Mechanism for Improved Curie Temperature and Magnetic Entropy Change in Sm-Doped Fe88Zr8B4 Amorphous Alloys.

In the present work, Fe88Zr8-xSmxB4 (x = 2, 4) amorphous alloys (AAs) were successfully synthesized into the shape of 40-micrometer-thick ribbons and their magnetic properties were measured. The Fe88Zr8-xSmxB4 (x = 2, 4) AAs exhibited a rather high maximum magnetic entropy change (-ΔSmpeak): ~3.53 J/(K × kg) near 317 K for x = 2 and ~3.79 J/(K × kg) near 348 K for x = 4 under 5 T. The effects of a Sm substitution for Zr on the Curie temperature (Tc) and -ΔSmpeak were studied and compared to those of Nd and Pr substitutions, for the purpose of revealing the mechanism involved in more detail.

Excellent Magnetocaloric Performance of the Fe87Ce13-xBx (x = 5, 6, 7) Metallic Glasses and Their Composite.

The novel Fe87Ce13-xBx (x = 5, 6, 7) metallic glass (MG) ribbons were fabricated in this work. The compositional dependence of glass forming ability (GFA), magnetic and magnetocaloric properties of these ternary MGs, and the mechanism involved was investigated. The GFA and Curie temperature (Tc) of the MG ribbons were found to improve with the boron content, and the peak value of magnetic entropy change (-ΔSmpeak) reaches a maximum of 3.88 J/(kg × K) under 5 T when x = 6. Based on the three results, we designed an amorphous composite that exhibits a table-shape magnetic entropy change (-ΔSm) profile with a rather high average -ΔSm (-ΔSmaverage~3.29 J/(kg × K) under 5 T) from 282.5 K to 320 K, which makes it a potential candidate for the highly efficient refrigerant in a domestic magnetic refrigeration appliance.

The exchange between anions and cations induced by coupled plasma and thermal annealing treatment for room-temperature ferromagnetism.

Two-dimensional (2D) materials, with outstanding magnetic properties at room temperature, are highly desirable for the future spintronic and nanoscale electronic industry. However, most of the 2D systems are not of magnetic nature due to thermal fluctuations. Herein, we propose a novel strategy to induce robust room-temperature ferromagnetism in the originally nonmagnetic 2D ReS2 by the exchange between anions and cations. The vacancies are created by argon plasma treatment, which lowers the formation energy of point defects. The subsequent annealing facilitates the movement of the cations into the anion sites, giving rise to antisite defects, which leads to a significant increase in the magnetization. First-principles calculations demonstrate that the point defect with respect to the antisite substitution from Re to S is responsible for the extraordinary room-temperature ferromagnetism. This work opens a new door to the design of spin electronic structures by controllable antisite defects.

A High Precision and Multifunctional Electro-Optical Conversion Efficiency Measurement System for Metamaterial-Based Thermal Emitters.

In this study, a multifunctional high-vacuum system was established to measure the electro-optical conversion efficiency of metamaterial-based thermal emitters with built-in heaters. The system is composed of an environmental control module, an electro-optical conversion measurement module, and a system control module. The system can provide air, argon, high vacuum, and other conventional testing environments, combined with humidity control. The test chamber and sample holder are carefully designed to minimize heat transfer through thermal conduction and convection. The optical power measurements are realized using the combination of a water-cooled KBr flange, an integrating sphere, and thermopile detectors. This structure is very stable and can detect light emission at the µW level. The system can synchronously detect the heating voltage, heating current, optical power, sample temperatures (both top and bottom), ambient pressure, humidity, and other environmental parameters. The comprehensive parameter detection capability enables the system to monitor subtle sample changes and perform failure mechanism analysis with the aid of offline material analysis using scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. Furthermore, the system can be used for fatigue and high-low temperature impact tests.

