No description available.

<p dir="ltr"><b>Tellurium (Te) stands out as an elemental narrow-bandgap semiconductor characterized by its distinctive chiral crystal structure. The interplay between fundamental symmetries and the topological properties of electrons has garnered significant attention in the scientific community. With its unique chiral crystal structure featuring three Tellurium atoms spiraling within a single unit cell, Tellurium offers a singular material system. This system provides an exceptional opportunity to explore the novel quantum and topological transport properties of electrons. Hydrothermally grown two-dimensional (2D) Te with a thickness of several nanometers gives us an opportunity to precisely control the carrier density and the carrier type in Te using gate voltage. In this dissertation, the spin-orbit coupling (SOC) of Te is quantitatively analyzed using the weak anti-localization effect. The strong SOC also gives rise to the Weyl point at the band edge of the conduction band. The topological nontrivial band structure of Te is characterized by a π phase shift in the Shubnikov-de Haas (SdH) oscillations. Due to the high mobility, the quantum Hall effect is measured with low spin and valley Landau levels controlled by an electric and magnetic field. Bilayer charge transferable quantum Hall states of Weyl fermions is observed in a wide Te quantum well. The topological phase transition from a semiconductor to Weyl semimetal under high pressure is studied up to 2.47 GPa. The chirality of 2D Te is separated by the hot sulfuric acid etching technique. The spin configuration and topological charge of the Weyl node exhibit a reversal in different chiralities, leading to an inverse in nonlinear responses, encompassing both electrical (nonreciprocal transport in the longitudinal direction and nonlinear planar Hall effect in the transvers direction) and optical phenomena (circular photogalvanic effect and circular photovoltaic effect). Our results unveil the topological nature of the Tellurium (Te) band structures, offering a promising avenue for controlling charge and spin transport within the chiral degree of freedom.</b></p>

В работе был проведен комплекс исследований состава, структуры, магнитных свойств и гигантского магнитного импеданса (ГМИ) многослойных пленочных структур, необходимый для их использования как чувствительных элементов датчиков слабых магнитных полей. Структуры получались методом ионно-плазменного распыления. Исследовались и пленочные структуры в стандартной геометрии, и элементы в виде полосок в геометрии ГМИ элементов. ГМИ конфигурация представляет собой трехслойную пленку типа магнитный слой - проводящий немагнитный слой - магнитный слой при равной толщине каждого из слоев. Магнитный слой может быть заменен многослойной структурой. Для анализа особенностей структуры использовали электронную микроскопию и рентгено-фазовый анализ. Показано, что образцы имеют нанокристаллическую структуру, разделение слоев пермаллоя немагнитными прослойками влияет на особенности его тек-стуры и размер зерна. Точный состав пленок Fe19Ni81 был определен с помощью приставки энергодисперсионной рентгеновской спектроскопии и рентгенофлуоресцентного анализа. Магнитные свойства были исследованы с помощью магнито-оптического эффекта Керра и вибрационной магнитометрии. Несмотря на присутствие специально прикладываемого во время напыления технологического магнитного поля, формируемая магнитная анизотро-пия имела сложный характер. Так, ось магнитной анизотропии поверхностного слоя у большинства образцов располагалась перпендикулярно длинной стороне прямоугольного элемента (соосно с ориентацией технологического поля), а ось эффективной магнитной анизотропии ориентировалась вдоль длиной стороны элемента. Исследование ГМИ в диапазоне частот 1 - 400 МГц позволило отобрать элементы для тестирования прототипа биодатчика для определения положения магнитной вставки в виде феррогеля/эпоксидной смолы с магнитными наночастицами. Показана возможность детектирования полей рассеяния магнитных частиц с помощью используемого элемента и проведены оценки погрешности ГМИ откликов прототипа биодатчика. Созданы и опробованы в ряде биологических экспериментов не имеющие коммерческих аналогов магнитные матрицы на основе постоянных магнитов. Исследовано распределение магнитного поля магнитных систем: катушек Гельмгольца, коммерческого электромагнита, 24 и 48 луночных магнитных матриц. / In this work, composition, structural and magnetic properties, giant magneto impedance (GMI) of thin film multilayered structures for sensors of weak magnetic fields were investigated. Multilayered films were fabricated by magnetron sputtering technique. They were investigated both in the standard thin film geometry and in the GMI-stripe geometry. Magnetoimpedance stripe geometry is a three-layered film that consists of magnetic layer - conductive non-magnetic layer - magnetic layer with an equal thickness of each one of three layers. Magnetic layers can be substituted by the multilayered structures. Structure of multilayered films was studied by electronic microscopy and XRD analysis. All samples have a nanocrystalline structure, separation of permalloy layers by non-magnetic spacers influences the permalloy structure and grain size. Composition of permalloy in films Fe19Ni81 was checked by energy-dispersive x-ray spectroscopy and x-ray fluorescence spectros-copy. Magnetic properties were investigated by magneto-optical Kerr effect and vibrating sample magnetometer. Despite the presence of a certain magnetic field applied during the sputtering deposition, the formed magnetic anisotropy was quite complex. The magnetic anisotropy axis of surface layers most of the samples was perpendicular to the long side of the GMI-stripe ele-ments. However, the effective magnetic anisotropy axis of the whole structure was along to the long side of elements. Investigation of GMI in the frequency range 1 - 400 MHz allowed to select the GMI-stripe elements for biosensor prototype testing for detection of the magnetic test-samples such as ferrogels or epoxy composites with magnetic micro- or nanoparticles. The possibility of detection of the stray fields of magnetic micro- and nanoparticles by selected GMI element was confirmed, and the experimental errors were carefully analyzed. Unique magnetic matrices based on permanent magnets were designed, fabricated and tested in biological experiments. Magnetic field distribution in space was measured for Helmholtz coils, commercial elec-tromagnet, 24 and 48 well magnetic matrices.

