TY  - THES
AU  - Hoppe, Michael
TI  - Magnetic, structural, and electronic properties of NiFe$_{2}$O$_{4}$ ultrathin films
VL  - 118
PB  - Universität Duisburg
VL  - Dr.
CY  - Jülich
M1  - FZJ-2016-01885
SN  - 978-3-95806-122-4
T2  - Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien / Key Technologies
SP  - VII, 118 S.
PY  - 2016
N1  - Universität Duisburg, Diss., 2015
AB  - The 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 film 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 offer a promising approach for the optimized performance of spintronic devices. They can realize a highly effective spin-filter effect, where spin-polarized electron currents are generated by a spindependent tunneling process. For this purpose, the spinel ferrite NiFe$_{2}$O$_{4}$ 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 NiFe$_{2}$O$_{4}$ is investigated in detail. Therefore, NiFe$_{2}$O$_{4}$ thin films are deposited on Nb-doped SrTiO$_{3}$ (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 NiFe$_{2}$O$_{4}$ thin films. 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 NiFe$_{2}$O$_{4}$ films. Here, an enhanced saturation magnetization M$_{S}$ for ultrathin NiFe$_{2}$O$_{4}$ films ($\textit{d}$ < 4nm) that coincides with a reduced out-of-plane lattice constant under compressive in-plane epitaxial strain is observed. The films 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 NiFe$_{2}$O$_{4}$ film thickness is found – thus ruling out a cationic inversion that nominally could account for an enhanced M$_{S}$. The spin and orbital contribution to the net magnetization are investigated element-specific by recording high-quality low noise XMCD spectra and evaluating them using the sum rules. The 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 NiFe$_{2}$O$_{4}$ films and SrTiO$_{3}$ substrates is discovered, which originates from a ferromagnetic ordering of the Ti electrons. The underlying mechanism is explained by superexchange interaction across the interface which imposes the ferromagnetic order of the electron in NiFe$_{2}$O$_{4}$ onto the Ti electrons. The given results open the path for a future integration of NiFe$_{2}$O$_{4}$ into spin filter tunnel junctions. Additionally, the observed interfacial Ti ferromagnetism renders NiFe$_{2}$O$_{4}$/SrTiO$_{3}$ heterostructures as a intriguing system for exploring the interplay between the various degrees of freedom in transition metal oxides.
LB  - PUB:(DE-HGF)3 ; PUB:(DE-HGF)11
UR  - https://juser.fz-juelich.de/record/283571
ER  -