Hexanuclear Co4 Dy2 , Zn4 Dy2 , and Co4 Y2 Complexes with Defect Tetracubane Cores: Syntheses, Structures, and Magnetic Properties.

A hexanuclear heterometallic cluster of composition [Dy2 Co4 (L)4 (NO3 )2 (OH)4 (C2 H5 OH)2 ] ⋅ 2 C2 H5 OH (1) was synthesized by employing a Schiff base 2-(((2-hydroxy-3-methoxybenzyl) imino)methyl)-4-methoxyphenol (H2 L) as ligand and utilizing Dy(NO3 )3 ⋅ 6H2 O and Co(NO3 )2 ⋅ 6H2 O as metal ion sources. X-ray single-crystal diffraction analysis indicated that complex 1 contains a defect tetracubane core and possesses central symmetric structure, with two DyIII ions being in the central body position of the molecule and four CoII ions being arranged at the outer sites. Magnetic studies reveal that complex 1 behaves as single-molecule magnet (SMM) with energy barrier of 27.50 K. To investigate the individual contribution of DyIII and CoII ions to the SMM behavior, another two complexes of formulae [Dy2 Zn4 (L)4 (NO3 )2 (OH)4 ] ⋅ 4CH3 OH (2) and [Y2 Co4 (L)4 (NO3 )2 (OH)4 (C2 H5 OH)2 ] ⋅ 2 C2 H5 OH (3) were prepared. Complexes 1 and 3 are isomorphous. The coordination geometries of DyIII ions in 1 and 2 are different. The DyIII ions are eight-coordinated in 2 and nine-coordinated in 1. Complex 2 exhibits SMM behavior with energy barrier of 69.67 K, but complex 3 does not display SMM property. These results reveal that the SMM behaviors of 1 and 2 are mainly originated from DyIII ions. It might be the higher symmetry of DyIII ions in 2 that results in the higher energy barrier.

Evolution from a single relaxation process to two-step relaxation processes of Dy2 single-molecule magnets via the modulations of the terminal solvent ligands.

To explore the influences of magnetic interactions on the relaxation dynamics of single-molecule magnets (SMMs) and to understand the relationship between single-ion relaxation and the relaxation of a molecular entity, it is very important to design dinuclear lanthanide-based SMMs with two-step relaxation processes. Here, three Dy2 complexes of compositions [Dy2(L)2(NO3)2(MeOH)2] (1), [Dy2(L)2(NO3)2(EtOH)2] (2), and [Dy2(L)2(NO3)2(DMF)2]·0.5EtOH (3) (H2L = 2-(((2-hydroxy-3-methoxybenzyl)imino)methyl)-4-methoxyphenol) were successfully synthesized via elaborately introducing different terminal solvent ligands. X-ray single-crystal diffraction analyses revealed that complexes 1-3 are isostructural. The two DyIII ions of 1 and 2 both adopt D2d symmetry, while the two DyIII centres of 3 display D2d and D4d symmetries, respectively. The magnetic property studies of 1-3 indicated that all three complexes exhibit single-molecule magnet (SMM) behaviours with energy barriers of 104 K for 1, 98.94 K for 2, and 76.28 K and 45.54 K for 3 under zero dc field. The target of assembling Dy2 SMMs with two-step relaxation processes was achieved by gradually increasing the sizes of the terminal solvent ligands. Complex 3 exhibits two-step relaxation processes. Complete active space self-consistent field (CASSCF) calculations were performed on 1-3 to rationalize the observed differences in the magnetic behaviour. It is found that both the angles θ between the magnetic axis and the vector connecting two DyIII ions and the symmetries of DyIII ions are vital factors that affect the energy barriers of 1-3. The high local symmetries of the central metals in 1 and 2 make the complexes act as SMMs with higher energy barriers, while the smaller θ angle and different symmetries of the two DyIII ions render complex 3 as a SMM with a two-step relaxation process. This work demonstrates a new methodology for preparing SMMs with two-step relaxation processes by fine-tuning the terminal solvent ligands.

