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| Home > Publications database > Synthesis and Electronic Ordering Phenomena of Calcium Ruthenate Thin Films |
| Dissertation / PhD Thesis | PUBDB-2018-05523 |
;
2018
Universität Stuttgart
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Please use a persistent id in citations: doi:10.18419/OPUS-10069 doi:10.3204/PUBDB-2018-05523
Abstract: The prospect to harness the wide range of electronic phenomena foundin transition metal oxides by synthesizing thin film structures andtheir implementation into next-generation technologies has sparked aresearch activity of ever increasing pace. Most prominent are thin filmtechniques such as heterostructuring and strain engineering, whichhave been shown to open up new paths to study the rich physics inthese compounds.This work constitutes a study of the thin film synthesis of the lay-ered orthorhombic transition metal oxide compound Ca$_2$RuO$_4$(CRO)and of the impact of biaxial epitaxial strain on the magnetic and or-bital ordering phenomena previously found by spectroscopic methods.The electronic structure of CRO and other transition metal oxideswith4d-valence electrons is of significant topical interest, because theenergy scales of spin-orbit-coupling, exchange energy and crystal fieldare comparable. As a result, the antiferromagnetic Mott insulatorCRO plays host to an array of novel ground states and is highly sus-ceptible to external perturbations as has been shown by numerousstudies on single crystals involving e.g pressure, chemical substitutionand electrical current.Embarking on the strategy to exploit this sensitivity of CRO, weemploy strain engineering via thin film growth to tune its properties.The details of the synthesis route using pulsed laser deposition arepresented. Growth was accomplished with two distinct sets of high-quality samples – a-axis oriented films on LaSrAlO$_4$ and NdCaAlO$_4$substrates with (110) cut and c-axis oriented films on LaAlO$_3$, LaSrAlO$_4$ and YAlO$_3$substrates with (001) cut. Structural charac-terization shows that the epitaxy exerts strains of 1 %–3 %on thelattice constants of CRO compared to the bulk system. These largedistortions manifest themselves in the electrical transport properties,which shows that the electronic state can be profoundly tuned froman insulator to a metal.Comprehensive resonant elastic X-ray scattering and magnetome-try studies confirm the presence of antiferromagnetism in CRO onLaAlO$_3$, NdCaAlO$_4$ and LaSrAlO$_4$(110) at T$_N$= 150 K, similar tothe antiferromagnetic state found in bulk. Detailed polarization anal-ysis of resonant peaks reveals a b-axis oriented magnetic moment forCRO on LaSrAlO$_4$(110), as found in bulk. On the other side, CRO on NdCaAlO$_4$shows an unusual signal that is compatible with a magnetic moment 45◦ away from the c-axis, approximately along the ($\bar{1}$02)direction of the orthorhombic unit cell of CRO, which has not beenreported for the bulk system. Furthermore, magnetoresistance andmagnetometry measurements show a weak ferromagnetic signal alongthe c-axis within the antiferromagnetic phases. We show that, whilea moment direction along the b-axis is compatible with a representa-tion analysis of the bulk Pbca space group, the presence of a weakferromagnetic component along the c-axis and the peculiar magneticmoment direction along the $\bar{1}$(02) for CRO on NdCaAlO$_4$ cannot bedescribed by a single irreducible representation of the space groupPbca. Thus, we suggest either an exotic scenario with more than onecritical irreducible representation or that the thin films exhibit anunusual symmetry reduction from the bulk space group Pbca. Thefinding of a strain-induced moment direction switching points to anon-trivial effect of epitaxial strain on the magnetic interactions in CRO.Magnetometry, magnetoresistance and polarized neutron reflectom-etry experiments were conducted to investigate a low-temperatureferromagnetic phase in the c-axis oriented samples. Similar to pres-surized bulk, we find ferromagnetic moment amplitudes of the order0.1$\pi_{B}$/Ru and an enhancement under compressive strain. Polarizedneutron reflectometry was used to exclude impurity phases or surfaceeffects as the underlying mechanism. Analogous to the pressurizedbulk system, we found a giant positive magnetoresistance in CRO onLaAlO$_3$. Moreover, a combined nuclear magnetic resonance and resonantelastic X-ray scattering study was dedicated to study the impact of epitaxial strain on quadrupolar order previously found in bulk CRO.Due to the small mass of the thin films, the nuclear magnetic resonance experiments were conducted using the NMR-technique, whichdetects the spin lattice relaxation through the asymmetry of the-decay of highly-polarized $^8$Li-isotopes. The temperature dependenceof the relaxation rate of the c-axis oriented films showed an increase towards higher temperatures, starting at$\approx$ 200 K. More detailed measurements on metallic CRO on LaSrAlO$_4$(001) revealed an anomaly at 300 K, which is reminiscent of a phase transition. Since resonant X-ray scattering on bulk CRO showed orbital ordering phenomena in thesame temperature range, we similarly searched for quadrupolar order-ing using synchrotron radiation tuned to the Ru-L absorption edges. However, the resonant peaks detected in our experiments could befully described by Templeton scattering, a phenomenon arising frompurely structural effects. For reference purposes, we performed a sim-ilar analysis for the single crystal showing that the results previouslyinterpreted as antiferro-orbital order bear many signatures of Temple-ton scattering as well. Thus, this work motivates a reevaluation ofthe original scattering study. An alternative scenario for the NMR anomaly based on diffusion of the $^8$Li-ions is discussed.The synthesis of CRO on a variety of substrates enables furtherstudies of the influence of large structural distortions on the elec-tronic states in CRO, which are currently inaccessible by pressuretechniques for single crystals. This opens up new opportunities totest theories regarding the electronic states in CRO, which are intensively debated due to multiple competing energy scales. Specifically, arecently proposed strong spin-orbit coupling picture for CRO predictsa high sensitivity towards the crystal field splitting in this material,which is directly tuned via the epitaxial strain. The large tunabilityof the electronic properties CRO by epitaxial strain might be usefulfor future thin film devices.
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