arXiv subjects

Zbigniew Galazka

Publications and source records attributed to Zbigniew Galazka.

At least 19 recordsLinked to original sources

arXiv2026

Two Strategies to Measure Spin-Orbit-Torque Efficiency Acting on the Insulating Magnet Li$_{0.5}$Al$_{0.7}$Fe$_{1.8}$O$_4$

For insulating magnets, widely-used electrically-based techniques to measure spin-orbit-torque efficiency can suffer from poor signal-to-noise ratios and unwanted artifacts, while optical measurements based on conventional magneto-optical Kerr effect (MOKE) imaging often lack sufficient sensitivity. Here we analyze two strategies for measuring the spin-torque efficiency acting on an insulating magnet, focusing on the torque from Pt acting on thin films of the low-damping magnet lithium aluminum ferrite (LAFO). First, lateral + longitudinal spin-torque ferromagnetic resonance (ST-FMR) allows separation of the spin-torque signal from artifacts due to spin-pumping and resonant heating. With this technique, we measure a spin-torque efficiency $ξ^\text{ST-FMR}_\text{DL}\sim\ 0.07$, comparable to but a bit less than for Pt acting on metallic ferromagnets. Second, we attempted optical measurements of spin-orbit-torque-induced magnetic deflection using Sagnac interferometry. We find that Sagnac interferometry provides sufficient sensitivity to measure signals from 14.4 nm LAFO thin films, but the inferred torque efficiency is much weaker than the result from lateral + longitudinal ST-FMR. We speculate that there may be another contribution to the Sagnac signal other than current-induced magnetic deflection, perhaps an out-of-plane component of itinerant electron spins. This becomes relevant for insulating magnets in which the magneto-optic coupling coefficient is very weak.

cond-mat.mes-hall↗

arXiv2026

Complete Raman Tensor Determination in Birefringent $β$-Ga$_2$O$_3$ by Single-Stage Hyperspectral Analysis of Polarization Angle-Resolved Raman Spectra

The low symmetry of the monoclinic phase of Ga$_2$O$_3$ leads to pronounced optical anisotropy and, consequently, to birefringence, which strongly affects the Raman response. Because Raman scattering is fundamentally sensitive to the polarizability of a material, this anisotropy must be carefully accounted for in order to extract quantitative information - an effort that has only recently been shown to be feasible in such media. Here, we report Raman measurements from all three principal crystal planes $(100)$, $(010)$, and $(001)$, as well as from the $(\overline{2}01)$-plane of a $β$-Ga$_2$O$_3$ single crystal. By combining polarization angle-resolved Raman spectroscopy (PARRS) with a newly developed fitting procedure and explicitly accounting for birefringence, we achieve full spectral separation and quantitatively determine the energies and relative Raman tensor elements of all 15 Raman-active modes.

cond-mat.mtrl-sci↗

arXiv2026

Tuning perpendicular magnetic anisotropy in ultra-low damping Li$_{0.5}$Al$_{x}$Fe$_{(2.5-x)}$O$_4$ thin films for efficient spin-orbit torque switching

Ultra-thin magnetic insulator films that simultaneously exhibit ultra-low magnon damping, perpendicular magnetic anisotropy (PMA), and low spin-orbit torque (SOT) switching current densities are highly desirable, albeit challenging, for next-generation spintronic technologies that exploit spin waves to transport information without dissipative charge currents. Here, we demonstrate this combination of properties in ferrimagnetic spinel Li$_{0.5}$Al$_{x}$Fe$_{(2.5-x)}$O$_4$ (LAFO) thin films. Through this model system, we find that PMA can be tuned by epitaxial strain in the form of chemical composition and substrate choice and that low SOT switching current densities correlate with small but finite PMA. Ultra-low damping is stabilized primarily by having only Fe$^{3+}$ as magnetically active cations with secondary effects due to increased disorder from Al substitution distribution. SOT efficiency is governed by interface quality and independent of chemical composition. By varying the Al concentration, we systematically tune the saturation magnetization and magnetic anisotropy while maintaining ultra-low Gilbert damping parameters as low as $2\times10^{-4}$ and composition-independent damping-like SOT efficiencies. We identify an optimal composition LAFO x=0.7 (Li$_{0.5}$Al$_{0.7}$Fe$_{1.8}$O$_4$), which combines ultra-low damping, stable PMA with small anisotropy fields, and low critical current densities for SOT switching, establishing it as a promising material platform for energy-efficient spin-wave and spintronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Lattice dynamics and complete polarization analysis of Raman-active modes in LaInO$_3$

