Temperature and Magnetic Field-Induced Spin Reorientation in Rare-Earth Perovskite ErFe<Subscript>0.75</Subscript>Cr<Subscript>0.25</Subscript>O<Subscript>3</Subscript>

We report the synthesis of a single-phase rare-earth perovskite ErFe_0.75Cr_0.25O₃ polycrystalline and its magnetic properties. A transition occurs at temperature T _N = 120 K below which we observe a weak magnetic moment from the canted antiferromagnetism. Interestingly, ErFe_0.75Cr_0.25O₃ reveals the compensation-like behavior at T _comp?like = 27 K, where the net magnetic moments of transition-metal ions are antiparallel and equal to the induced net moment of Er³⁺ ions, and the paramagnetic contribution of Er³⁺ moment presenting a nonzero magnetization. The temperature-dependent magnetization measurement shows a spin reorientation transition from Γ₄ to Γ₁ at 6 K. Furthermore, it is also observed that there is a spin-flop transition at low temperature induced by external magnetic field in Γ₁ state (antiferromagnetic state). The interaction between (Fe/Cr)-3d and Er-4f electrons drives an extremely interesting spin reorientation transition which is highly sensitive to magnetic field and temperature.     

Ab-initio Calculation of Electronic and Magnetic Properties of Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

The electronic and magnetic properties of Mn_1−x Cr _x Te of NiAs-type and zinc-blend type structures are theoretically investigated using the first-principles KKR-CPA method. It is concluded that at low concentration region of Cr, an antiferromagnetic state is the ground-state, while a ferrimagnetic state is more stable at higher concentration region of Cr in the both types of structures. In particular, a new type of half-metallic ferrimagnets is found in zinc-blend type Mn_1−x Cr _x Te. On the other hand, a nearly half-metallic behavior is observed in NiAs-type Mn_1−x Cr _x Te, where the Fermi level locates slightly above the minority spin band gap. The features of the half-metallicity of Mn_1−x Cr _x Te in zinc-blend structure are also seen in the results of the conductivity calculations using the Kubo–Greenwood formula.     

Effects of gate sidewall recess on Al<Subscript>0.2</Subscript>Ga<Subscript>0.8</Subscript>As/ In<Subscript>0.15</Subscript>Ga<Subscript>0.85</Subscript>As PHEMTs by citric-based selective etchant

In this report, an effective and simple method of selective gate sidewall recess is proposed to expose the low barrier channel at mesa sidewalls during device isolation for Al_0.2Ga_0.8As/ In_0.15Ga_0.85As PHEMTs (pseudomorphic high electron mobility transistors) by using a newly developed citric-acid-based etchant with high selectivity (> 250) for GaAs/Al_0.2Ga_0.8As or In_0.15Ga_0.85As/Al_0.2G_0.8 As interfaces. After sidewall recess, a revealed cavity will exist between the In_0.15Ga_0.85As layers and gate metals. Devices with 1 × 100μm² exhibit a very low gate leakage current of 2.4μA/mm even at V_GD = −10 V and high gate breakdown voltage over 25 V. As compared to that of no sidewall recess, nearly two orders of reduction in magnitude of gate leakage current and 100% improvement in gate breakdown voltage can be achieved.     

CuCr<Subscript>2</Subscript>S<Subscript>4</Subscript>-Based Quaternary Cation-Substituted Magnetic Phases

CuCr₂S₄-based solid solutions containing solute atoms on the Cu or Cr site were studied with the aim of changing the band structure of the spinel to semiconducting, while maintaining a high magnetic ordering temperature. Cu_{1 ? x}M_xCr₂S₄ and CuCr_{2 ? x}M_xS₄ (M = Al, Ga, In, Fe, Co, As) quaternary phases were synthesized, and CuCr₂S₄, Cu_0.3Co_0.7Cr₂S₄, and Cu_0.5Fe_0.5Cr₂S₄ single crystals were prepared using high-temperature solution growth and chemical vapor transport. The magnetic materials were characterized by a variety of techniques.     

Magnetic phase diagram of solid solutions in the CoCr<Subscript>2</Subscript>S<Subscript>4</Subscript>–Cu<Subscript>0.5</Subscript>In<Subscript>0.5</Subscript>Cr<Subscript>2</Subscript>S<Subscript>4</Subscript> system

The magnetic properties of CoCr₂S₄–Cu_0.5In_0.5Cr₂S₄ solid solutions have been studied in the temperature range 5–300 K at different ac magnetic field frequencies (100, 500, and 1000 Hz) and an amplitude of 79.6 A/m. We have determined the temperatures of the magnetic transformations in the system, identified their nature, and constructed the magnetic phase diagram of the solid solutions.     

