Photoemission electron microscopy of magneto-ionic effects in La0.7Sr0.3MnO3

One of the most challenging tasks in future nanoelectronics is the realization of novel low-power consumption devices for the application in non-volatile memories, processing, transduction and sensing units.  Electric-field-control of the magnetism, which is based on the magnetoelectric (ME) effect, may pave the way towards an alternative concept to Si-based electronics relying on dissipative electrical currents.
Numerous studies reported on magnetoelectric materials showing ME coupling via strain, charge carrier doping and interfacial exchange processes. Recently, the field of magneto-ionics received increased attention as the voltage-driven chemical intercalation of ionic species opened a novel route towards the control of magnetism in bulk materials.
This thesis reports on electric-field-induced magnetic switching in a La0.7Sr0.3MnO3 (LSMO) thin film prototype system. As a strongly correlated magnetic oxide LSMO can appear in multiple magnetic states depending on the prevalent oxidation state of the manganese cations. The migration of oxygen vacancies and the local redistribution of the oxygen concentration profoundly alters the balance of the double-exchange and superexchange interactions between manganese and oxygen ions, thus providing a granular control of chemical, structural and magnetic phase transitions. 
10 nm thick La0.7Sr0.3MnOfilms were epitaxially grown on TiO2-terminated Nb(0.5%):SrTiO3 (001) substrates by means of pulsed laser deposition (PLD). The morphology and the crystal structure were characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD) revealing  an epitaxial, single crystalline film and an atomically flat surface. Vibrating   sample magnetometry (VSM) and X-ray magnetic circular dichroism (XMCD) were used to investigate the magnetic properties. Further, the electronic structure and the chemical properties were probed by hard X-ray photoemission spectroscopy (HAXPES) and X-ray absorption spectroscopy (XAS).
Local conductivity atomic force microscopy (LC-AFM) was utilized to electrically modulate the resistance of LSMO thin films. To investigate the micro-scale redox processes present in the electrically modified areas, we performed a combined and element-selective X-ray photoemission electron microscopy (XPEEM) study in XAS and XMCD mode. The results demonstrate the direct interplay between resistivity, chemical composition, and magnetic ordering driven by an oxygen exchange process across the LSMO film surface. Significant chemical modifications are observed in the high resistive state, where the incorporation of oxygen vacancies leads to a distinct valence change from Mn3+/4+ to Mn2+/3+. Further, a direct correlation between oxygen deficiency and a degradation of the ferromagnetic properties is found.
In this light, the results of this thesis provide novel insights into the vacancy-driven magneto-ionic control of magnetoelectric oxides for the example LSMO. Moreover, they open up novel routes towards a multiphase-control of physical and chemical properties in magnetic oxides and novel ionotronic devices.


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