In this study, we present a comprehensive analysis of the Raman active phonon modes in orthorhombic LaInO$_3$ based on a combination of polarization-angle resolved Raman spectroscopy and density functional theory calculations. By using backscattering from multiple crystallographic surface orientations and employing a full symmetry analysis, we identify and assign most of the Raman-active $Γ$-point phonons to their irreducible representations of the D$_{\rm{2h}}$ point group. A multidimensional hyperspectral fitting procedure allows us to extract the relative Raman tensor elements from the angular dependence of the scattering intensities, even for strongly overlapping modes. First-principles calculations yield the phonon dispersion along high-symmetry directions, the phonon densities of states, and atomic displacement patterns, which are found to be in good agreement with the experimental mode frequencies.

cond-mat.mtrl-sci↗

arXiv2026

Microscopic Origin of Temperature-Dependent Anisotropic Heat Transport in Ultrawide-Bandgap Rutile GeO2

Ultrawide-bandgap rutile GeO2 is emerging as a promising semiconductor for power electronics, where efficient heat dissipation is essential to suppress self-heating and ensure device reliability. However, the temperature dependence and microscopic origin of its anisotropic heat transport have remained experimentally unresolved. Here, temperature-dependent time-domain thermoreflectance measurements combined with first-principles phonon transport calculations are used to quantify the thermal conductivity of single-crystal rutile GeO2 from 80 to 350 K along [001] and [110]. At 295 K, the thermal conductivity reaches 47.5 W m^-1 K^-1 along [001] and 32.5 W m^-1 K^-1 along [110], corresponding to an anisotropy ratio of 1.46, in good agreement with theory. Rather than following a simple T^(-1) law, the thermal conductivity exhibits an approximate T^(-1.4) dependence, indicating additional scattering beyond purely three-phonon-limited transport. Mode-resolved analysis reveals that the room-temperature anisotropy originates from the combined effect of larger phonon group velocities along [001] and direction-dependent phonon lifetimes. Upon cooling, depopulation of high-frequency phonons progressively suppresses their contribution to heat transport and reduces the anisotropy. The temperature-dependent thermal boundary conductance of Al/rutile GeO2 interfaces is further resolved, and the scaled conductance indicates predominantly elastic interfacial transport. These findings establish the microscopic basis of bulk and interfacial heat transport in rutile GeO2 and position this material as a promising thermally robust platform for ultrawide-bandgap electronics.

cond-mat.mtrl-sci↗

arXiv2025

Unravelling the oxygen influence in cubic bixbyite In$_2$O$_3$ on Raman active phonon modes by isotope studies

