rather details of the interfaces are accessed via a special scanning electron microscope technique for buried interface imaging, revealing important details of the interfaces which explain the spin transport results. By additional measurements of CoFe-Cu lateral spin valves with varying fabrication methods for both the CoFe wires and the Cu bridge, the reliability and reproducibility of the previous measurements are confirmed in a different system. While samples with high quality interfaces are expected to show no differences in the temperature behaviour of the conventional non-local and the spin absorption signal, a significantly stronger decrease of the spin absorption signal with increasing temperatures for samples with low quality interfaces is observed. The last part of the thesis is about the fabrication of Co2MnSi based lateral spin valves. Although no spin signal has been seen for these samples, the fabrication for samples based on thin films has been significantly improved and opens a route to potentially produce lateral spin valves with high spin signals based on the findings and improvements.
DDC: 530 Physik
530 Physics
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 08 Physik, Mathematik u. Informatik
Place: Mainz
URN: urn:nbn:de:hebis:77-diss-1000032722
Version: Original work
Publication type: Dissertation
License: In Copyright
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Extent: 201 Seiten
Appears in collections:JGU-Publikationen

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