<p dir="ltr">Since the discovery of graphene in 2004, numerous other materials with intriguing electronic, optical, and magnetic properties have been found to be layered and exfoliatable down to atomic thickness. Owing to their weak interlayer coupling, mediated only by van der Waals forces, this new class of 2-dimensional materials, also known as van der Waals (vdW) materials, allows layer-by-layer stacking, overcoming some of the limitations of growth techniques. In particular, the growing inventory of vdW materials has expanded to include magnetic materials, further broadening the possibilities of novel devices based on stacked heterostructures. These magnetic heterostructures can find applications in spintronics and memory devices and may be combined with other vdW materials with optical properties for applications in optoelectronics. In this thesis, we assembled heterostructures via mechanical transfer or growth to modify the magnetism in these vdW materials. We used various optical and electrical techniques to probe the modified magnetism or its effects on the novel heterostructure. Thus, we observed the emergence of the magnetic proximity effect on the topological insulator BiSbTeSe₂ after dry transferring a thin flake of Cr₂Ge₂Te₆ on top, taking steps towards the observation of novel topological phases, such as the quantum Hall insulator. Additionally, we demonstrated an increased Curie temperature and magnetic anisotropy, effectively enhancing the magnetism, in thin flakes of Cr₂Ge₂Te₆ and Cr₂Si₂Te₆ after sputtering NiO or MgO. Finally, noting that the effect of modified magnetism in Cr2Ge2Te6 after sputtering NiO or MgO is induced due to wrinkle formation and strain, we further reproduce similar wrinkle formation on other 2D materials such as hBN, graphite, and 2D antiferromagnets (XPS₃, (X= Mn, Fe, Ni), CrSBr, RuCl₃). We used polarized Raman spectroscopy to characterize the induced biaxial strain in hBN and showed that such wrinkle formation can lead to moderately (up to 1.4% strain) spatially inhomogeneous and anisotropic strain profiles. These efforts demonstrate the versatility of tailoring the properties of these vdW materials.</p>