Designing asymmetric Dy2 single-molecule magnets with two-step relaxation processes by the modification of the coordination environments of Dy(iii) ions.

The reaction of Dy(NO3)3·6H2O and an asymmetric Schiff-base linker 5-chloro-2-(((2-hydroxy-3-methoxybenzyl)imino)methyl) phenol (H2L) afforded a dinuclear compound [Dy2L2(HL)(NO3)(EtOH)]·0.5C2H5OH (1). Complex 1 features two inequivalent Dy(iii) sites, where three ligand sets (one HL- moiety and two L2- groups) are shared by two Dy(iii) ions. The strategic introduction of CH3COOH in the reaction system used for synthesizing 1 induces the replacement of the HL- ligand by the CH3COO- ion, consequently resulting in the generation of [Et3NH][Dy2L2(NO3)(CH3COO)2] (2). In complex 2, two Dy(iii) centers adopt NO7 (D2d geometry) and NO6 (C2v) coordination sphere, respectively. DC magnetic susceptibility studies for the two complexes indicate ferromagnetic interactions. Complexes 1 and 2 exhibit single-molecule magnet behavior with two-step slow relaxation processes due to the possession of inequivalent metal sites. The energy barriers of 69.19 and 45.73 K for 1, and 92.77 and 72.95 K for 2 are determined. Theoretical calculations reveal that the two-step relaxation processes in both 1 and 2 mainly originate from the single-ion magnetism of two distinct Dy(iii) ions with inequivalent coordination environments.

Investigation of catalyst-assisted growth of nonpolar GaN nanowires via a modified HVPE process.

The growth of nonpolar GaN nanowires along the [101[combining macron]0] orientation has been demonstrated via a modified hydride vapor phase epitaxy (HVPE) process using GaCl3 and NH3 as precursors. The morphology and structure evolution as a dependence of the growth parameters was thoroughly studied to elucidate the nucleation and crystallization of nonpolar GaN nanowires. It has been found that the V/III ratio and temperature are critically important for the formation of high-quality nonpolar GaN nanowires. The existence of a cubic GaN (c-GaN) transition layer between the Au catalyst and hexagonal GaN (h-GaN) nonpolar nanowires was demonstrated by high-resolution transmission electron microscopy (HRTEM) characterization, which plays an important role in the initial nucleation of nonpolar GaN nanowires and the formation of stacking faults (SFs) in the GaN nanowires grown at lower temperature. Optical investigations show that the defect-related visible emission of nonpolar GaN nanowires is closely related to the growth process and can be selectively tailored. The synthetic strategy using GaCl3 as the Ga precursor to study the vapor phase epitaxy process in this work will provide a simple and efficient approach to obtain nonpolar GaN nanowires and will thus pave a solid way for fundamental research on high-quality nonpolar GaN nanowires in optoelectronic nanodevices.

Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode.

Driven by the motivation to quantitively control and monitor trace metal ions in water, the development of environmental-friendly electrodes with superior detection sensitivity is extremely important. In this work, we report the design of a stable, ultrasensitive and biocompatible electrode for the detection of trace Ag+ and Cu2+ ions by growing n-type GaN micropillars on conductive p-type GaN substrate. The electrochemical measurement based on cyclic voltammetry indicates that the GaN micropillars exhibit quasi-reversible and mass-controlled reaction in redox probe solution. In the application of trace Ag+ and Cu2+ determination, the GaN micropillars show superior sensitivity and excellent conductivity by presenting a detection limit of 3.3 ppb for Ag+ and 3.3 ppb for Cu2+. Comparative studies on the electrochemical response of GaN micropillars and GaN film in the simultaneous Ag+ and Cu2+ detection reveal that GaN micropillars show three orders of magnitude higher stripping peak current than GaN film. It is assumed that the microarray morphology with large active area and the hydrophilia nature of GaN micropillars are responsible for the excellent sensitivity. This work will open up some opportunities for GaN nanostructure electrodes in the application of trace metal ions detection.