Ab-initio spin polarized electronic structure calculations for Ti<Subscript>x</Subscript>Ga<Subscript>n</Subscript>As<Subscript>m</Subscript> photovoltaic materials

A half-metallic isolated band in the band-gap of GaAs and GaP semiconductors has been found for Ti and Sc transition metal impurities and proposed as highly-efficient photovoltaic materials. In this paper, we have investigated by first principle calculations, the spin polarized and non-polarized dispersion band structures and lattice constants of Ga₃As₄Ti and Ga₄As₃Ti alloy semiconductor compounds. We have carried out a comparative study of these compounds in order to identify the basic features of the isolated intermediate band formation in the semiconductor band-gap. We use an ab-initio fully self-consistent density functional theory method in the local density approximation (LDA), with norm-conserving, non-local pseudopotentials for core electrons. To assess the results, we first determined the electronic properties of GaAs and compared them with the experimental results. We find that spin wave functions of the polarized Ga_nAs_mTi compounds noticeably modify the nature and properties of the intermediate band that have already shown in the corresponding paramagnetic compounds.     

Properties of Mg(Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>O<Subscript>4 + δ</Subscript> solid solutions in stable and metastable states

Using pyrohydrolytic synthesis, we have obtained a continuous series of Mg(Fe_{1 − x} Ga _x )₂O_{4 + δ} solid solutions, as checked by X-ray diffraction. The magnetization and coercivity of the samples have been determined from field dependences in applied magnetic fields of ±5 T, and their conductivity has been assessed using current-voltage measurements at 300 K in electric fields from −200 to +200 V. The effective band gap of the solid solutions has been evaluated from their absorption spectra obtained by diffuse reflectance measurements. The optimal composition for spintronic applications is Mg(Fe_0.8Ga_0.2)₂O_{4 + δ}.     

Structural studies of a new electroceramic composite: Pb(Fe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> (PFN)-Cr<Subscript>0.75</Subscript>Fe<Subscript>1.25</Subscript>O<Subscript>3</Subscript>(CRFO)

A study of the structural characteristics of the composites [Pb(Fe_0.5Nb_0.5)O₃(PFN)] _x -[Cr_0.75Fe_1.25O₃(CRFO)]_100?x (x = 0 (CRFO100), 10, 50, 90, 100) was performed in this work. The compounds PFN100 and CRFO100 were prepared by conventional solid-state method and investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and ⁵⁷Fe Mössbauer Spectroscopy techniques. The X-ray analysis shows that PFN100 is tetragonal and the CRFO100 phase has a trigonal symmetry. The refinement of all the composites was also performed and discussed in this paper. The Mössbauer spectrum for the composite samples shows a paramagnetic doublet and a sextet probably assigned to a magnetic phase associated to Fe⁺³. For the sample PFN100, only a magnetic field of 49.5 T (isomer shift (δ) = 0.21 mm/s) was detected. For the composite sample, the δ and Δ are typical of Fe ions at sites of octahedral coordination.     

Ab initio study on fracture toughness of Ti<Subscript>0.75</Subscript>X<Subscript>0.25</Subscript>C ceramics

Ab initio density functional theory calculations have been performed to evaluate the fracture toughness for selected Ti_0.75X_0.25C ceramics (X = Ta, W, Mo, Nb and V). The calculated Young’s modulus E, surface energy γ and fracture toughness K _IC of pure TiC are in a good agreement with experimental data and other theoretical calculations. The results for Ti_0.75X_0.25C system show that alloying additions increase Young’s modulus, and all but vanadium increase surface energy. It was observed that tungsten has the most significant effect on increasing Young’s modulus, while tantalum on increasing surface energy of the Ti_0.75X_0.25C system. Surface energy plays a dominated role in determining the trend of fracture toughness. Overall, tantalum and tungsten are the most effective alloying elements in increasing the fracture toughness of Ti_0.75X_0.25C ceramics.     

