arXiv subjects

Oguz Yildirim

Publications and source records attributed to Oguz Yildirim.

4 recordsLinked to original sources

arXiv2025

On Tailoring Structural and Optoelectronic Properties of TiO2 Thin Films Synthesized via 'Room' Temperature High Power Impulse Magnetron Sputtering (HiPIMS)

Titanium dioxide (TiO2) is a key material in optoelectronic and energy conversion technologies, including solar cells and photocatalysis. However, integrating TiO2 into flexible or temperature-sensitive devices requires deposition techniques that avoid high-temperature processing while maintaining control over both phase composition and crystallinity. In this work, we demonstrate the synthesis of nanocrystalline TiO2 thin films using High Power Impulse Magnetron Sputtering (HiPIMS) at nearroom temperature. By systematically varying total pressure and oxygen flow, we achieve tunable anatase to rutile phase ratios and control over crystalline quality, as evidenced by Raman and photoluminescence trends. The observed optical trends in both refractive index and emission are directly linked to the underlying structural evolution, with compositional analysis verifying stoichiometric consistency across all deposition conditions. Our findings establish HiPIMS as a powerful low temperature method for tailoring TiO2 thin films and enabling their application in flexible photovoltaics, photoelectrochemical water splitting, and other energy-related systems.

cond-mat.mtrl-sci↗

arXiv2022

Tuning the coexistence regime of incomplete and tubular skyrmions in ferro/ferri/ferromagnetic trilayers

The development of skyrmionic devices requires a suitable tuning of material parameters in order to stabilize skyrmions and control their density. It has been demonstrated recently that different skyrmion types can be simultaneously stabilized at room temperature in heterostructures involving ferromagnets, ferrimagnets and heavy metals, offering a new platform of coding binary information in the type of skyrmion instead of the presence/absence of skyrmions. Here, we tune the energy landscape of the two skyrmion types in such heterostructures by engineering the geometrical and material parameters of the individual layers. We find that a fine adjustment of the ferromagnetic layer thickness and thus its magnetic anisotropy, allows the trilayer system to support either one of the skyrmion types or the coexistence of both and with varying densities.

cond-mat.mtrl-sci↗

arXiv2021

Mapping the Structure of Oxygen-Doped Wurtzite Aluminum Nitride Coatings From Ab Initio Random Structure Search and Experiments

Machine learning is changing how we design and interpret experiments in materials science. In this work, we show how unsupervised learning, combined with ab initio modeling, improves our understanding of structural metastability in multicomponent alloys. We use the example case of Al-O-N alloys where the formation of aluminum vacancies in wurtzite AlN upon the incorporation of substitutional oxygen can be seen as a general mechanism of solids where crystal symmetry is reduced to stabilize defects. The ideal AlN wurtzite crystal structure occupation cannot be matched due to the presence of an aliovalent hetero-element into the structure. The traditional interpretation of the c-lattice shrinkage in sputter-deposited Al-O-N films from X-ray diffraction (XRD) experiments suggests the existence of a solubility limit at 8at.% oxygen content. Here we show that such naive interpretation is misleading. We support XRD data with a machine learning analysis of ab initio simulations and positron annihilation lifetime spectroscopy data, revealing no signs of a possible solubility limit. Instead, the presence of a wide range of non-equilibrium oxygen-rich defective structures emerging at increasing oxygen contents suggests that the formation of grain boundaries is the most plausible mechanism responsible for the lattice shrinkage measured in Al-O-N sputtered films.

cond-mat.mtrl-sci↗

arXiv2015

Structure and Giant Inverse Magnetocaloric Effect of Epitaxial Ni-Co-Mn-Al Films

The structural, magnetic, and magnetocaloric properties of epitaxial Ni-Co-Mn-Al thin films with different compositions have been studied. The films were deposited on MgO(001) substrates by co-sputtering on heated substrates. All films show a martensitic transformation, where the transformation temperatures are strongly dependent on the composition. The structure of the martensite phase is shown to be 14M. The metamagnetic martensitic transformation occurs from strongly ferromagnetic austenite to weakly magnetic martensite. The structural properties of the films were investigated by atomic force microscopy and temperature dependent X-ray diffraction. Magnetic and magnetocaloric properties were analyzed using temperature dependent and isothermal magnetization measurements. We find that Ni$_{41}$Co$_{10.4}$Mn$_{34.8}$Al$_{13.8}$ films show giant inverse magnetocaloric effects with magnetic entropy change of 17.5\,J\,kg$^{-1}$K$^{-1}$ for $μ_0 ΔH=5\,\text{T}$.

cond-mat.mtrl-sci↗