Cu3P-Ni2P Hybrid Hexagonal Nanosheet Arrays for Efficient Hydrogen Evolution Reaction in Alkaline Solution.

The development of efficient and low-cost hydrogen evolution reaction electrocatalysts has been regarded as a promising approach to produce sustainable and clean fuels to solve the energy crisis and environmental problems. Herein, 3D hybrid Cu3P-Ni2P hexagonal nanosheet arrays are successfully prepared on nickel foam (Cu3P-Ni2P/NF). Benefiting from synergistic effects and strong chemical coupling existing at the interface, the Cu3P-Ni2P/NF electrode exhibits a low overpotential of 103 mV at a current density of 10 mA cm-2, which is 47 and 100 mV less than that for Ni2P/NF and Cu3P/NF, respectively. It also shows excellent electrochemical durability for long-term reaction in alkaline medium. The excellent electrocatalytic activity makes the Cu3P-Ni2P/NF as a promising cathode toward efficient hydrogen evolution via electrochemical water splitting.

Stoner-enhanced paramagnetism in tungsten tetraboride.

We demonstrate that tungsten tetraboride (WB4), a heavy transition metallic compound without magnetic atoms, is an exchange-enhanced paramagnet revealed by the magnetization and specific heat measurements. WB4 has a small effective magnetic moment of 0.53 µB/fu. The high magnetic susceptibility in the magnitude of 1 memu (mol·Oe)(-1) below 10 K obeys quadratic temperature dependence. The upturn behavior of C(P)/T versus T(2) at low temperatures is attributed to the electron-paramagnon interactions. A high Stoner enhancement parameter, Z = 0.93, was derived to explain the enhanced paramagnetism based on the Stoner model.

Field-induced spin-flop in antiferromagnetic semiconductors with commensurate and incommensurate magnetic structures: Li2FeGeS4 (LIGS) and Li2FeSnS4 (LITS).

Li2FeGeS4 (LIGS) and Li2FeSnS4 (LITS), which are among the first magnetic semiconductors with the wurtz-kesterite structure, exhibit antiferromagnetism with TN ≈ 6 and 4 K, respectively. Both compounds undergo a conventional metamagnetic transition that is accompanied by a hysteresis; a reversible spin-flop transition is dominant. On the basis of constant-wavelength neutron powder diffraction data, we propose that LIGS and LITS exhibit collinear magnetic structures that are commensurate and incommensurate with propagation vectors km = [1/2, 1/2, 1/2] and [0, 0, 0.546(1)], respectively. The two compounds exhibit similar magnetic phase diagrams, as the critical fields are temperature-dependent. The nuclear structures of the bulk powder samples were verified using time-of-flight neutron powder diffraction along with synchrotron X-ray powder diffraction. (57)Fe and (119)Sn Mössbauer spectroscopy confirmed the presence of Fe(2+) and Sn(4+) as well as the number of crystallographically unique positions. LIGS and LITS are semiconductors with indirect and direct bandgaps of 1.42 and 1.86 eV, respectively, according to optical diffuse-reflectance UV-vis-NIR spectroscopy.

Structural and relative stabilities, electronic properties and possible reactive routing of osmium and ruthenium borides from first-principles calculations.

First-principles calculations are employed to provide a fundamental understanding of the structural features and relative stability, mechanical and electronic properties and possible reactive route for osmium and ruthenium borides. The structural searches and calculations of the formation enthalpy identify a low-energy monoclinic phase for OsB3 with P2(1)/m symmetry, an orthorhombic phase for OsB4 with Pmmn symmetry, an orthorhombic phase for RuB3 with Pnma symmetry and a hexagonal phase for RuB4 with P63/mmc symmetry. Also, the structure transition at high pressure is also predicted for MB3 and MB4 (M = Os and Ru). Moreover, among the borides, orthorhombic RuB3 and OsB4 phases are predicted to be potential hard materials with estimated Vickers hardness values of 26.3 and 31.3 GPa, respectively. The analysis on the electronic properties and crystal orbital Hamilton population shows that the directional boron-boron networks, together with the strong metal-boron bonds, are responsible for their excellent mechanical properties. Relative enthalpy calculations with respect to possible constituents are also investigated to assess the prospects for phase formation and an attempt at high-pressure synthesis is suggested to obtain osmium and ruthenium tri- and tetra-borides.