Effect of Ba Substitution for Sr in the Pb-based 1212 Cuprate Containing Sulfur of (Pb<Subscript>0.75</Subscript>S<Subscript>0.25</Subscript>)Sr<Subscript>2</Subscript>(Y,Ca)Cu<Subscript>2</Subscript>O<Subscript><Emphasis Type="Italic">z</Emphasis></Subscript>

We already reported on the synthesis of new Pb-based 1212 layered cuprates containing sulfur in the (Pb_0.75S_0.25)Sr₂(Y,Ca)Cu₂O _z system. There none of the samples showed superconductivity until after annealing under high O₂ pressure. More recently, we have discovered that the samples could be substituted by Ba for Sr up to x=0.6 in the composition of (Pb_0.75S_0.25)(Sr_2−x Ba _x )(Y_0.4Ca_0.6) Cu₂O _z . Then if the Ba-substitution is performed for Sr, some of the almost-single phase samples are found to show resistivity drops and magnetic anomalies without annealing under high O₂ pressure. Among the samples, a sample with the nominal composition of (Pb_0.75S_0.25)(Sr_1.5Ba_0.5)(Y_0.4Ca_0.6) Cu₂O _z is found to show the highest onset temperatures of a resistivity drop at about 35 K and a magnetic anomaly at about 31.5 K. These are higher than the highest values for the superconducting Ba-free sample of (Pb_0.75S_0.25)Sr₂ (Y_0.4Ca_0.6)Cu₂O _z which was annealed under high O₂ pressure.     

Photoluminescence of heterostructures with highly strained Ga<Subscript>0.76</Subscript>In<Subscript>0.24</Subscript>As quantum wells separated by GaAs<Subscript>y</Subscript>P<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript> compensating barriers

Based on the results of model calculations and the data of photoluminescence measurements and transmission electron microscopy, the optimum composition (GaAs_0.85P_0.15) of compensating barriers for a structure with four highly strained Ga_0.76In_0.24 As quantum wells (QWs) has been established that excludes the relaxation of elastic stresses in these QWs. Laser heterostructures with four such QWs separated by said compensating barriers have been grown by means of hydride metalorganic chemical vapor deposition. Laser diodes with short (～100 μm) resonators based on these heterostructures operate at λ = 1060 nm with an output radiation power of 100 mW in the continuous regime.     

Optoelectronic and thermoelectric response of Ca<Subscript>5</Subscript>Al<Subscript>2</Subscript>Sb<Subscript>6</Subscript> to shift of band gap from direct to indirect

The structural, optoelectronic and thermoelectric properties of Ca₅Al₂Sb₆ under applied external pressures have been studied using the full potential linear augmented plane wave method. WIEN2k code is used with considering the generalized gradient approximation (GGA), modified Becke–Johnson (MBJ) and modified Becke–Johnson?+?spin orbit (mBJ?+?SO) functionals based on density functional theory (DFT). From electronic results, the size of the band gap decreases with increasing pressure and the nature of the band gap shift from direct to the indirect. In high pressure (>35.7 GPa by mBJ?+?SO), the band gap is also completely disappeared and the nature of compound is changed to the metallic. The calculated anisotropic optical properties such as the static dielectric function, increase with decreasing the size of band gap and increasing of pressure. As a novel result, the thermoelectric performance of n-type and p-type doping of Ca₅Al₂Sb₆ is related to the value of pressure. According to the thermoelectric results, the n-type one has the highest ZT in comparison with the p-type Ca₅Al₂Sb₆ material.     

Structural,Magnetic, and Optoelectronic Properties of TbNi<Subscript>5</Subscript>, TbNi<Subscript>3</Subscript>Ti<Subscript>2</Subscript> and TbNi<Subscript>3</Subscript>V<Subscript>2</Subscript> Compounds

The spin-polarized, electronic, magnetic, and optical properties of TbNi₅, TbNi₃Ti₂, and TbNi₃V₂ intermetallic compounds have been calculated by employing the full-potential linear augmented plane waves (FP-LAPW) within the density functional theory (DFT) and implemented in the WIEN2k package. In this approach, the generalized gradient approximation with Hubbard U-correction (GGA + U) was chosen as exchange-correlation potential. The electronic structure such as band structure and density of states have been investigated and compared among them. The frequency dependences of dielectric function, optical absorption, reflectivity, and optical conductivity are determined. The optical spectra are changed due to the substitution of nickel with titanium and vanadium. Total and local magnetic moments of Tb, Ni, Ti, and V are also estimated; it is shown that the total magnetic moment of the three alloys is vigorously contributed by the local magnetic moment of terbium.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

Modeling of the electrical properties of the mixed ionic-electronic conductor BiMg<Subscript>0.25</Subscript>Cu<Subscript>0.75</Subscript>NbO<Subscript>5</Subscript>

A physically justified equivalent circuit of an ionic-electronic conductor has been proposed which accounts for the unusual electrical behavior of BiMg_0.25Cu_0.75NbO₅. The problem of separating the electronic and ionic contributions to its conductance has been resolved by analyzing the frequency dependences of its impedance. A new approach has been proposed for interpreting the temperature dependences of capacitance and conductance for this material.     

