Critical behavior of the van der Waals bonded ferromagnet Fe3−x_{3-x}GeTe2_2

The critical properties of the single-crystalline van der Waals bonded ferromagnet Fe$_{3-x}$GeTe$_2$ were investigated by bulk dc magnetization around the paramagnetic (PM) to ferromagnetic (FM) phase transition. The Fe$_{3-x}$GeTe$_2$ single crystals grown by self-flux method with Fe deficiency $x \approx 0.36$ exhibit bulk FM ordering below $T_c = 152$ K. The M\"{o}ssbauer spectroscopy was used to provide information on defects and local atomic environment in such crystals. Critical exponents $\beta = 0.372(4)$ with a critical temperature $T_c = 151.25(5)$ K and $\gamma = 1.265(15)$ with $T_c = 151.17(12)$ K are obtained by the Kouvel-Fisher method whereas $\delta = 4.50(1)$ is obtained by a critical isotherm analysis at $T_c = 151$ K. These critical exponents obey the Widom scaling relation $\delta = 1+\gamma/\beta$, indicating self-consistency of the obtained values. With these critical exponents the isotherm $M(H)$ curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation $m = f_\pm(h)$, where $m$ and $h$ are renormalized magnetization and field, respectively. The exponents determined in this study are close to those calculated from the results of the renormalization group approach for a heuristic model of three-dimensional Heisenberg ($d = 3, n = 3$) spins coupled with the attractive long-range interactions between spins that decay as $J(r)\approx r^{-(3+\sigma)}$ with $\sigma=1.89$.Comment: arXiv admin note: substantial text overlap with arXiv:1706.07324, arXiv:1803.0448

Critical current density and vortex pinning in tetragonal FeS1−x_{1-x}Sex_{x} (x=0,0.06x=0,0.06)

We report critical current density ($J_c$) in tetragonal FeS single crystals, similar to iron based superconductors with much higher superconducting critical temperatures ($T_{c}$'s). The $J_c$ is enhanced 3 times by 6\% Se doping. We observe scaling of the normalized vortex pinning force as a function of reduced field at all temperatures. Vortex pinning in FeS and FeS$_{0.94}$Se$_{0.06}$ shows contribution of core-normal surface-like pinning. Reduced temperature dependence of $J_c$ indicates that dominant interaction of vortex cores and pinning centers is via scattering of charge carriers with reduced mean free path ($\delta$$l$), in contrast to K$_x$Fe$_{2-y}$Se$_2$ where spatial variations in $T_{c}$ ($\delta$$T_{c}$) prevails.Comment: 5 pages, 4 figure

Investigation of the structure and hyperfine interactions in the intermetallic y'-Ni3Al phase doped with hafnium and iron