Synthesis, crystal structure, and electronic properties of the tetragonal (RE(I)RE(II))3SbO3 phases (RE(I) = La, Ce; RE(II) = Dy, Ho).

In our efforts to tune the charge transport properties of the recently discovered RE(3)SbO(3) phases (RE is a rare earth), we have prepared mixed (RE(I)RE(II))(3)SbO(3) phases (RE(I) = La, Ce; RE(II) = Dy, Ho) via high-temperature reactions at 1550 °C or greater. In contrast to monoclinic RE(3)SbO(3), the new phases adopt the P4(2)/mnm symmetry but have a structural framework similar to that of RE(3)SbO(3). The formation of the tetragonal (RE(I)RE(II))(3)SbO(3) phases is driven by the ordering of the large and small RE atoms on different atomic sites. The La(1.5)Dy(1.5)SbO(3), La(1.5)Ho(1.5)SbO(3), and Ce(1.5)Ho(1.5)SbO(3) samples were subjected to elemental microprobe analysis to verify their compositions and to electrical resistivity measurements to evaluate their thermoelectric potential. The electrical resistivity data indicate the presence of a band gap, which is supported by electronic structure calculations.

Does the real ReN2 have the MoS2 structure?

Rhenium nitride (ReN(2)) with the hexagonal MoS(2) structure was recently synthesized by metathesis reaction under high pressure. Here the calculated elastic and thermodynamic stabilities and chemical bonding show that the MoS(2) phase is unstable based on first-principles calculations. Meanwhile, the MoS(2)-type ReN(2) compound may be stabilized by nitrogen-vacancies from X-ray diffraction and supercell calculations. Structure searches identify a monoclinic C2/m phase for ReN(2), which is energetically more stable than previous predictions and MoS(2) structure over a wide range of pressures. Above 130 GPa, a tetragonal P4/mbm phase becomes favorable from enthalpy calculations. Both phases have superior mechanical properties, and their syntheses would have important applications fundamentally and technologically.

Electronically induced ferromagnetic transitions in Sm5Ge4-type magnetoresponsive phases.

The correlation between magnetic and structural transitions in Gd(5)Si(x)Ge(4-x) hampers the studies of valence electron concentration (VEC) effects on magnetism. Such studies require decoupling of the VEC-driven changes in the magnetic behavior and crystal structure. The designed compounds, Gd(5)GaSb(3) and Gd(5)GaBi(3), adopt the same Sm(5)Ge(4)-type structure as Gd(5)Ge(4) while the VEC increases from 31 e(-)/formula in Gd(5)Ge(4) to 33 e(-)/formula in Gd(5)GaPn(3) (Pn: pnictide atoms). As a result, the antiferromagnetic ground state in Gd(5)Ge(4) is tuned into the ferromagnetic one in Gd(5)GaPn(3). First-principles calculations reveal that the nature of interslab magnetic interactions is changed by introducing extra p electrons into the conduction band, forming a ferromagnetic bridge between the adjacent (∝)(2)[Gd(5)T(4)] slabs.

Electron-deficient Eu(6.5)Gd(0.5)Ge6 intermetallic: a layered intergrowth phase of the Gd5Si4- and FeB-type structures.

