Fabrication and analysis of nanostructures in the quantum material SrRuO${}_3$

Nanoscale confinement of materials in one or more dimensions allows for the realization of properties that do not exist in other substances in nature. This makes such objects extremely interesting as systems to study new physical effects and for potential applications in devices. New perspectives in this field arise if one considers using materials with inherently correlated electron systems. Even in bulk, these substances possess rich phase diagrams with a large number of competing ground states. The abundance of the functional properties is expected to enrich the physics of nanoobjects created from such materials. In this work, I utilize the top-down approach to create and study 0D- and 1D-confined nanostructures of correlated ferromagnetic metallic perovskite SrRuO${}_3$. A specific emphasis is put on the modification of its magnetic properties upon confinement from the thin film to the nanostructured form. In Chapter 3, I establish a basis for this research by developing the technology for patterning complex epitaxially-grown heterolayers. Using a combination of electron beam lithography and dry etching with Ar ions I succeeding fabricating nanostructures with sizes of <20 nm, which was never achieved before for this class of materials. The top-down approach allows us the creation of nanostructures with arbitrary configurations and shape. At the same time, the stability of the process grants us the possibility to structure large areas containing up to several billions elements per sample. The second highlight of the chapter is the optimization of growth conditions of SrRuO${}_3$ on (100) SrTiO${}_3$ to achieve the highest possible material quality while using pulsed laser deposition as a growth technique. We implement a newly developed technology of in situ thermal annealing for preparation of the substrate surface before epitaxial growth. The technology allows for a higher quality and better reproducibility of the deposited material compared to the commonly used chemical treatment of substrates. The properties of both the initial thin film and the nanodots of SrRuO${}_3$ are examined in Chapter 4 using various methods. I show that the quality of our thin films is comparable to or even exceeds the best films reported in the literature. After the patterning to nanodots, the material preserves the epitaxial structure and does not develop visible defects or dead layers down to structure sizes of 30 nm. This result is crucial as it proves that epitaxial layers of complex materials can be patterned to such small scales while maintaining structural integrity. The results of SQUID magnetometry performed on the nanodots are presented in Chapter 5. I show that the magnetic properties of SrRuO${}_3$ change in many respects after patterning. One intriguing effect is the enhancement of the Curie temperature of the ferromagnetic transition in the nanodots with sizes smaller than $\approx$ 200 nm. Although a modification of T${}_C$ in nanopatterned structures is not surprising, usually only a decrease is observed which is associated with the reduction of the number of nearest neighbors when one of the dimension of a system approaches several nanometers. In my samples, I see a gradual increase of T${}_C$ with shrinking dot size, starting from T${}_C$ = 149±1K in thin films and dots > 200nm and reaching a maximum of T${}_C$$\approx$ 157K at a dot size of 35-40 nm. To explain such a behavior, we study the structural properties of the nanodots using STEM-based geometric phase analysis. This reveals a reduction of compressive epitaxial strain in small dots. The amount of the relieved strain increases with decreasing dot size and happens at the same length scale as the observed T${}_C$ enhancement. Literature data showing a dependence of the magnetism in SrRuO${}_3$ on the amount of applied strain confirm this interpretation. Analyzing the behavior of the nanodots hysteresis loops, I track the evolution of M(B) with reducing structure size. I see that the hysteresis curves start to deviate from the thin film shape at dot sizes of $\approx$ 10 μm and smaller. I observe a gradual reduction of the remanence to saturation magnetization ratio from M${}_{rem}$/M${}_{sat}$ = 0.85 in the thin films to M${}_{rem}$/M${}_{sat}$ = 0.51 and M${}_{rem}$/M${}_{sat}$ = 0.44 in 200 and 50nm dots, respectively, as measured in the out-of-plane oriented magnetic field. This indicates that the magnetic easy axis reorients itself to a different angle. Using the slope of the loops at small fields, I estimate values of the anisotropy field and the anisotropy constant and conclude that they remain within the same order of magnitude after patterning. To describe the behavior of magnetic anisotropy in the dots quantitatively, different experimental approaches have to be used. Also, I discover that with changing periodicity of the dots, a modification of the hysteretic behavior in the region of magnetic fields from $\approx$ 0 to 2.5T takes place which implies the effects of an interaction between the dots. From the SQUID measurements alone, it was not possible to determine whether these effects appear due to demagnetizing checkerboard states or to vortex or multidomain states inside the dots. In addition to the nanodots exploration, I also studied electrical transport in nanowires fabricated of SrRuO${}_3$ as described in Chapter 6. For these devices, (110) DyScO${}_3$ was used a substrate material to avoid leakage currents caused by the conductivity of SrTiO${}_3$ appearing after ion irradiation. I successfully fabricated wires with widths down to 30nm and a thickness of 12nm and showed that the material remains conductive and ferromagnetic. R(T) curves measured in a temperature range down to 2K show the same characteristic features as the corresponding thin films. The wires are robust, sustaining a dissipated power up to $\approx$ 300 μW corresponding to a current of I $\approx$ 300 μA and a voltage drop of U $\approx$ 1 V. I(V) characteristics are smooth with a nonlinearity caused by the effect of resistive heating. At a dissipated power of P $\approx$ 50 μW, a small anomaly is observed in nanowires of all sizes which is well visible in the dI/dV signal. The anomaly apparently occurs when the local temperature of the device exceeds that of the ferromagnetic transition. The results of this work show that it is possible to utilize a top-down approach and conventional e-beam lithography to pattern epitaxial nanostructures of complex materials with structure sizes of just about 10-20nm while retaining their functional properties. At the same time, the material structure stays free of major defects and dead layers that deteriorate the nanostructure properties. Using the example of a single material, I demonstrate new interesting features caused by electronic and structural modifications happening upon spatial confinement even in single or weakly interacting nanostructures. This is not to mention their strongly bound ensembles, and in the long term even entire artificial solids composed of such objects. We anticipate many possibilities for the fabrication and exploration of such systems consisting of various combinations of materials from a broad family of substances with correlated electron systems.