arXiv subjects

Katarzyna Gas

Publications and source records attributed to Katarzyna Gas.

At least 19 recordsLinked to original sources

arXiv2026

Local magnetic correlations and light-sensitive centers in the Cr2AlC MAX phase

Cr2AlC MAX phase is synthesized by high-pressure solid-state annealing and investigated as a candidate platform for optically responsive magnetism. Structural characterization confirms the formation of the Cr2AlC phase, while magnetic and optical-magnetic properties are examined by superconducting quantum interference device (SQUID) magnetometry, electron spin resonance (ESR), and first principles calculations. SQUID magnetometry identifies Cr 2AlC as a weak, field-linear metallic paramagnet dominated by Pauli-like susceptibility of itinerant Cr-derived states. Its non-monotonic temperature dependence is described by an additional contribution from antiferromagnetically coupled Cr-Cr dimers, whereas the low-temperature Curie-like upturn originates from only a trace population of localized Cr centers. Under red-light illumination, SQUID magnetometry does not reveal an intrinsic macroscopic optomagnetic response. In contrast, ESR at 4 K shows a reversible light-induced reduction of a local magnetic signal, but the optically modified spin population corresponds only to several tens of ppm of the Cr sublattice. Ab initio Bethe-Salpeter equation (ai-BSE) calculations combined with the maximally localized Wannier function analysis suggest that optical excitation can redistribute spin polarization between neighboring Cr sites with the opposite local moments. The combined experiment-theory approach therefore establishes the hierarchy of magnetic contributions in Cr 2AlC and identifies the microscopic origin of its local optical sensitivity. This provides a reference for designing MAX phases and related MXenes in which defects, surface terminations or reduced dimensionality may enhance optically active magnetic states.

cond-mat.mtrl-sci↗

arXiv2026

Phase-Controlled Epitaxy and Anisotropic Antiferromagnetism of Polar Wurtzite MnTe

Altermagnetic spintronics requires materials in which compensated magnetic order, symmetry-controlled electronic responses, and epitaxial tunability can be combined in experimentally accessible thin films. MnTe is a key material in this context, but experimental studies have focused mainly on the stable NiAs-type polymorph, whereas the polar wurtzite phase remains largely unexplored. Here we demonstrate molecular-beam epitaxy growth and investigate properties of nearly phase-pure wurtzite MnTe deposited directly on GaAs(111)B, and show that small changes in the growth conditions strongly modify the phase composition, from a multiphase state with endotaxial NiAs-type inclusions embedded in wurtzite MnTe matrix to an almost single-phase polar wurtzite layer.

cond-mat.mtrl-sci↗

arXiv2025

Self-organization mechanism in Bridgman-grown MnBi2Te4/(Bi2Te3)n: influence on layer sequence and magnetic properties

The growth of high-quality magnetic topological insulator crystals by the Bridgman method remains challenging due to thermodynamic limitations inherent to this technique. Nevertheless, this approach continues to provide bulk materials with significantly reduced free carrier concentrations compared to epitaxial methods. Here, we investigate the Inverted Vertical Bridgman growth of MnBi2Te4/(Bi2Te3)n crystals, with particular emphasis on the structural ordering of MnBi2Te4 septuple layers within the Bi2Te3 quintuple-layer matrix and its influence on magnetic properties. Through a detailed analysis of growth dynamics, we identify four distinct stages, including a turbulent flow regime promoting pure MnBi2Te4 phase, rapid MnTe precipitation reducing Mn content in the melt, a stationary growth phase supporting ordered stacking of septuple and quintuple layers, and a final stage marked by flow cessation and defect formation. We demonstrate that septuple layer spacing is inversely correlated with MnTe supersaturation due to the diffusion-limited incorporation in stationary growth phase. Magnetic characterization reveals antiferromagnetic ordering in pure MnBi2Te4 phase and in MnBi2Te4/(Bi2Te3)n heterostructure, with ferromagnetism emerging for wider septuple layer spacing. We determine critical temperatures for observed antiferromagnetic and ferromagnetic phase transitions and magnetic anisotropy constants for ferromagnetic samples. Our findings highlight key growth parameters governing magnetic and structural quality, offering a pathway to scalable synthesis of layered topological insulators with tunable magnetic properties.

