Ozan Kirilmaz (PhD thesis)

Ozan Kirilmaz (PhD thesis): Thin Film Growth and Characterization of the Transition Metal Oxides Magnetite and Layered Perovskite Iridates

Abstract

This thesis describes the growth and characterization of both the all-oxide heterostructure Fe₃O₄/ZnO and the spin-orbit coupling driven layered perovskite iridates.

As for Fe₃O₄/ZnO, the 100% spin-polarized Fe₃O₄ is a promising spin electrode candidate for spintronic devices. However, the single crystalline ZnO substrates exhibit different polar surface termination which, together with substrate preparation method, can drastically affect the physical properties of Fe₃O₄/ZnO heterostructures. In this thesis two different methods of substrate preparation were investigated: a previously used in situ method involving sputtering and annealing treatments and a recent ex situ method containing only the annealing procedure. For the latter, the annealing treatment was performed in dry and humid O2 gas ow for the O- and Zn-terminated substrates, respectively, to produce atomically at surfaces as verified by atomic force microscopy (AFM). With these methods, four different ZnO substrates were fabricated and used further for Fe₃O₄ lm growth. Fe₃O₄ films of 20 nm thickness were successfully grown by reactive molecular beam epitaxy. AFM measurements reveal a higher lm surface roughness for the samples with in situ prepared substrates. Moreover, X-ray photoelectron spectroscopy (XPS) measurements indicate signicant Zn substitution within the Fe₃O₄ lm for these samples, whereas the samples with ex situ prepared substrates show stoichiometric Fe₃O₄ films. X-ray diffraction measurements conrm the observations from XPS, revealing additional peaks due to Zn substitution in Fe₃O₄ films grown on in situ prepared ZnO substrates. Conductivity, as well as magnetometry, measurements show the presence of Zn-doped ferrites in films grown on in situ prepared substrates. Such unintentionally intercalated Zn-doped ferrites dramatically change the electrical and magnetic properties of the films and, therefore, are not preferred in a high-quality heterostructure. X-ray re ectivity (XRR) measurements show for the lm grown on ex situ prepared Zn-terminated substrate a variation of lm density close to the interface which is also conrmed by transmission electron microscopy (TEM). Using polarized neutron re ectometry, magnetic depth proles of the films grown on ex situ prepared substrates clearly indicate Fe₃O₄ layers with reduced magnetization at the interfaces. This result is consistent with earlier observations made by resonant magnetic X-ray re ectometry (RMXR), but in contrast to the ndings from XRR and TEM of this thesis. A detailed TEM study of all four samples shows that the sample with ex situ prepared O-terminated substrate has the sharpest interface, whereas those with ex situ prepared Zn-terminated as well as in situ prepared substrates indicate rougher interfaces. STEM-EELS composition proles of the samples reveal the Zn substitution in the films with in situ prepared substrates and therefore conrm the presence of Zn-doped ferrites. Moreover, a change of the Fe oxidation state of the rst Fe layer at the interface which was observed in previous studies done by RMXR, was not veried for the samples with in situ prepared substrates thus leaving the question of a possible presence of the magnetically dead layer open.
Furthermore, density functional theory calculations were performed to determine the termination dependent layer sequences which are ...-Zn-O-(interface)-[Fe(octa)-O-Fe(tetra)-Fe(octa)-Fe(tetra)-O]-[...]-... and ...-O-Zn-(interface)-[O-Fe(octa)-O-Fe(tetra)-Fe(octa)-Fe(tetra)]-[...]-... for the samples with O- and Zn-terminated substrates, respectively. Spin density calculations show that in case of O-termination the topmost substrate layers imitate the spin polarization of lm layers close to the interface. Here, the rst O layer is affected much stronger than the rst Zn layer. Due to the strong decrease of this effect toward deeper substrate layers, the substrate surface is supposed to be sensitive to the contiguous spin polarization of the lm. Thus, the topmost O layer of the O-terminated substrate could play the most essential role for effective spin injection into ZnO.

The 5d transition metal oxides Ba2IrO4 (BIO) and Sr2IrO4 (SIO) are associated with the Ruddlesden-Popper iridate series with phase type "214" (RP{214), and due to the strong spin-orbit coupling belong to the class of Mott insulators. Moreover, they show many similarities of the isostructural high Tc-cuprate superconductors, e.g. crystal structure, magnetism and electronic band structure. Therefore, it is of great interest to activate a potential superconducting phase in (RP{214) iridates. However, only a small number of publications on PLD grown (RP{214) iridates in the literature exists. Furthermore, published data of soft X-ray angle resolved photoemission spectroscopy (SX-ARPES) experiments mainly originate from measurements which were performed on single crystals or MBE grown films of SIO and BIO. In this thesis La-doped SIO films (La0:2Sr1:8IrO4, further referred as LSIO) were used to pursue a potential superconducting phase. A set of characterization methods was used to analyze the quality of the PLD grown BIO, SIO and LSIO films. AFM measurements demonstrate that thick PLD grown (RP{214) iridate films have rougher surfaces, indicating a transition from a 2D layer-bylayer growth (which is demonstrated by RHEED oscillations) to a 3D island-like growth mode. In addition, chemical depth proling XPS measurements indicate an increase of the O and Ir relative concentrations in the topmost lm layers. Constant energy k-space
maps and energy distribution curves (EDCs) measured by SX-ARPES show for every grown lm only weak energy band dispersions, which are in strong contrast to the results obtained on the MBE grown films and single crystals from the literature. In this thesis, a subsequent TEM study reveals missing SrO layers within the grown films which occur mainly in the topmost layers, conrming the results and suggestions from XPS and SX-ARPES data: the PLD grown films have defects and, therefore, incoherently scatter photoelectrons. Nevertheless, the LSIO lm shows small additional spectral weight between the highsymmetry M points close to the Fermi level which can be attributed to quasiparticle states which, in turn, indicates the formation of a Fermi-arc. However, neither conductivity measurements nor valence band analysis via XPS conrm an activation of a superconducting phase or presence of spectral weight of quasiparticle states at the Fermi level in this LSIO film.

It is possible that these discovered diffculties in growth are responsible for the low number of SX-ARPES publications on PLD grown (RP{214) iridate films. For further investigations of (RP{214) iridate films by SX-ARPES, their PLD growth recipes have to be improved to create high quality single crystalline films without imperfections.

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