In this study, we performed comprehensive investigations on the Raman active phonon modes in cubic bixbyite In$_2$O$_3$, an important oxide based, wide-bandgap semiconductor. Fundamental insights into the lattice dynamics are revealed, by determining the atomistic contribution to all modes and their frequencies by density functional perturbation theory calculations. Those simulations were performed for different compositions of $^{16}$O and $^{18}$O isotope ratios, including their pure states. An increasing red-shift of the mode frequencies with increasing $^{18}$O content for all modes, due to the increased atomic mass, is revealed. For the seven lowest energy modes, this relative shift is below 1%, whereas for the remaining 15 higher energetic modes a shift of about 5.5% was identified. All modes have energy contributions of both indium and oxygen lattice sites, except for one, which corresponds to a pure oxygen vibrational state. Applying Raman spectroscopy, those results could be verified experimentally with excellent agreement. Investigated samples consisted of a bulk single crystal with $^{16}$O isotopes and a MBE grown thin film as the $^{18}$O sample. Time-of-flight secondary ion mass spectrometry confirms the purity of the oxygen isotope in the sample. These isotopologue studies allow for a direct experimental access to fundamental material properties in cubic In$_2$O$_3$ by means of Raman spectroscopy. For example, we speculate, that the presence of oxygen vacancies in In$_2$O$_3$ would result in a shift of modes that are dominated by O-vibrations, e. g., $E_{g}^{(4)}$ or $A_{g}^{(4)}$, towards lower frequencies.

cond-mat.mtrl-sci↗

arXiv2022

A consistent picture of excitations in cubic BaSnO$_{3}$ revealed by combining theory and experiment

Among the transparent conducting oxides, the perovskite barium stannate is most promising for various electronic applications due to its outstanding carrier mobility achieved at room temperature. However, most of its important characteristics, such as band gaps, effective masses, and absorption edge, remain controversial. Here, we provide a fully consistent picture by combining state-of-the-art {\it ab initio} methodology with forefront electron energy-loss spectroscopy and optical absorption measurements. Valence electron energy-loss spectra, featuring signals originating from band gap transitions, are acquired on defect-free sample regions of a BaSnO$_{3}$ single crystal. These high-energy-resolution measurements are able to capture also very weak excitations below the optical gap, attributed to indirect transitions. By temperature-dependent optical absorption measurements, we assess band-gap renormalization effects induced by electron-phonon coupling. Overall, we find for the effective electronic mass, the direct and the indirect gap, the optical gap, as well as the absorption onsets and spectra, excellent agreement between both experimental techniques and the theoretical many-body results, supporting also the picture of a phonon-mediated mechanism where indirect transitions are activated by phonon-induced symmetry lowering. This work demonstrates a fruitful connection between different high-level theoretical and experimental methods for exploring the characteristics of advanced materials.

cond-mat.mtrl-sci↗

arXiv2022

First and Second Order Raman Spectroscopy of Monoclinic $β-\mathrm{Ga}_2\mathrm{O}_{3}$

We employ a combined experimental-theoretical study of the first- and second-order Raman modes of monoclinic $β$-Ga$_{2}$O$_{3}$. The investigated materials is of particular interest due to its deep-UV bandgap paired with a high critical field strength, offering promising applications in power-electronics. A crucial prerequisite for the future development of Ga$_{2}$O$_{3}$-based devices is a detailed understanding of the lattice dynamics as they are important for the elasticity (through acoustic phonons), thermal conductivity (through the heat transferred by phonons), the temperature-dependence of the bandgap (impacted by electron-phonon coupling) or the free carrier transport (via phonon scattering). Polarized micro-Raman spectroscopy measurements on the (010) and ($\bar{2}01$) planes enable the determination of the phonon frequencies of all 15 first-order and more than 40 second-order Raman modes. The experimental results are correlated with calculations of the mode frequencies, phonon dispersion relation and phonon density of states using density functional perturbation theory (DFPT). By applying a group-theoretical analysis, we are able to distinguish between overtones and combinational modes and identify the high symmetry points in the Brillouin zone which contribute to the observed second order modes. Based on these information, we demonstrate the simultaneous determination of Raman-, IR-, and acoustic phonons in $β$-Ga$_{2}$O$_{3}$ by second-order Raman spectroscopy.

cond-mat.mtrl-sci↗

arXiv2021

Optical phonon modes, static and high frequency dielectric constants, and effective electron mass parameter in cubic In$_2$O$_3$