cond-mat.mtrl-sci↗

arXiv2025

Reliable Magnetometry for Antiferromagnets and Thin Films: Correcting Substrate Artifacts in Mn3Sn/MgO Systems

The rapid progress in antiferromagnetic and altermagnetic spintronics has led to increased interest in magnetic materials with vanishing net magnetization but strong spin-dependent transport properties. As thin films of such materials become central to device concepts, precise magnetic characterization is essential, for quantify intrinsic moments, and interpret transport signatures such as the anomalous Hall effect. In this work, we show that commercial MgO substrates, commonly used in epitaxial growth, often produce substantial parasitic magnetic signals that can match or exceed the response of weakly magnetic films. We identify two major components: a low-field ferromagnetic-like contribution originating from epi-ready surface, and a temperature-dependent paramagnetic background associated with dilute bulk impurities. These artifacts vary between samples and cannot be corrected using standard linear background subtraction or a measured reference substrate. To address this, we develop and put forward a compensation scheme which combines two complementary, non-destructive measurement protocols. We demonstrate up to 97% efficacy without requiring prior measurements of the bare substrate. The proposed framework enables reliable extraction of intrinsic magnetic signals and provides a general strategy for high-fidelity magnetometry in weakly magnetic thin-film systems, including emerging classes of materials such as topological phases and two-dimensional magnets. We also report the diamagnetic susceptibility of crystalline MgO, chi_MgO = -4.0 x 10^-7 emu/g/Oe.

cond-mat.mtrl-sci↗

arXiv2024

Spin Hall magnetoresistance in Pt/(Ga,Mn)N devices

Diluted magnetic semiconductors (DMS) have attracted significant attention for their potential in spintronic applications. Particularly, magnetically-doped GaN is highly attractive due to its high relevance for the CMOS industry and the possibility of developing advanced spintronic devices which are fully compatible with the current industrial procedures. Despite this interest, there remains a need to investigate the spintronic parameters that characterize interfaces within these systems. Here, we perform spin Hall magnetoresistance (SMR) measurements to evaluate the spin transfer at a Pt/(Ga,Mn)N interface. We determine the transparency of the interface through the estimation of the real part of the spin mixing conductance finding $G_r = 2.6\times 10^{14} \, Ω^{-1} m^{-2}$, comparable to state-of-the-art yttrium iron garnet (YIG)/Pt interfaces. Moreover, the magnetic ordering probed by SMR above the (Ga,Mn)N Curie temperature TC provides a broader temperature range for the efficient generation and detection of spin currents, relaxing the conditions for this material to be applied in new spintronic devices.

cond-mat.mes-hall↗

arXiv2024

Coexistence of Antiferromagnetic Cubic and Ferromagnetic Tetragonal Polymorphs in Epitaxial CuMnSb

High-resolution transmission electron microscopy and superconducting quantum interference device magnetometry shows that epitaxial CuMnSb films exhibit a coexistence of two magnetic phases, coherently intertwined in nanometric scales. The dominant $α$~phase is half-Heusler cubic antiferromagnet with the Néel temperature of 62~K, the equilibrium structure of bulk CuMnSb. The secondary phase is its ferromagnetic tetragonal $β$ polymorph with the Curie temperature of about 100~K. First principles calculations provide a consistent interpretation of experiment, since (i) total energy of $β$--CuMnSb is higher than that of $α$--CuMnSb only by 0.12~eV per formula unit, which allows for epitaxial stabilization of this phase, (ii) the metallic character of $β$--CuMnSb favors the Ruderman-Kittel-Kasuya-Yoshida ferromagnetic coupling, and (iii) the calculated effective Curie-Weiss magnetic moment of Mn ions in both phases is about $5.5~μ_\mathrm{B}$, favorably close to the measured value. Calculated properties of all point native defects indicate that the most likely to occur are $\mathrm{Mn}_\mathrm{Cu}$ antisites. They affect magnetic properties of epilayers, but they cannot induce the ferromagnetic order in CuMnSb. Combined, the findings highlight a practical route towards fabrication of functional materials in which coexisting polymorphs provide complementing functionalities in one host.

