Magnetic, structural, and electronic properties of NiFe2O4 ultrathin films

¿e physical properties of transition-metal oxides are strongly determined by the competition of charge, spin, and orbital degrees of freedom. Continuing progress in the deposition techniques of oxides nowadays allows to grow thin lm heterostructures with atomically sharp interfaces. Tailoring the interface between oxides opens up the possibility to explore novel nanoelectronic functionalities and even to discover new phenomena only existing at the interface. In this framework, oxides featuring simultaneously magnetic and insulating properties oer a promising approach for the optimized performance of spintronic devices. ¿ey can realize a highly eective spin-lter eect, where spin-polarized electron currents are generated by a spindependent tunneling process. For this purpose, the spinel ferrite NiFe2O4 is a very auspicious material since it possesses both features even at room temperature. In this thesis, the sensitive interplay between magnetic, electronic and structural properties in the ferrimagnetic oxide NiFe2O4 is investigated in detail. ¿erefore, NiFe2O4 thin lms are deposited on Nb-doped SrTiO3 (001) substrates via pulsed laser deposition (PLD) and the growth conditions of the deposition process are carefully evaluated. Based upon this, a procedure is deduced, that allows the reproducible growth of high-quality, epitaxial, and single-crystalline NiFe2O4 thin lms. With the aim towards fabricating tunnel barriers, special emphasis is placed on the impact of reduced dimensionality in the crossover from bulk-like to ultrathin NiFe2O4 lms. Here, an enhanced saturationmagnetization MS for ultrathin NiFe2O4 lms (d < 4nm) that coincides with a reduced out-of-plane lattice constant under compressive in-plane epitaxial strain is observed. ¿e lms are investigated by complementing bulk- and surface-sensitive analyses using HAXPES, XANES and XMCD spectroscopy techniques. Hereby, a bulk-like cationic coordination of the inverse spinel lattice independent of the NiFe2O4 lm thickness is found – thus ruling out a cationic inversion that nominally could account for an enhanced MS. ¿e spin and orbital contribution to the net magnetization are investigated element-specic by recording high-quality low noise XMCD spectra and evaluating them using the sum rules.¿e resulting moments agree with the magnetic structure of an inverse spinel. However, they give no explanation for the observed enhanced MS. Instead, a novel magnetism at the interface between the NiFe2O4 lms and SrTiO3 substrates is discovered, which originates from a ferromagnetic ordering of the Ti electrons.¿e underlyingmechanism is explained by superexchange interaction across the interface which imposes the ferromagnetic order of the electron in NiFe2O4 onto the Ti electrons. ¿e given results open the path for a future integration of NiFe2O4 into spin lter tunnel junctions. Additionally, the observed interfacial Ti ferromagnetism renders NiFe2O4/SrTiO3 heterostructures as a intriguing system for exploring the interplay between the various degrees of freedom in transition metal oxides.