У овом раду истраживана су поликристална интерметална једињења γ′–Ni3Al са примесама атома хафнијума и γ′–Ni3Al са примесама атома гвожђа. Испитивања локалне структуре, магнетних и електричних особина γ′–Ni3Al једињења допираних атомима Hf или атомима Fe извршена су методама дифракције Х–зрачења, мерењима магнетизације, као и применом временски разложених пертурбованих угаоних корелација и спектроскопије Mössbauer–овог ефекта, које су омогућиле истраживања хиперфиних интеракција на језгрима примеса Hf и на језгрима примеса Fe. Ова експериментална истраживања испитиваних система су употпуњена прорачунима енергије дефеката, као и параметара структуре и хиперфиних интеракција на основу првих принципа. Главни циљ експерименталних истраживања и теоријских прорачуна интерметалне γ′– Ni3Al фазе са примесама атома хафнијума, односно гвожђа био је да се одреди преферентно позиционирање атома Hf, односно атома Fe у γ′–Ni3Al решетки, као и њихов утицај на кристалну решетку у коју се уграђују. Aнализoм дифракције Х–зрака испитиваних Hf допираних Ni3Al узорака утврђено је присуство кубне γ′ фазе (ознака L12) у испитиваним Hf–Ni3Al легурама, као и њене две тетрагоналне трансформације L60 и D022. Анализа је показала да је кубна L12 фаза доминантна у односу на тетрагоналне фазе испитиваних Hf–Ni3Al узорака. Мерења хиперфиних интеракција на проби 181Та, језгру–потомку 181Hf, у легурама 0.2, 0.5 и 5 at.% Hf – Ni3Al извршена су у температурском опсегу од 78 K до 1230 K. Измерене вредности хиперфиних параметара приписане су одређеним позицијама атома Hf у присутним Ni3Al фазама: L12, D022 и L60. Присуствo 181Ta–сигнала који је константан у времену кореспондира атомима Hf на позицији Al атома у доминантној L12 фази. За све три испитиване легуре, измерена нижа вредност спин независне електричне iii квадруполне фреквенције додељена је електричној квадруполној интеракцији која дејствује на атоме Hf који су заменили атоме Al у тетрагоналној D022 фази. Измерена виша вредност фреквенције кореспондира атомима Hf на позицијама атома Al у тетрагоналној L60 фази. Резултати прорачуна параметара хиперфиних интеракција, електронских и структурних особина Hf–Ni3Al легура из првих принципа изведених методом проширених равних таласа са локалним орбиталама коришћењем WIEN2k (“An Augmented Plane Wave Plus Local Orbitals Program”) кода коректно репродукују вредности добијене у експерименталним мерењима. Прорачуни енергије дефеката базирани на теорији функционала густине изведени су VASP (“the Vienna ab–initio simulation package”) кодом. Прорачунате електронске структуре проба у разматрању са тачкастим групама симетрије њихових позиција помогли су у идентификовању порекла хиперфиних интеракција и утврдили да је преферентна позиција хафнијума на позицији коју заузимају атоми Al у Ni3Al. Карактеризација узорака γ′–Ni3Al допираних гвожђем извршена је дифракцијом Х–зрачења и мерењима магнетизације. Локална електронска и магнетна структура, хиперфине интеракције пробе 57Fe, као и фазни састав легура Ni2.82Fe0.18Al и Ni2.64Fe0.36Al истражени су спектроскопијом Mössbauеr–овог ефекта. Резултати прорачуна електронских особина Fe–Ni3Al легура из првих принципа методом пројектора проширених таласа као и резултати прорачуна енергије дефеката објашњавају електронску структуру, хиперфине интеракције и преферентну позицију атома Fe у легури γ′–Ni3Al са мањком никла. Вредност израчунате највеће компоненте тензора градијента електричног поља, Vzz = 1.6 1021 Vm-2 на основу вредности параметара добијених из Mössbauеr–ових спектара, указује да у обе испитиване легуре атоми гвожђа замењују атоме никла у γ′–Ni3Al.The dissertation addresses the local crystal structure and hyperfine interactions of hafnium or iron doped polycrystalline intermatallic γ′–Ni3Al alloys. The local structure, magnetic and electrical properties of γ′–Ni3Al compounds doped with Hf atoms or Fe atoms were investigated by the X ray diffraction, magnetization, time differential perturbed angular correlation spectroscopy and Mössbauer–effect spectroscopy. The last two experimental methods provided insight into hyperfine interactions of Hf– and Fe–nucleus. This was complemented by the calculations of the energy of the defects, as well as with ab initio calculations of structural and hyperfine interactions parameters. The main goal of the experimental and theoretical investigations of the intermetallic γ′–Ni3Al phase doped with Hf or Fe atoms was to determine the site preference of dopant atoms in the γ′–Ni3Al matrix, as well as their influence on the host lattice. The X–ray diffraction measurements of the Hf doped Ni3Al samples revealed the presence of the cubic γ′ phase (usually denoted by L12) and its two tetragonal distortions: D022 and L60. The refinement of XRD data showed the dominant presence of the cubic L12 phase in the Hf–Ni3Al samples, with the minority presence of the two tetragonal phases. The measurements of the hyperfine interactions of the 181Та–probe in the 0.2, 0.5 and 5 at.% Hf – Ni3Al alloys (181Та–nucleus is the daughter–nucleus of the 181Hf–parent nucleus) were done in the temperature range 78 K to 1230 K. The measured hyperfine parameters values were assigned to the various Hf atom sites in the different crystal structures (cubic and tetragonal) present in the Ni3Al alloys. The presence of 181Та–signal constant in time is ascribed to Hf atoms at the Al sites in dominant L12 phase. For all investigated alloys, the lower value of the spin independent electric quadrupole frequency was ascribed to the electric quadrupole interaction of the Hf nuclei substituting on the Al sites in the tetragonal D022 phase in the Hf–Ni3Al v alloys. The higher value of the spin independent electric quadrupole frequency corresponded to the electric quadrupole interactions of the Hf nuclei substituting on the Al sites in the tetragonal L60 phase in the Hf–Ni3Al alloys. The ab initio calculations of electronic and structural properties and hyperfine parameters of the 181Ta ion probe in the Hf–Ni3Al alloys, performed using the full potential augmented plane wave plus local–orbital method as implemented in the WIEN2k code, correctly reproduce the experimental results. The DFT (density functional theory) calculations of point defect energies were done by using the Vienna ab initio simulation package VASP. The ab initio calculations enable discussion on the structural and electronic properties of the Ni3Al polycrystalline alloys. The joint experimental and theoretical investigations enabled us to identify the observed hyperfine interactions and showed that hafnium additions prefer aluminum sites in Ni3Al. The structural and magnetic characterizations of the Fe doped γ′–Ni3Al samples were done by the X–ray diffraction and magnetization measurements. The local electronic and magnetic structure, the hyperfine interactions of the 57Fe–probes, as well as the phase composition of the Ni2.82Fe0.18Al and Ni2.64Fe0.36Al alloys were investigated by means of the Mössbauer–effect spectroscopy. The ab initio calculations performed with the projector augmented wave method and the calculations of the energies of iron point defects were done to elucidate the electronic structure, the hyperfine interactions and site preference of Fe doped Ni–deficit γ′–Ni3Al. The value of calculated electric field gradient tensor Vzz = 1.6 1021 Vm-2 matches well with the results of Mössbauer spectroscopy and indicates weak preference of iron dopant atoms for Ni sites