cond-mat.mtrl-sci↗

arXiv2023

Wurtzite vs rock-salt MnSe epitaxy: electronic and altermagnetic properties

Newly discovered altermagnets are magnetic materials exhibiting both compensated magnetic order, similar to antiferromagnets, and simultaneous non-relativistic spin-splitting of the bands, akin to ferromagnets. This characteristic arises from the specific symmetry operations that connect the spin sublattices. In this report, we show with ab initio calculations that the semiconductive MnSe exhibits altermagnetic spin-splitting in the wurtzite phase as well as a critical temperature well above room temperature. It is the first material from such space group identified to possess altermagnetic properties. Furthermore, we demonstrate experimentally through structural characterization techniques that it is possible to obtain thin films of both the intriguing wurtzite phase of MnSe and the more common rock-salt MnSe using molecular beam epitaxy on GaAs substrates. The choice of buffer layers plays a crucial role in determining the resulting phase and consequently extends the array of materials available for the physics of altermagnetism.

cond-mat.mtrl-sci↗

arXiv2022

A Simplified Method of the Assessment of Magnetic Anisotropy of Commonly Used Sapphire Substrates in SQUID Magnetometers

Solid state wafers are indispensable components in material science as substrates for epitaxial homo- or hetero-structures or carriers for two-dimensional materials. However, a reliable determination of magnetic properties of nanomaterials in volume magnetometry is frequently affected by unexpectedly rich magnetism of these substrates, including significant magnetic anisotropy. Here, we describe a simplified experimental routine of magnetic anisotropy assessment, which we exemplify and validate for epi-ready sapphire wafers from various sources. Both the strength and the sign of magnetic anisotropy is obtained from carefully designed temperature dependent measurements, which mitigate all known pitfalls of volume SQUID magnetometry and are substantially faster than traditional approaches. Our measurements indicate that in all the samples two types of net paramagnetic contributions coexists with diamagnetism. The first one can be as strong as 10% of the base diamagnetism of sapphire [-3.7(1) x 10-7 emu/gOe], and, when exceeds 2% mark, it exhibits pronounced magnetic anisotropy with the easy axis oriented perpendicularly to the face of c-plane wafers. The other is much weaker but exhibit ferromagnetic-like appearance. These findings form an important message that non-standard magnetism of common substrates can significantly influence the results of precise magnetometry of nanoscale materials and its existence must be taken for granted by both industry and academia.

cond-mat.mtrl-sci↗

arXiv2022

In Situ Compensation Method for Precise Integral SQUID Magnetometry of Miniscule Biological, Chemical, and Powder Specimens Requiring the Use of Capsules

Steadily growing interest in magnetic characterization of organic compounds for therapeutic purposes or of other irregularly shaped specimens calls for refinements of experimental methodology to satisfy experimental challenges. Encapsulation in capsules remains the method of choice, but its applicability in precise magnetometry is limited. This is particularly true for minute specimens in the single milligram range as they are outweighed by the capsules and are subject to large alignment errors. We present here a completely new experimental methodology that permits 30-fold in situ reduction of the signal of capsules by substantially restoring the symmetry of the sample holder that is otherwise broken by the presence of the capsule. In practical terms it means that the standard 30 mg capsule is seen by the magnetometer as approximately a 1 mg object, effectively opening the window for precise magnetometry of single milligram specimens. The method is shown to work down to 1.8 K and in the whole range of the magnetic fields. The method is demonstrated and validated using the reciprocal space option of MPMS-SQUID magnetometers; however, it can be easily incorporated in any magnetometer that can accommodate straw sample holders (i.e., the VSM-SQUID). Importantly, the improved sensitivity is accomplished relying only on the standard accessories and data reduction method provided by the SQUID manufacturer, eliminating the need for elaborate raw data manipulations.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic Constitution of Topologically Trivial Thermoelectric PbTe:Cr

