Mennig, Julius

English

Kölner UniversitätsPublikationsServer

AbstractThe subject of this thesis at hand is the generation of pure spin currents in lateral spinvalves.Pure spin currents result from the accumulation of spins and give rise to spin transport in theabsence of charge transport. The diﬀusive character of spin transport may be the key to prospec-tive concepts for information transmission and processing with signiﬁcantly reduced dissipation.The investigation of pure spin currents also contributes to a deeper understanding of spin-basedphenomena such as magnetoresistance (e.g. GMR, TMR) and magnetization dynamics (e.g. spin-transfer torque STT) by addressing fundamental spin transport and relaxation processes.In order to create a pure spin current, a charge current is injected into a ferromagnet/ nonmagnetinterface. The multi-terminal geometry for local and non-local electrical transport measurementsand the size of the diﬀusion channel needed for the generation and detection of pure spin cur-rents require a sequence of laterally connected ferromagnetic and nonmagnetic leads on thesubmicron scale, the so called lateral spinvalve. Besides local and non-local electrical transportmeasurements, imaging of the ferromagnetic parts and the diﬀusion channel by scanning electronmicroscopy with polarization analysis (SEMPA) is employed as an analysis tool. The intent ofapplying SEMPA to the nonmagnetic diﬀusion channel is to directly image the spin accumulati-on, which would provide new knowledge about the dynamics, propagation, and relaxation of spincurrents. The small information depth of SEMPA and the crucial importance of clean interfacesand surfaces for the creation of pure spin currents are the motivation to realize the entire samplefabrication and investigation in situ in a complex ultra-high vacuum system.A novel multi-stage fabrication process based on thermal evaporation and structuring with anfocused ion beam system (UHV-FIB) is developed. Thereby, the design of a sample layout givingrise to a single-domain magnetization patterns in the two ferromagnetic leads and allowing forhigh-resolution SEMPA imaging even during the current-induced generation of a pure spin cur-rent turned out to be the major challenge. The development steps to achieving these goals and thederived experience and know-how are presented in detail. An important aspect for interpretingthe non-local transport signal in a spinvalve is the distinct understanding of the remagnetiza-tion processes in the nanoscale magnets. This is achieved by a combined analysis of 2-pointmeasurements of the anisotropic magnetoresistance (AMR) eﬀect and SEMPA images of the fer-romagnets. For Co/Cu spinvalves the successful detection of pure spin currents is evidenced bycorrelating non-local electrical transport measurements to AMR and SEMPA data. The observedeﬀect size is in agreement with published work of other groups and indicates the high qualityof the ferromagnet/nonmagnet interfaces in our spinvalves. Imaging of the spin accumulation inthe nonmagnet by SEMPA did not show not the assumed results. Likely reasons as well as otheropen questions are ﬁnally discussed.v

Reine Spinströme in lateralen Spinventilen: in situ Erzeugung und Nachweis

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