The structure and electrochemical properties of fayalite Fe2SiO4

Fayalite has been found various applications in many fields. Here is presented its use as anode material for lithium ion batteries. The syntheses of Fe2SiO4 and its composite with carbon are conducted through solid-state reaction at 850 °C under inert atmosphere of argon, using cheap and abundant precursors (Fe(NO3)3×9H2O and amorphous silica). Citric acid served as carbon source. The phase-purity of synthesized powders is checked by X-ray powder diffraction. The crystal structure of the powders is refined in orthorhombic Pbnm space group. Half-cell configuration, with lithium metal as counter electrode and fayalite as working electrode, is used for electrochemical measurements: galvanostatic cycling and electrochemical impedance spectroscopy

Structural and electrochemical study of lithium iron (II) pyrophosphate

Lithium iron(II) pyrophosphate, Li2FeP2O7, attracts attention of researchers for application as a cathode material in rechargeable lithium batteries. Li2FeP2O7 has somewhat higher voltage than commercial LiFePO4 (3.5 and 3.4 V, respectively), thus enables higher energy density, and also provides the possibility of two-electron reaction during intercalation. Within this study, pristine Li2FeP2O7 and its composite with carbon Li2FeP2O7/C were synthesized, with the carbon being formed by the pyrolysis of organic precursor in situ during formation of Li2FeP2O7 at high temperature. The polymer of methylcellulose was used as carbon source because of its ability to reversibly, depending on temperature, dissolve or gel in water. The structural, electrical and electrochemical characteristics of prepared powders were investigated by means of X-ray diffraction analysis, Mossbauer spectroscopy, impedance spectroscopy and galvanostatic charge/discharge testing. The results imply that in situ formation of carbon alters lattice parameters, decreases crystallite size, and facilitates lithium ion intercalation/deintercalation processes. The Ministry of Education, Science and Technological Development of the Republic of Serbia provided financial support for this study under Grant No. III 4500

A semi-classical over-barrier model for charge exchange between highly charged ions and one-optical electron atoms