A complete set of all optical phonon modes predicted by symmetry for bixbyite structure indium oxide is reported here from a combination of far-infrared and infrared spectroscopic ellipsometry, as well as first principle calculations. Dielectric function spectra measured on high quality, marginally electrically conductive melt grown single bulk crystals are obtained on a wavelength-by-wavelength (a.k.a. point-by-point) basis and by numerical reduction of a subtle free charge carrier Drude model contribution. A four-parameter semi-quantum model is applied to determine all sixteen pairs of infrared-active transverse and longitudinal optical phonon modes, including the high-frequency dielectric constant, $\varepsilon_{\infty}=4.05\pm 0.05$. The Lyddane-Sachs-Teller relation then gives access to the static dielectric constant, $\varepsilon_{\mathrm{DC}}=10.55\pm 0.07$. All experimental results are in excellent agreement with our density functional theory calculations and with previously reported values, where existent. We also perform optical Hall effect measurements and determine for the unintentionally doped $n$-type sample a free electron density of $n=(2.81 \pm 0.01)\times 10^{17}$~cm$^{-3}$, mobility of $μ=(112 \pm 3)$~cm$^{2}$/(Vs), and an effective mass parameter of $(0.208\pm0.006)m_e$. Density and mobility parameters compare very well with results of electrical Hall effect measurements. Our effective mass parameter, which is measured independently of any other experimental technique, represents the bottom curvature of the $Γ$ point in In$_2$O$_3$ in agreement with previous extrapolations. We use terahertz spectroscopic ellipsometry to measure the quasi-static response of In$_2$O$_3$, and our model validates the static dielectric constant obtained from the Lyddane-Sachs-Teller relation.

cond-mat.mtrl-sci↗

arXiv2021

Fingerprints of optical absorption in the perovskite LaInO$_{3}$: Insight from many-body theory and experiment

We provide a combined theoretical and experimental study of the electronic structure and the optical absorption edge of the orthorhombic perovskite LaInO$_{3}$. Employing density-functional theory and many-body perturbation theory, we predict a direct electronic quasiparticle band gap of about 5 eV and an effective electron (hole) mass of 0.31 (0.48) m$_{0}$. We find the lowest-energy excitation at 0.2 eV below the fundamental gap, reflecting a sizeable electron-hole attraction. Since the transition from the valence band maximum (VBM, $Γ$ point) is, however, dipole forbidden the onset is characterized by weak excitations from transitions around it. The first intense excitation appears about 0.32 eV above. Interestingly, this value coincides with an experimental value obtained by ellipsometry (4.80 eV) which is higher than the onset from optical absorption spectroscopy (4.35 eV). The latter discrepancy is attributed to the fact that the weak transitions that define the optical gap are not resolved by the ellipsometry measurement. The absorption edge shows a strong dependency on the light polarization, reflecting the character of the involved valence states. Temperature-dependent measurements show a redshift of the optical gap by about 120 meV by increasing the temperature from 5 to 300 K. Renormalization due to zero-point vibrations is extrapolated from the latter measurement to amount to 150 meV. By adding the excitonic binding energy of 0.2 eV obtained theoretically to the experimental optical absorption onset, we determine the fundamental band gap at room temperature to be 4.55 eV.

cond-mat.mtrl-sci↗

arXiv2021

Thermal conductivity of bulk In$_{2}$O$_{3}$ single crystals

The transparent semiconductor In$_{2}$O$_{3}$ is a technologically important material. It combines optical transparency in the visible frequency range and sizeable electric conductivity. We present a study of thermal conductivity of In$_{2}$O$_{3}$ crystals and find that around 20 K, it peaks to a value as high as 5,000 WK$^{-1}$m$^{-1}$, comparable to the peak thermal conductivity in silicon and exceeded only by a handful of insulators. The amplitude of the peak drastically decreases in presence of a type of disorder, which does not simply correlate with the density of mobile electrons. Annealing enhances the ceiling of the phonon mean free path. Samples with the highest thermal conductivity are those annealed in the presence of hydrogen. Above 100 K, thermal conductivity becomes sample independent. In this intrinsic regime, dominated by phonon-phonon scattering, the magnitude of thermal diffusivity, $D$ becomes comparable to many other oxides, and its temperature dependence evolves towards $T^{-1}$. The ratio of $D$ to the square of sound velocity yields a scattering time which obeys the expected scaling with the Planckian time.

