Signatures of nearly compensated magnetism and spin glass behavior in highly frustrated ß-Mn-type Mn50Fe25+xAl25-x Heusler alloys.

The present study examines the effect of Fe/Al concentration on the structural and magnetic properties of Mn-rich Mn50Fe25+xAl25-x (x = 5, 10, 15) Heusler alloys through x-ray diffraction, temperature- and field-dependent DC magnetization, thermoremanent magnetization, magnetic memory effect, AC susceptibility measurements, and DFT calculations. The samples crystallize in a cubic ß-Mn structure. The trend shows a reduction in lattice parameters (unit cell volume) with the increasing Fe proportion. These alloys exhibit strong antiferromagnetic interactions with large frustration parameters, indicating the presence of competing magnetic interactions. The DC magnetization data reveal spin glass-like features with a peak at spin glass freezing temperature (Tf). The observation of bifurcation in temperature-dependent zero-field-cooled and field-cooled magnetization curves, exponential dependence of the temperature variation of remanence and coercivity, magnetic relaxation, and magnetic memory effect below Tf support the spin-glass character of these alloys. The frequency dependence of Tf is also examined in the context of dynamic scaling laws, such as the Vogel-Fulcher law and critical slowing down model, which further supports the presence of spin glass behavior. In the theoretical DFT calculations, the electronic structure is found to be metallic and similar for both spin projections. Moreover, the antiferromagnetic arrangement of the magnetic moments, in line with the experimental observations, is stabilized by exchange interactions, resulting in an almost compensated total magnetic moment of 0.02-0.38 µB/f.u. This is probably caused by the frustrated structure and non-stoichiometric compositions of Mn50Fe25+xAl25-x.

Electronic Structure and Transport Properties of Bi2Te3 and Bi2Se3 Single Crystals.

The electrical resistivity and the Hall effect of topological insulator Bi2Te3 and Bi2Se3 single crystals were studied in the temperature range from 4.2 to 300 K and in magnetic fields up to 10 T. Theoretical calculations of the electronic structure of these compounds were carried out in density functional approach, taking into account spin-orbit coupling and crystal structure data for temperatures of 5, 50 and 300 K. A clear correlation was found between the density of electronic states at the Fermi level and the current carrier concentration. In the case of Bi2Te3, the density of states at the Fermi level and the current carrier concentration increase with increasing temperature, from 0.296 states eV-1 cell-1 (5 K) to 0.307 states eV-1 cell-1 (300 K) and from 0.9 × 1019 cm-3 (5 K) to 2.6 × 1019 cm-3 (300 K), respectively. On the contrary, in the case of Bi2Se3, the density of states decreases with increasing temperature, from 0.201 states eV-1 cell-1 (5 K) to 0.198 states eV-1 cell-1 (300 K), and, as a consequence, the charge carrier concentration also decreases from 2.94 × 1019 cm-3 (5 K) to 2.81 × 1019 cm-3 (300 K).

Semimetallic, Half-Metallic, Semiconducting, and Metallic States in Gd-Sb Compounds.

The electronic and band structures of the Gd- and Sb-based intermetallic materials have been explored using the theoretical ab initio approach, accounting for strong electron correlations of the Gd-4f electrons. Some of these compounds are being actively investigated because of topological features in these quantum materials. Five compounds were investigated theoretically in this work to demonstrate the variety of electronic properties in the Gd-Sb-based family: GdSb, GdNiSb, Gd4Sb3, GdSbS2O, and GdSb2. The GdSb compound is a semimetal with the topological nonsymmetric electron pocket along the high-symmetry points Γ-X-W, and hole pockets along the L-Γ-X path. Our calculations show that the addition of nickel to the system results in the energy gap, and we obtained a semiconductor with indirect gap of 0.38 eV for the GdNiSb intermetallic compound. However, a quite different electronic structure has been found in the chemical composition Gd4Sb3; this compound is a half-metal with the energy gap of 0.67 eV only in the minority spin projection. The molecular GdSbS2O compound with S and O in it is found to be a semiconductor with a small indirect gap. The GdSb2 intermetallic compound is found to have a metallic state in the electronic structure; remarkably, the band structure of GdSb2 has a Dirac-cone-like feature near the Fermi energy between high-symmetry points Г and S, and these two Dirac cones are split by spin-orbit coupling. Thus, studying the electronic and band structure of several reported and new Gd-Sb compounds revealed a variety of the semimetallic, half-metallic, semiconducting, or metallic states, as well topological features in some of them. The latter can lead to outstanding transport and magnetic properties, such as a large magnetoresistance, which makes Gd-Sb-based materials very promising for applications.