A novel electron-poor Eu(6.5)Gd(0.5)Ge6 compound adopts the Ca7Sn6-type structure (space group Pnma, Z = 4, a = 7.5943(5) Å, b = 22.905(1) Å, c = 8.3610(4) Å, and V = 1454.4(1) ų). The compound can be seen as an intergrowth of the Gd5Si4-type (Pnma) R5Ge4 (R = rare earth) and FeB-type (Pnma) RGe compounds. The phase analysis suggests that the Eu(7-x)Gd(x)Ge6 series displays a narrow homogneity range of stabilizing the Ca7Sn6 structure at x ≈ 0.5. The structural results illustrate the structural rigidity of the ²(∝)[R5X4] slabs (X = p-element) and a possibility for discovering new intermetallics by combining the ²(∝)[R5X4] slabs with other symmetry-approximate building blocks. Electronic structure analysis suggests that the stability and composition of Eu(6.5)Gd(0.5)Ge6 represents a compromise between the valence electron concentration, bonding, and existence of the neighboring EuGe and (Eu,Gd)5Ge4 phases.

Decoupling the electrical conductivity and Seebeck coefficient in the RE2SbO2 compounds through local structural perturbations.

Compromise between the electrical conductivity and Seebeck coefficient limits the efficiency of chemical doping in the thermoelectric research. An alternative strategy, involving the control of a local crystal structure, is demonstrated to improve the thermoelectric performance in the RE(2)SbO(2) system. The RE(2)SbO(2) phases, adopting a disordered anti-ThCr(2)Si(2)-type structure (I4/mmm), were prepared for RE = La, Nd, Sm, Gd, Ho, and Er. By traversing the rare earth series, the lattice parameters of the RE(2)SbO(2) phases are gradually reduced, thus increasing chemical pressure on the Sb environment. As the Sb displacements are perturbed, different charge carrier activation mechanisms dominate the transport properties of these compounds. As a result, the electrical conductivity and Seebeck coefficient are improved simultaneously, while the number of charge carriers in the series remains constant.

Suppression of antiferromagnetic interactions through Cu vacancies in Mn-substituted CuInSe2 chalcopyrites.

Stoichiometric and Cu-poor Cu(0.95-x)Mn(0.05)InSe(2) (x = 0-0.20) compounds were synthesized by high-temperature, solid-state reactions. The presence of copper vacancies is revealed by Rietveld refinements of combined neutron and x-ray powder diffraction data. The antiferromagnetic interaction is depressed by the copper deficiency, which may be explained as the competition between the antiferromagnetic Mn-Se-Mn superexchange interaction and the hole-mediated ferromagnetic exchange induced by the copper vacancy. The introduction of copper vacancies is proposed to be a viable route to impart carrier-mediated ferromagnetic exchange in the chalcopyrite-based dilute magnetic semiconductors.

Crystal structure, coloring problem and magnetism of Gd(5-x)Zr(x)Si4.

Ternary Gd(5-x)Zr(x)Si(4) silicides were synthesized by arc melting of the constituent elements and subsequent heat treatments. The Gd(5-x)Zr(x)Si(4) phases adopt the orthorhombic Gd(5)Si(4)-type (space group Pnma) structure for x≤ 0.25 and the tetragonal Zr(5)Si(4)-type (space group P4(1)2(1)2) structure for x≥ 1.0, respectively. The samples with intermediate compositions contain two phases. Single-crystal X-ray diffraction reveals a preferential site occupancy for Zr on the three metal sites in the order of M3 > M2 > M1. Size arguments based on the local coordination environments suggest that the larger Gd atoms preferentially occupy the larger M1 site, while the smaller Zr atoms tend to occupy the smaller metal sites, M2 and M3. Tight-binding linear-muffin-tin orbital calculations illustrate a role of the metal-silicon bonds in the metal site occupation. An increase in the valence electron concentration through the Zr substitution weakens the Si-Si interactions but enhances the metal-silicon and metal-metal interactions. The Curie temperature of Gd(5-x)Zr(x)Si(4) decreases gradually with the increasing Zr content.