cond-mat.mtrl-sci↗

arXiv2020

The anisotropic quasi-static permittivity of single-crystal beta-Ga2O3

The quasi-static anisotropic permittivity parameters of electrically insulating gallium oxide (beta-Ga2O3) were determined by terahertz spectroscopy. Polarization-resolved frequency domain spectroscopy in the spectral range from 200 GHz to 1 THz was carried out on bulk crystals along different orientations. Principal directions for permittivity were determined along crystallographic axes c, and b, and reciprocal lattice direction a*. No significant frequency dispersion in the real part of dielectric permittivity was observed in the measured spectral range. Our results are in excellent agreement with recent radio-frequency capacitance measurements as well as with extrapolations from recent infrared measurements of phonon mode and high frequency contributions, and close the knowledge gap for these parameters in the terahertz spectral range. Our results are important for applications of beta-Ga2O3 in high-frequency electronic devices

cond-mat.mtrl-sci↗

arXiv2020

Offcut-related step-flow and growth rate enhancement during (100) $β$-Ga2O3 homoepitaxy by metal-exchange catalyzed molecular beam epitaxy (MEXCAT-MBE)

In this work we investigate the growth of $β$-Ga2O3 homoepitaxial layers on top of (100) oriented substrates via indium-assisted metal exchange catalyzed molecular beam epitaxy (MEXCAT-MBE) which have exhibited prohibitively low growth rates by non-catalyzed MBE in the past. We demonstrate that the proper tuning of the MEXCAT growth parameters and the choice of a proper substrate offcut allow for the deposition of thin films with high structural quality via step-flow growth mechanism at relatively high growth rates for $β$-Ga2O3 homoepitaxy (i.e., around 1.5 nm/min, $\approx$45% incorporation of the incoming Ga flux), making MBE growth on this orientation feasible. Moreover, through the employment of the investigated four different (100) substrate offcuts along the [00-1] direction (i.e., 0$^\circ$, 2$^\circ$, 4$^\circ$, 6$^\circ$) we give experimental evidence on the fundamental role of the (-201) step edges as nucleation sites for growth of (100)-oriented Ga2O3 films by MBE.

cond-mat.mtrl-sci↗

arXiv2020

Charge carrier density, mobility and Seebeck coefficient of melt-grown bulk ZnGa2O4 single crystals

The temperature dependence of the charge carrier density, mobility and Seebeck coefficient of melt-grown, bulk ZnGa2O4 single crystals was measured between 10 K and 310 K. The electrical conductivity at room temperature is about s = 286 S/cm due to a high electron concentration of n = 3.26*10^(19) cm^(-3), caused by unintenional doping. The mobility at room temperature is mu = 55 cm^2/Vs, whereas the scattering on ionized impurities limits the mobility to mu =62 cm^2/Vs for temperatures lower than 180 K. The Seebeck coefficient relative to aluminum at room temperature is S_(ZnGa2O4-Al) = (-125+-2) muV/K and shows a temperature dependence as expected for degenerate semiconductors. At low temperatures, around 60 K we observed a maximum of the Seebeck coefficient due to the phonon drag effect.

cond-mat.mtrl-sci↗

arXiv2019

Signatures of free carriers in Raman spectra of cubic In$_2$O$_3$

We discuss the influence of free carriers on the Raman scattering in $n$-type In$_2$O$_3$. For high-quality cubic single crystals, electronic single-particle excitations are revealed as a relatively broad Raman feature in the frequency range below 300~cm$^{-1}$. Furthermore, discrete phonon lines in the same frequency range exhibit asymmetric lineshapes characteristic for Fano resonances. The two observed spectral features contain the potential to be utilized for the quantitative determination of the free carrier concentration in $n$-type In$_2$O$_3$ using Raman spectroscopy as a contactless experimental technique.