In this paper we report on detailed temperature and magnetic field dependence of m agnetization of IV-VI semiconductor PbTe doped with mixed valence transition metal Cr$^{2+/3+}$. The material is studied solely by an integral superconducting quantum interference device magnetometer in order to quantitatively determine the contribution of single substitutional Cr$^{3+}$ as well as of various Cr-Te magnetic nanocrystals, including their identification. The applied experimental procedure reveals the presence of about $10^{19}$~cm$^{-3}$ paramagnetic Cr$^{3+}$ ions formed via self-ionization of Cr$^{2+}$ resonant donors. These are known to improve the thermoelectric figure of merit parameter zT of this semiconductor. The magnetic finding excellently agrees with previous Hall effect studies thus providing a new experimental support for the proposed electronic structure model of PbTe:Cr system with resonant Cr$^{2+/3+}$ state located (at low temperatures) about 100 meV above the bottom of the conduction band. Below room temperature a ferromagnetic-like signal points to the presence of Cr-rich nanocrystalline precipitates. Two most likely candidates, namely: Cr$_2$Te$_3$ and Cr$_5$Te$_8$ are identified upon dedicated temperature cycling of the sample at the remnant state. As an ensemble, the nanocrystals exhibits (blocked) superparamagnetic properties. The magnetic susceptibility of both n- and p-type PbTe in the temperature range $100 < T < 400$~K has been established. These magnitudes are essential in proper accounting for the high temperature magnetic susceptibility of PbTe:Cr.

cond-mat.mtrl-sci↗

arXiv2021

Anomalous Hall Effect in Bismuth

We report the occurrence of ferromagnetic-like anomalous Hall effect (AHE) below $30$ mT in bismuth single and policrystals. The signatures of ferromagnetism in transport are not corroborated in magnetization measurements, thus suggesting the induction of non-intrinsic magnetism at surfaces and grain boundaries in bismuth. The suppression of the AHE with the increase of magnetic field and temperature coincides with previous reports of superconductivity in Bi, suggesting an interplay between the two phenomena.

cond-mat.str-el↗

arXiv2021

Improved-Sensitivity Integral SQUID Magnetometry of (Ga,Mn)N Thin Films in Proximity to Mg-doped GaN

Nominally 45 nm GaN:Mg/ 5 nm (Ga,Mn)N / 45 nm GaN:Mg trilayers structures prepared by molecular beam epitaxy on GaN-buffered Al2O3 substrates are investigated to verify whether the indirect co-doping by holes from the cladding layers can alter the spin-spin interaction in (Ga,Mn)N. The four investigated structures, differing with the Mg doping level, are carefully characterized at the nanoscale by HRTEM, EDX, and by SIMS. HRTEM decisively excluded a presence of foreign Mn-rich phases. The structures, up to medium Mg doping, show no Mg over-doping effects. Magnetic studies of these structures are aided by the employment of a dedicated experimental approach of the in situ compensation of the magnetic contribution from the substrate, allowing up to about fifty-fold reduction of this contribution. This technique, dedicated to these structures, simultaneously provides a tenfold reduction of temporal instabilities of the magnetometric unit and lowers the experimental jitter to merely $5 \times 10^{-7}$~emu at 70~kOe, vastly increasing the precision and the credibility of the results of the standard integral SQUID magnetometry in high magnetic fields. The magnetic characteristics of the trilayers structures established here prove identical with the already known properties of the thick (Ga,Mn)N single layers, namely (i) the low temperature ferromagnetism among Mn$^{3+}$ ions driven by superexchange and (ii) purely paramagnetic response at higher temperatures. The possible cause of the lack of any effects brought about by the adjacent Mg-doping is a presence of residual Mn in the cladding layers, resulting in the deactivation of the p-type doping intended there. This finding points out that a more intensive technological effort has to be exerted to promote the co-doping-driven carrier-mediated ferromagnetic coupling in Mn-enriched GaN, especially at elevated temperatures.