Absolute total cross sections for electron capture between slow, highly charged ions and alkali targets have been recently measured. It is found that these cross sections follow a scaling law with the projectile charge which is different from the one previously proposed basing on a classical over-barrier model (OBM) and verified using rare gases and molecules as targets. In this paper we develop a "semi-classical" (i.e. including some quantal features) OBM attempting to recover experimental results. The method is then applied to ion-hydrogen collisions and compared with the result of a sophisticated quantum-mechanical calculation. In the former case the accordance is very good, while in the latter one no so satisfactory results are found. A qualitative explanation for the discrepancies is attempted.Comment: RevTeX, uses epsf; 6 pages text + 3 EPS figures Journal of Physics B (scehduled March 2000). This revision corrects fig.

A study of defect structures in Fe-alloyed ZnO: Morphology, magnetism, and hyperfine interactions

In order to study the effect of Fe cation substitution on the local structure, defect formation, and hyperfine interactions in ZnO, Mössbauer spectroscopy measurements of the microwave processed Zn1−xFexO (x=0.05, 0.10, 0.15, and 0.20) nanoparticles, together with ab initio calculations, were performed. Complementary information on the distribution of particle size and morphology, as well as magnetic properties, were obtained by X-ray diffraction, transmission electron microscopy, and squid-magnetometry. The selected model for analyzing the Mössbauer spectra of our samples is a distribution of quadrupole splittings. The fitting model with two Lorentz doublets was rejected due to its failure to include larger doublets. The Fe3+ ions do not yield magnetic ordering in the samples at room temperature. The results from first-principles calculations confirm that the major component of the Mössbauer spectra corresponds to the Fe-alloyed ZnO with Zn vacancy in the next nearest neighbor environment. The magnetic measurements are consistent with the description of the distribution of iron ions over the randomly formed clusters in the ZnO host lattice. While at room temperature all the samples are paramagnetic, magnetic interactions cause a transition into a cluster spin-glass state at low temperatures

On the similarity of dust flows in the inner coma of comets

The atmosphere of a comet is formed by the sublimation, due to solar illumination, of its volatile component and the dust particles ejected from its nucleus and entrained by the gas flow. Contemporary dusty-gas coma models take into account numerous physical processes occurring in the coma and a complex geometry of the nucleus. For the description of the dusty-gas flow in the coma, such models introduce a large number of governing parameters characterizing physical properties and processes. The relative role of these processes is not easy to ascribe therefore a relevant inter-comparison of model results becomes difficult. The present work introduces a set of universal, dimensionless parameters, which characterize the dust motion in the inner cometary coma. This approach allows one to: (i) reduce the number of parameters for analysis; (ii) reveal dust flows similarities; (iii) rescale the available numerical solutions. The present work demonstrates application of this approach to a realistic coma model. Description of dust motion with dimensionless parameters allows us to make a parametric study for a broad range of conditions and to find simple analytic approximations (via a polynomial function) of the numerical results suitable for rough estimations of dust density in the coma

Enhanced superconductivity and electron correlations in intercalated ZrTe3

Charge density waves (CDWs) with superconductivity, competing Fermi surface instabilities, and collective orders have captured much interest in two-dimensional van der Waals (vdW) materials. Understanding the CDW suppression mechanism, its connection to the emerging superconducting state, and electronic correlations provides opportunities for engineering the electronic properties of vdW heterostructures and thin-film devices. Using a combination of the thermal transport, x-ray photoemission spectroscopy, Raman measurements, and first-principles calculations, we observe an increase in electronic correlations of the conducting states as the CDW is suppressed in ZrTe3 with 5% Cu and Ni intercalation in the vdW gap. As superconductivity emerges, intercalation brings not only decoupling of quasi-one-dimensional conduction electrons with phonons as a consequence of intercalation-induced lattice expansion but also a drastic increase in Zr2+ at the expense of Zr4+ metal atoms. These observations not only demonstrate the potential of atomic intercalates in the vdW gap for ground-state tuning but also illustrate the crucial role of the Zr metal valence in the formation of collective electronic orders