cond-mat.mtrl-sci↗

arXiv2019

Vectorial observation of the spin Seebeck effect in epitaxial NiFe$_2$O$_4$ thin films with various magnetic anisotropy contributions

We have developed a vectorial type of measurement for the spin Seebeck effect (SSE) in epitaxial NiFe$_2$O$_4$ thin films which have been grown by pulsed laser deposition on MgGa$_2$O$_4$ (MGO) with (001) and (011) orientation as well as CoGa$_2$O$_4$ (011) (CGO), thus varying the lattice mismatch and crystal orientation. We confirm that a large lattice mismatch leads to strain anisotropy in addition to the magnetocrystalline anisotropy in the thin films using vibrating sample magnetometry and ferromagnetic resonance measurements. Moreover, we show that the existence of a magnetic strain anisotropy in NiFe$_2$O$_4$ thin films significantly impacts the shape and magnitude of the magnetic-field-dependent SSE voltage loops. We further demonstrate that bidirectional field-dependent SSE voltage curves can be utilized to reveal the complete magnetization reversal process, which establishes a vectorial magnetometry technique based on a spin caloric effect.

cond-mat.mes-hall↗

arXiv2019

The electrical conductivity tensor of $β$-Ga2O3 analyzed by van der Pauw measurements: Inherent anisotropy, off-diagonal element, and the impact of grain boundaries

The semiconducting oxide $β$-Gallium Oxide ($β$-Ga$_{2}$O$_{3}$) possesses a monoclinic unit cell whose low symmetry generally leads to anisotropic physical properties. For example, its electrical conductivity is generally described by a polar symmetrical tensor of second rank consisting of four independent components. Using van der Pauw measurements in a well-defined square geometry on differently-oriented high-quality bulk samples and the comparison to finite element simulations we precisely determine the ratio of all elements of the $β$-Ga$_{2}$O$_{3}$ 3-dimensional electrical conductivity tensor. Despite the structural anisotropy a nearly isotropic conductivity at and above room temperature was found with the principal conductivities deviating from each other by less than 6% and the off-diagonal element being $\approx3$% of the diagonal ones. Analysis of the temperature dependence of the anisotropy and mobility of differently doped samples allows us to compare the anisotropy for dominant phonon-scattering to that for dominant ionized-impurity scattering. For both scattering mechanisms, the conductivites along the $a$ and $b$-direction agree within 2%. In contrast, the conductivity along $c$-direction amounts to $0.96\times$ and up to $1.12\times$ that along the $b$-direction for phonon and ionized impurity scattering, respectively. The determined transport anisotropies are larger than the theoretically predicted effective mass anisotropy, suggesting slightly anisotropic scattering mechanisms. We demonstrate that significantly higher anisotropies can be caused by oriented extended structural defects in the form of low-angle grain boundaries for which we determined energy barriers of multiple 10 meV.

physics.app-ph↗

arXiv2018

Temperature dependence of the Seebeck coefficient of epitaxial $β$-Ga$_2$O$_3$ thin films

The temperature dependence of the Seebeck coefficient of homoepitaxial metal organic vapor phase (MOVPE) grown, silicon doped $β$-Ga$_2$O$_3$ thin films was measured relative to aluminum. For room temperature we found the relative Seebeck coefficient of $S_{β\text{-Ga}_2\text{O}_3\text{-Al}}=(-300\pm20)\;μ$V/K. At high bath temperatures $T>240$ K, the scattering is determined by electron-phonon-interaction. At lower bath temperatures between $T=100$ K and $T=300$ K, an increase in the magnitude of the Seebeck coefficient is explained in the frame of Strattons formula. The influence of the different scattering mechanisms on the magnitude of the Seebeck coefficient is discussed and compared with Hall measurement results.

cond-mat.mtrl-sci↗