cond-mat.mtrl-sci↗

arXiv2021

Magnetic properties of wurtzite (Ga,Mn)As

Here we report on detailed studies of the magnetic properties of the wurtzite (Ga,Mn)As cylindrical shells. Ga$_{0.94}$Mn$_{0.06}$As shells have been grown by molecular beam epitaxy at low temperature as a part of multishell cylinders overgrown on wurtzite (Ga,In)As nanowires cores, synthesized on GaAs (111)B substrates. Our studies clearly indicate the presence of a low temperature ferromagnetic coupling, which despite a reasonably high Mn contents of 6\% is limited only to below 30~K. A set of dedicated measurements shows that despite a high structural quality of the material the magnetic order has a granular form, which gives rise to the dynamical slow-down characteristic to blocked superparamagnets. The lack of the long range order has been assigned to a very low hole density, caused primarily by numerous compensation donors, arsenic antisites, formed in the material due to a specific geometry of the growth of the shells on the nanowire template. The associated electrostatic disorder has formed a patchwork of spontaneously magnetized (macrospin) and nonmagnetic (paramagnetic) volumes in the material. Using high field results it has been evaluated that the total volume taken by the macrospins constitute about 2/3 of the volume of the (Ga,Mn)As whereas in the remaining 1/3 only paramagnetic Mn ions reside. By establishing the number of the uncoupled ions the two contributions were separated. The Arrott plot method applied to the superparamagnetic part yielded the first experimental assessment of the magnitude of the spin-spin coupling temperature within the macrospins in (Ga,Mn)As, $T_{\mathrm{C}}=28$~K. In a broader view our results constitute an important contribution to the still ongoing dispute on the true and the dominant form(s) of the magnetism in this model dilute ferromagnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2020

Molecular beam epitaxy of the half-Heusler antiferromagnet CuMnSb

We report growth of CuMnSb thin films by molecular beam epitaxy on InAs(001) substrates. The CuMnSb layers are compressively strained ($0.6~\text{%}$) due to lattice mismatch. The thin films have a $ω$ full width half max of $7.7^{''}$ according to high resolution X-ray diffraction, and a root mean square roughness of $0.14~\text{nm}$ as determined by atomic force microscopy. Magnetic and electrical properties are found to be consistent with reported values from bulk samples. We find a Néel temperature of $62~\text{K}$, a Curie-Weiss temperature of $-65~\text{K}$ and an effective moment of $5.9~μ_{\text{B}}/\text{f.u.}$. Transport measurements confirm the antiferromagetic transition and show a residual resistivity at $4~\text{K}$ of $35~μΩ\cdot \text{cm}$.

cond-mat.mtrl-sci↗

arXiv2019

Raman scattering studies of the lateral Mn distribution in MBE-grown Ga1-xMnxN epilayers

Recent interest in very thin single phase Ga1-xMnxN dilute magnetic layers increased needs for precise, non-destructive, and relatively fast characterization methods with key issues being the macroscopic lateral Mn distribution and the absolute values of Mn concentration x. We report on resonantly enhanced UV Raman scattering studies of high quality Ga1-xMnxN layers grown on GaN templated sapphire by molecular beam epitaxy with 4 < x < 9%. The main advantage of the UV excitation is the restriction of the light penetration depth to nearly a hundred nanometers, eliminating signal from the GaN buffer. Under this conditions we determine the dependence of the 1LO phonon frequency on x, what allows for a fine mapping of its lateral distribution over the entire surface of the samples. Our Raman scanning clearly confirms substantial lateral distribution of Mn atoms across the layer, which is radial with respect to its center. From the established distributions in two deliberately chosen layers the magnitude of the optimal growth temperature for most efficient Mn atoms incorporation in epitaxial GaN has been confirmed. It is shown that the combination of the 1LO line width and its energy provides assessment of the crystalline quality of the investigated layers.