<p dir="ltr">InSbAs is a promising platform for exploring topological superconductivity and spin-based device applications, thanks to its strong spin-orbit coupling (SOC) and high effective <i>g</i>-factor. This thesis investigates low-temperature transport of electrons confined in InSb_1-xAs_x quantum wells. Specifically, we study the properties of electrons confined in 2D and 0D by fabricating gated Hall bars and gate-defined quantum dots. Theoretical considerations suggest that InSbAs will have stronger SOC and a larger effective <i>g</i>-factor compared to InAs and InSb. Both the SOC and effective <i>g</i>-factor change as a function of arsenic mole fraction, but much remains to be understood in real material systems. Here, we study the dominant scattering mechanisms, effective mass, spin-orbit coupling strength, and the <i>g</i>-factor in InSb_1-xAs_x quantum wells grown by molecular beam epitaxy. </p><p dir="ltr">We explore 30 nm InSb_1-xAs_x quantum wells with arsenic mole fractions of <i>x</i> = 0.05, 0.13, and 0.19. The 2DEG properties were studied by fabricating gated Hall bars and placing them in a perpendicular magnetic field at low temperatures (T = 10 - 300 mK). All samples showed high-quality transport with mobility greater than 100,000 cm²/Vs. For the <i>x</i> = 0.05 sample, the 2DEG displays a peak mobility μ = 2.4 x 10⁵ cm²/Vs at a density of <i>n</i> = 2.5 x 10¹¹ cm^-². We investigated the evolution of mobility as a function of arsenic mole fraction and 2DEG density for all samples. As the arsenic mole fraction increases, peak mobility decreases, and the dependence of mobility on density becomes weaker, suggesting that short-range scattering becomes the dominant scattering mechanism. We extracted an alloy scattering rate of τ_alloy = 45 ns^-1 per % As, an important parameter for understanding the impact of disorder on induced superconductivity. The high mobility, strong spin-orbit coupling, and low effective mass in this material system resulted in a beating pattern in the Shubnikov de Haas oscillations, allowing for the extraction of the Rashba parameter as a function of density and arsenic mole fraction. We observed a gate tunable spin-orbit coupling and, as predicted by theory, an increase in spin-orbit coupling with increasing arsenic mole fraction. For the sample with x = 0.19, the highest Rashba parameter is α_R ~ 300 meVÅ, which is significantly higher than in InSb. </p><p dir="ltr">In addition, we explored 0D confinement by fabricating a gate-defined quantum dot in an InSb₀_.87As_0.13 quantum well. By studying the evolution of Coulomb blockade peaks and differential conductance peaks as a function of magnetic field, a nearly isotropic in-plane effective <i>g</i>-factor in the [1-10] and [110] crystallographic directions was extracted, ranging from 49-58. The values extracted are 1.8 times higher than in a quantum dot fabricated in pure InSb. Furthermore, this study produced the first demonstration of a tunable spin-orbit coupling in this material system. This was achieved by measuring the avoided crossing gap, mediated by spin-orbit coupling, between the ground state and excited state in a magnetic field. The avoided crossing gap indicates the strength of the spin-orbit coupling; the maximum energy separation extracted is Δ_SO ~100 μeV. </p><p dir="ltr">Our work should stimulate further investigation of InSbAs quantum wells as a promising platform for applications requiring strong spin-orbit coupling, such as topological superconductivity or spin-based devices.</p>

The spin-lattice relaxation rates are calculated for a rigid magnetic spin cluster in an elastic medium in the presence of a magnetic eld using the latticerame method. This rate is then compared with both the rate calculated using the quantum mechanical densitymatrix method and with the Glauber dynamics. These calculation results are used in the contribution of various heat baths, such as a phonon bath in various dimensions or a fermionic bath, to transition rates that enter into dynamic Monte Carlo simulations of molecular magnets and nanomagnets.

No description available.

<p>    </p> <p>Electrical steels are one type of soft magnetic material. They are based on Fe-Si alloys and are widely used for magnetic cores in transformers and electric motors. It is well known that Fe- 6.5Si wt% is the most efficient composition; however, at such a high silicon concentration (6.5wt.% = 12.1 at.% Si in Fe), the poor workability of the alloy makes it unacceptable for industrial production via conventional sheet steel rolling processes. This problem was approached in two different ways. First, a machining-based approach that suppresses the mechanisms that lead to cracking during conventional rolling was implemented for processing of thin metal strips. Two related machining-based sheet production technologies called free machining (FM), and hybrid cutting extrusion (HCE) were used to produce strips of high resistivity electrical steel. The maximum strip width achieved was 50 mm, and it was produced with a combination of FM and light rolling with a surface roughness comparable to cold-rolled sheet surfaces. Second, a new experimental alloy Fe-4Si-4Cr wt% was developed with improved magnetic properties compared to ~ Fe-3.2Si wt% and outstanding workability. Results report that the new experimental alloy has an electrical resistivity of 85 ± 3 𝜇Ω ∙ 𝑐𝑚 which is higher than Fe-6.5%Si. Also, the results on the Fe-4Si-4Cr workability show that this new alloy can withstand 75% cold-rolled reduction. The magnetic properties characterization was done via standard stacked toroid testing, and results show that Fe-4Si-4Cr experimental alloy exhibits excellent magnetic performance with a reduction in core losses of 33% at 400 Hz compared to commercial alloys with ~ Fe-3.2Si wt%. Recrystallization kinetics and texture evolution in the experimental alloy were evaluated for traditionally rolled and machining-based samples. Results were used to construct annealing maps. These maps represent the stages of the annealing process for a range of temperature versus time conditions, i.e., the annealing maps are a graphical summary showing the different stages of the annealing process for the Fe-4Si-4Cr experimental alloy in the two conditions. Despite the significant differences in the deformation texture of the two conditions, the recrystallization kinetics were similar. Finally, the two conditions retained the as-deformed texture in the intermediate annealing but to a lesser degree after completing a full anneal. In the case of the rolled sample, it is possible to trace the original texture fibers (γ-fiber, the partial α-fiber, and the θ -fiber) in the fully annealed data, but the texture intensity is just 2.5 mrd. On the other hand, the texture of the fully annealed HCE sample changes as compared to the as-deformed condition, located close to (110)[112] with a surprisingly strong peak of ~ 25 mrd. </p>