Magnetic,Optoelectronic, and Thermodynamic Properties of Sr<Subscript>2</Subscript>Cr<Emphasis Type="Italic">X</Emphasis>O<Subscript>6</Subscript> (<Emphasis Type="Italic">X</Emphasis> = La and Y): Half-Metallic and Ferromagnetic Behavior

The effects of spin polarization on the structure, magnetic, and optoelectronic properties of Cr-based series of double perovskites Sr₂CrXO₆ (X = La and Y) have been studied by using the full-potential linearized augmented plane-wave method (FP-LAPW), based on the density functional theory (DFT) as implemented in the Wien2k code, within the generalized gradient approximation (GGA), GGA + U, and GGA plus Trans-Blaha-modified Becke–Johnson (TB-mBJ) as the exchange correlation. Our results show a similar half-metallic ferromagnetic ground state for both materials. From the electronic properties, it is found that Sr₂CrYO₆ has a direct band gap at (Γ-Γ) direction and Sr₂CrLaO₆ has an indirect band gap at (Γ-W) direction. Furthermore, we have computed the optic and thermodynamic properties which are investigated for the first time. Consequently, the magnetic, optoelectronic, and thermodynamic properties show these compounds are promising for high technological applications, namely spintronic materials.     

Specific absorption coefficient of chromium in (TeO<Subscript>2</Subscript>)<Subscript>0.80</Subscript>(MoO<Subscript>3</Subscript>)<Subscript>0.20</Subscript> glass

We have prepared (TeO₂)_0.80(MoO₃)_0.20 glass samples containing 0.01 to 0.11 wt % chromium and determined their optical transmission in the range from 450 to 2800 nm. The glasses have been shown to have a strong absorption band centered at 660 nm. From the attenuation coefficient as a function of Cr³⁺ concentration in the glasses, we have evaluated their specific absorption coefficient, which has been shown to be 190 ± 2 cm^–1/wt % at the maximum of the absorption band.     

Correlation between the physical properties and the novel applications of Mg<Subscript>0.7</Subscript>Cu<Subscript>0.3</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nano-ferrites

Nano-ferrite of the general formula Mg_0.7Cu_0.3Fe₂O₄ was prepared by citrate-gel auto combustion method. The structure was studied by X-ray diffraction, Brunauer–Emmet–Teller, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy analyses. The crystallite size of the investigated nano ferrite was ?39 nm. The magnetic hysteresis measurements at different temperatures (100, 170, 240, and 300 K) were performed using a vibrating sample magnetometer. A correlation between magnetic behavior and lattice strain has been established. Arrott plot has been employed to understand the magnetic behavior of nano-crystalline Mg_0.7Cu_0.3Fe₂O₄. The magnetic susceptibility was carried out using Faraday’s method. Magnetic constants such as Curie temperature, effective magnetic moment, saturation magnetization, and coercivity were obtained and reported. Based on UV diffuse reflectance spectroscopy studies, the optical band gaps are in the range from (1.3–1.9 eV), hence the investigated samples could act as visible light driven photo catalysts.     

Magnetism and Half-Metallicity in the Fe<Subscript>2</Subscript>ZrP Heusler Alloy

The electronic structure and magnetic properties of the Fe₂ZrP full-Heusler compound has been investigated by using ab initio calculations with the full-potential linearized augmented plane wave (FLAPW) method. The exchange-correlation functionals are taken into account within the generalized gradient approximation (GGA). Energetically, the AlCu₂Mn-type structure of the full-Heusler Fe₂ZrP is energetically more preferable than that of the CuHg₂Ti-type structure and it exhibits half-metallic ferrimagnet. The calculated total spin moment is found as 1 μ _B at the equilibrium lattice constant a ₀=5.90Å which remarkably agrees with the Slater-Pauling rule of M _t = Z _t ?24. While the spin-up band is metallic, the spin-down band has a semiconductor behavior with a gap of 0.593 eV and the spin-flip gap is 0.129 eV. The negative formation energy is shown as an evidence of the thermodynamic stability of alloy. The dependencies of the magnetic and electronic properties on the lattice constant are also discussed. The estimated Curie temperature is 752 K in the mean field approximation.