cond-mat.mtrl-sci↗

arXiv2019

Enhanced ferromagnetism in cylindrically confined MnAs nanocrystals embedded in wurtzite GaAs nanowire shells

Nearly 30% increase of the ferromagnetic phase transition temperature has been achieved in strained MnAs nanocrystals embedded in a wurtzite GaAs matrix. Wurtzite GaAs exerts tensile stress on hexagonal MnAs nanocrystals, preventing a hexagonal to orthorhombic structural phase transition, which in the bulk MnAs is combined with the magnetic one. This effect results in a remarkable shift of the magneto-structural phase transition temperature from 313 K in the bulk MnAs to above 400 K in the tensely strained MnAs nanocrystals. This finding is corroborated by the state of the art transmission electron microscopy, sensitive magnetometry and the first-principles calculations. The effect relies in defining a nanotube geometry of molecular beam epitaxy grown core-multishell wurtzite (Ga,In)As/(Ga,Al)As/(Ga,Mn)As/GaAs nanowires where the MnAs nanocrystals are formed during the thermal-treatment-induced phase separation of wurtzite (Ga,Mn)As into the GaAs:MnAs granular system. Such a unique combination of two types of hexagonal lattices provides possibility of attaining quasi-hydrostatic tensile strain in MnAs (impossible otherwise), leading to the substantial ferromagnetic phase transition temperature increase in this compound.

cond-mat.mtrl-sci↗

arXiv2019

Electrical characteristics of vertical-geometry Schottky junction to magnetic insulator (Ga,Mn)N heteroepitaxially grown on sapphire

Schottky barrier height and the ideality factor $η$ are established for the first time in the single phase (Ga,Mn)N using a vertical geometry device. The material has been heteroepitaxially grown on commercially available low threading dislocation density GaN:Si template. The observed above 10M$Ω$ resistances already at room temperature are indicative that a nearly conductive-dislocation-free electrical properties are achieved. The analysis of temperature dependence of the forward bias I-V characteristics in the frame of the thermionic emission model yields Ti-(Ga,Mn)N Schottky barrier height to be slightly lower but close in character to other metal/GaN junctions. However, the large magnitudes of the ideality factor $η$>1.5 for T$\leqslant$300K, point to a sizable current blocking in the structure. While it remains to be seen whether it is due to the presence of (Ga,Mn)N barrier or due to other factors which reduce the effective area of the junction, an existence of a substantial serial resistance may hold the key to explain similar observations in other devices of a corresponding structure and technological relevance.

physics.app-ph↗

arXiv2019

In situ compensation method for high-precision and high-sensitivity integral magnetometry

An ongoing process of miniaturization of spintronics and magnetic-films-based devices, as well as a growing necessity for basic material research place stringent requirements for sensitive and accurate magnetometric measurements of minute magnetic constituencies deposited on large magnetically responsive carriers. However, the most popular multipurpose commercial superconducting quantum interference device (SQUID) magnetometers are not object-selective probes, so the sought signal is usually buried in the magnetic response of the carrier, contaminated by signals from the sample support, system instabilities and additionally degraded by an inadequate data reduction. In this report a comprehensive method based on the in situ magnetic compensation for mitigating all these weak elements of SQUID-based magnetometry is presented. Practical solutions and proper expressions to evaluate the final outcome of the investigations are given. Their universal form allows to employ the suggested design in investigations of a broad range of specimens of different sizes, shapes and compositions. The method does not require any extensive numerical modelling, it relies only on the data taken from the standard magnetometer output. The solution can be straightforwardly implemented in every field where magnetic investigations are of a prime importance, including in particular emerging new fields of topological insulators, 3D-Dirac semimetals and 2D-materials.

cond-mat.mtrl-sci↗