Nearly compensated ferrimagnetic behaviour and giant exchange bias of hexagonal Mn2PtAl: experimental and theoretical studies.

We have investigated the Mn2PtAl Heulser alloy to unravel its structural, magnetic, calorimetric and electronic structure properties. At room temperature, the alloy crystallizes in a hexagonal structure. Magnetization reveals a weak martensitic transition at 307 K, followed by a long range ferrimagnetic transition at 90 K. Griffiths phase-like signature and positive Weiss temperature in dc-magnetization, isothermal magnetic hysteresis loops and a frequency-independent peak confirm a nearly compensated ferrimagnetic order of Mn2PtAl. The theoretical electronic structure calculations also reveal the ferrimagnetic ground state of Mn2PtAl and Mn ions (occupying different sites) with a very small total magnetic moment. A giant exchange bias field of 2.73 kOe, at a temperature of 3 K and a cooling field of 70 kOe, has been estimated and is attributed to the unidirectional anisotropy associated with possible ferromagnetic clusters formed by the field cooling process in the ferrimagnetic matrix.

Experimental and Theoretical Investigations of Fe-Doped Hexagonal MnNiGe.

We report a comprehensive investigation of MnNi0.7Fe0.3Ge Heusler alloy to explore its magnetic, caloric, and electrical transport properties. The alloy undergoes a ferromagnetic transition across T C ∼ 212 K and a weak-antiferromagnetic transition across T t ∼ 180 K followed by a spin-glass transition below T f ∼ 51.85 K. A second-order phase transition across T C with mixed short and long-range magnetic interactions is confirmed through the critical exponent study and universal scaling of magnetic entropy and magnetoresistance. A weak first-order phase transition is evident across T t from magnetization and specific heat data. The frequency dependent cusp in χAC(T) along with the absence of a clear magnetic transition in specific heat C(T) and resistivity ρ(T) establish the spin glass behavior below T f. Mixed ferromagnetic and antiferromagnetic interactions with dominant ferromagnetic coupling, as revealed by density functional calculations, are experimentally evident from the large positive Weiss temperature, magnetic saturation, and negative magnetic-entropy and magnetoresistance.

Effect of Doping on the Electronic Structure of the Earth's Lower Mantle Compounds: FeXO3 with X = C, Al, Si.

The effect of the mutual doping of C, Si, and Al atoms on the electronic structure and magnetic properties of FeXO3 (X = C, Al, Si) compounds, which are constituent compounds of the Earth's lower mantle, was studied. In our first principles calculations, it was found that doping with carbon for both FeSiO3 and FeAlO3 leads to the transition of the compound from a half-metallic state to a metallic one. The values of the magnetic moments of Fe were obtained for pure and doped compounds. For the doped compounds, there is a tendency of the Fe magnetic moment to increase with the growth in the number of substituted ions in the case of replacing Si with C and Si for Al; on the contrary, in the case of replacing Al with C and Si, a decrease in the magnetic moment was revealed. For FeXO3 (X = C, Al, Si), the obtained magnetic moment values were found to be in a good agreement with the known experimental data.

Common Topological Features in Band Structure of RNiSb and RSb Compounds for R = Tb, Dy, Ho.

The electronic and band structures of ternary RNiSb and binary RSb compounds for R = Tb, Dy, Ho, have been investigated using an ab initio method accounting for strong electron correlations in the 4f shell of the rare-earth metals. These ternary compounds are found to be semiconductors with the indirect gap of 0.21, 0.21, and 0.26 eV for Tb, Dy, and Ho(NiSb), respectively. In contrast, in all binary RSb compounds, bands near the Fermi energy at the Г and X points are shifted relatively to RNiSb and form hole and electron pockets, so the energy gap is closed in RSb. The band structure typical for semimetals is formed in all RSb compounds for R = Tb, Dy, Ho. For the first time, we identify similar features near the Fermi level in the considered binary semimetals, namely, the presence of the hole and electron pockets in the vicinity of the Г and X points, the nonsymmetric electron pocket along Γ-X-W direction and hole pockets along the L-Γ-X direction, which were previously found experimentally in the other compound of this series GdSb. The magnetic moment of all considered compounds is fully determined by magnetic moments of the rare earth elements, the calculated effective magnetic moments of these ions have values close to the experimental values for all ternary compounds.

Silver Flowerlike Structures for Surface-Enhanced Raman Spectroscopy.