[pt] Ferritas do tipo espinélio têm atraído atenção devido a suas propriedades magnéticas com possibilidade de aplicações em dispositivos spintrônicos e de memória magnética, além de aplicações em catálise. Este trabalho tem como objetivo sintetizar o espinélio de alumínio e ferro (FeAl2O4) pelo método de síntese reação de combustão e avaliar suas características estruturais e morfológicas bem como suas propriedades magnéticas devido a sua ampla possibilidade de aplicações. A síntese deste material foi feita utilizando soluções concentradas dos nitratos metálicos hidratados de ferro e alumínio, variando o tipo e quantidade de combustível no intuito de verificar sua influência na microestrutura do produto. Os combustíveis utilizados neste trabalho foram ureia e ácido cítrico, ambos em quantidade estequiométrica, dita ideal, calculada a partir da teoria química dos propelentes, além de reações com quantidades acima e abaixo do ideal. Uma síntese apenas utilizando os nitratos metálicos, sem combustível, foi feita para comparação. O produto final das diferentes sínteses foram caracterizados em termos de estrutura e composição por meio de difração de raios X (DRX), Microscopia Eletrônica de Varredura (MEV) e Microscopia Eletrônica Transmissão (MET). As propriedades magnéticas foram obtidas por magnetometria de amostra vibrante e a identificação das fases de ferro por espectroscopia Mössbauer. As análises indicaram que a amostra sintetizada sem combustível corresponde a óxidos de ferro e de alumínio, majoritariamente amorfo, com formação de partículas nanométricas de hematita (Fe2O3) e um comportamento paramagnético. As caracterizações das amostras produzidas com ureia apresentam maior cristalinidade em relação a sintetizada sem combustível, com a amostra em quantidade estequiométrica de ureia, resultando em um produto monofásico identificado como o espinélio FeAl2O4, misto. Já as amostras com excesso e deficiência de ureia tiveram como produto, além da formação do espinélio desejado, a formação de outra fase identificada como magnetita (Fe3O4), sendo todas as amostras sintetizadas com ureia manifestando comportamento ferromagnético. As amostras sintetizadas com ácido cítrico, em todas as condições, apresentaram um sistema trifásico constituído pelo espinélio, magnetita e hematita e mostraram comportamento também ferromagnético. Neste trabalho é mostrado que a síntese por reação de combustão se mostrou eficiente para produzir o FeAl2O4, alcançando melhores resultados utilizando ureia como combustível, em quantidade ideal. / [en] Spinel-like ferrites has attracted attention due to its magnetic properties with the possibility of spintronic and magnetic memory devices applications as well as catalysis applications. This work aims to synthesize aluminum and iron spinel (FeAl2O4) by combustion reaction synthesis method and evaluate its structural and morphological characteristics, as well as its magnetic properties due to its wide application possibilities. The synthesis of this material was made using concentrated solutions of hydrated iron and aluminum metal nitrates, varying the type and quantity of fuel in order to verify its influence on the product microstructure. The fuels used in this work were urea and citric acid, both in stoichiometric quantity, said ideal, calculated from the chemical theory of propellants, and reactions with quantities above and below ideal. A synthesis using only metal nitrates without fuel was made for comparison. The final product of the different syntheses were characterized in terms of structure and composition by Xray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Magnetic properties were obtained by vibrating sample magnetometry and the iron phase identification by Mössbauer spectroscopy. The analysis indicated that the sample synthetized without fuel corresponds to iron and aluminum oxides, mostly amorphous, with formation of nanometer hematite particles (Fe2O3) and a paramagnetic behavior. The characterizations of the samples produced with urea show higher crystallinity than the synthesized without fuel, with the sample in stoichiometric amount of urea, resulting in a single phase product identified as FeAl2O4 mixed spinel. The samples with excess and deficiency of urea had as product, besides the formation of the desired spinel, the formation of another phase identified as magnetite (Fe3O4), being all the samples synthesized with urea manifesting ferromagnetic behavior. The samples synthesized with citric acid, under all conditions, presented a triphasic system consisting of spinel, magnetite and hematite and showed also ferromagnetic behavior. In this work it is shown that combustion reaction synthesis was efficient to produce FeAl2O4, achieving better results using urea as fuel, in ideal quantity.

The rare mineral cyanochroite, K2Cu(SO4)2·6H2O, features isolated Cu2+ ions in distorted octahedral coordination, linked via a hydrogen-bond network. We have grown single crystals of cyanochroite as large as ∼0.5 cm3 and investigated structural and magnetic aspects of this material. The positions of hydrogen atoms deviate significantly from those reported previously based on X-ray diffraction data, whereas the magnetic response is fully consistent with free Cu2+ spins. The structure is not changed by deuteration. Density functional theory calculations support our refined hydrogen positions.