Micro- and nanoflowers are a class of materials composed of particles with high surface-to-volume ratio. They have been extensively studied in the last decade due to simple preparation protocols and promising applications in biosensing, as drug delivery agents, for water purification, and so on. Flowerlike objects, due to their highly irregular surface, may act also as plasmonic materials, providing resonant coupling between optical waves and surface plasmon excitations. This fact allows us to infer the possibility to use micro- and nanoflowers as effective surface-enhanced Raman scattering (SERS) substrate materials. Here, we report on the design and Raman enhancement properties of silver flowerlike structures, deposited on aluminum surface. A simple and cost-effective fabrication method is described, which leads to SERS substrates of high developed surface area. The morphology of the silver flowers on a nanoscale is characterized by self-organized quasiperiodic stacks of nanosheets, which act as plasmonic cavity resonators. The substrates were tested against rhodamine-6G (R6G) water solutions of concentration varying between 10-3 M and 10-7 M. Optimal SERS enhancement factors of up to 105 were established at R6G concentrations in the 10-6-10-7 M range.

Non-collinear antiferromagnetism to compensated ferrimagnetism in Ti(Fe1-xCox)2 (x = 0, 0.5 and 1) alloys: experiment and theory.

The manifestation of the structural and magnetic properties of Co substituted TiFe2 is investigated using powder X-ray diffraction, magnetization and density functional theory calculations. The alloys TiFe2 and TiFeCo crystallize in the hexagonal structure (P63/mmc) with a reduction in the lattice parameters of TiFeCo (by about 0.51% in a and 0.64% in c) when compared to TiFe2. On the other hand, TiCo2 crystallizes in the cubic structure (Fd3[combining macron]m). A structural transition from hexagonal to cubic is anticipated for a composition with x ∈ [0.5, 1]. The non-collinear antiferromagnetic (AFM) spin structure (formed by 6h Fe atoms) of TiFe2 with Néel temperature TN ∼ 275 K is reported at zero magnetic field H. Meanwhile, a magnetic field-induced collinear antiferromagnetic spin structure is suggested by magnetization measurements and supported by density functional theory calculations. The magnetization of TiFeCo shows a weak-ferromagnetic (FM)-like transition around 204 K, followed by a broad hump at 85.5 K and H = 200 Oe. Ferromagnetic interactions are weakened, causing the hump to disappear due to the possible transfer of electrons between Fe and Co. TiCo2 shows compensated ferrimagnetism with magnetization of the order of 10-5µB f.u.-1 and a linear increase of M with H at 5 K. The presence of a non-collinear AFM spin structure in TiFe2, a reduced magnetic moment in TiFeCo due to the charge transfer between Co and Fe, and compensated ferrimagnetism in TiCo2 promise a rich phase diagram of Ti(Fe1-xCox)2 alloys and the possible potential of these alloys for use in spintronics applications.

Effect of Electronic Correlations on the Electronic Structure, Magnetic and Optical Properties of the Ternary RCuGe Compounds with R = Tb, Dy, Ho, Er.

In this study, the ab initio and experimental results for RCuGe ternary intermetallics were reported for R = Tb, Dy, Ho, Er. Our theoretical calculations of the electronic structure, employing local spin density approximation accounting for electron-electron correlations in the 4f shell of Tb, Dy, Ho, Er ions were carried in DFT+U method. The optical properties of the RCuGe ternary compounds were studied at a broad range of wavelengths. The spectral and electronic characteristics were obtained. The theoretical electron densities of states were taken to interpret the experimental energy dependencies of the experimental optical conductivity in the interband light-absorption region. From the band calculations, the 4f shell of the rare-earth ions was shown to provide the major contribution to the electronic structure, magnetic and optical properties of the RCuGe intermetallics. The accounting for electron-electron correlations in Tb, Dy, Ho, Er resulted in a good agreement between the calculated and experimental magnetic and optical characteristics.

Collapse of magnetic moment drives the Mott transition in MnO.

The metal-insulator transition in correlated electron systems, where electron states transform from itinerant to localized, has been one of the central themes of condensed-matter physics for more than half a century. The persistence of this question has been a consequence both of the intricacy of the fundamental issues and the growing recognition of the complexities that arise in real materials, when strong repulsive interactions play the primary role. The initial concept of Mott was based on the relative importance of kinetic hopping (measured by the bandwidth) and onsite repulsion of electrons. Real materials, however, have many further degrees of freedom that, as is recently attracting note, give rise to a rich variety of scenarios for a 'Mott transition'. Here, we report results for the classic correlated insulator MnO that reproduce a simultaneous moment collapse, volume collapse and metallization transition near the observed pressure, and identify the mechanism as collapse of the magnetic moment due to an increase of crystal-field splitting, rather than to variation in the bandwidth.