New magnetic crystal,metastable metallic alloys Co1−xCux, prepared at strongly non-equilibrium conditions

Magnetic Co_1−xCu_x alloys in the form of single-crystal and polycrystalline films were grown under strongly non-equilibrium conditions using magnetron and triode sputtering techniques. It was found that these alloys could be obtained not only in the expected face-centered cubic structure, but also in a tetragonal phase, undetected earlier for present alloys. Results comparing some structural, magnetic and electric properties of these two phases are also presented here.     

Anomalous paramagnetic behavior in the paramagnetic phase of La2/3−xYxCa1/3MnO3 studied through EPR and its relation to colossal magnetoresistance

Electron paramagnetic resonance on La_2/3−xY_xCa_1/3MnO₃ in the paramagnetic (PM) regime is presented for 0≤x≤0.133. The resonance linewidth (ΔH_pp) decreases with cooling, reaches the minimum at T_min, and then anomalously increases with further cooling toward T_c. Our analysis on ΔH_pp(T) below T_min shows that the anomalous PM behavior below T_min is due to the appearance of a ferromagnetic (FM) phase within the PM matrix caused by the applied magnetic fields. The correlation between the anomalous PM and the colossal magnetoresistance is discussed. We argue that both are caused by the phase segregation in which the compound is phase-separated into a mixture of FM and PM regions.     

Structural, chemical and magnetic investigations of polycrystalline Zn1−xMnxO

Present study reports the detailed structural and magnetic, as well as chemical analysis of polycrystalline Zn_1−xMn_xO (where x=0, 0.005, 0.01, 0.03, 0.05 and 0.1) samples synthesized by the high-temperature solid state reaction route. X-ray diffraction studies reveal the presence of secondary phase for higher Mn-doping concentrations (x≥0.03). Secondary phase formation having spinel structure is confirmed and reported as an evidence for the first time using transmission electron microscopy study. Chemical investigations using X-ray photoelectron spectroscopy showed the presence of Mn in two valence states. From the observed results we are of the opinion that Zn²⁺ ions, mainly present at or near grain boundaries, diffuse into manganese oxide to form a stable spinel phase Zn_xMn_3−xO₄ at or near the grain boundaries of ZnO/Zn_1−xMn_xO. Magnetization measurements did not show any magnetic transition down to 5 K.     

Photoluminescence studies of Cd1−xZnxS nanocrystals

Cd_1−xZn_xS (0?x?0.5) nanocrystals have been synthesized using a simple chemical precipitation method. Morphological and crystallographic analyses have been done using transmission electron microscope (TEM) and X-ray diffraction (XRD). Room temperature energy and time resolved photoluminescence spectra of these synthesized nanophosphors have been studied using xenon lamp spectroflourometer and high peak power, pulsed N₂-laser excitation, respectively. Photoluminescence spectra are composed of broad peaks ranging from green to red region of the visible spectrum. Important optical parameters: excited state lifetime, trap-depth and decay constant values have been calculated from recorded luminescence decay curves. These nanophosphors show typical lifetime shortening and high quantum yield with increasing concentration of Zn.     

Magnetic and electrical properties of new magnetic phase in Si1−xMnx crystals

We have investigated the magnetic and electrical transport properties of Si_1−xMn_x single crystals grown by the vertical Bridgman method. The alloys with Mn concentrations up to x=0.64 have weak ferromagnetic ordering around T_C∼30 K. However, Si_0.25Mn_0.75 alloys show weak ferromagnetic ordering at 70 K and antiferromagnetic ordering at 104 K, which is confirmed by magnetization and electrical transport studies.     

Phase diagram of Fe1−xCox ultrathin film

Concentration-driven reorientation phase transitions in ultrathin magnetic films of FeCo alloy have been studied. It is established that, in addition to the easy-axis and easy-plane phases, a spatially inhomogeneous phase (domain structure), a canted phase, and also an “in-plane easy-axis” phase can exist in the system. The realization of the last phase is associated with the competition between the single-ion anisotropy and the magnetoelastic interaction. The critical values of Co concentration corresponding to the phase transitions are evaluated, the types of phase transitions are determined, and the phase diagrams are constructed.     

Magnetoresistance and electron paramagnetic resonance study of La0.67−yYyBa0.33MnO3/LaAlO3 thin films

Magnetoresistive La_0.67−yY_yBa_0.33MnO₃/LaAlO₃ thin films were prepared by the sol-gel spin-coating method. Our resistivity and the electron spin resonance (ESR) measurements indicate that the main factor determining the metal-insulator transition temperature T_m is the cation disorder represented by the cation radii variance σ², and that ferromagnetic insulating regions coexist in the ferromagnetic metallic phase. In the paramagnetic phase, the dissociation energy of spin clusters and the polaron hopping energy obtained from the ESR intensity and linewidth also displayed a prominent dependence on σ². Polaron localization due to Jahn-Teller distortions appears to be responsible simultaneously for the decrease in the ferromagnetic order and for the increase in the orbital order.     

Hard magnetic properties of ErCo7−xCux [0.3⩽x⩽1] system

We report the observation of excellent hard magnetic properties on purely single phase ErCo_7−xCu_x compounds with x=0.3, 0.5, 0.8 and 1. Cu substitution leads to a decrease in the saturation magnetization, but enhances the uniaxial anisotropy in this system. The large anisotropy field (∼100 kOe) is attributed to the Er and the Co sublattices. Domain wall pinning effect seems to play a crucial role in determining the temperature and field dependences of magnetization in these compounds. The hard magnetic properties obtained at room temperature (RT) are comparable to the best results obtained in other RCo₇ based materials.     

Photoluminescence at room temperature in polycrystalline BaxSr1−xTiO3 ceramics obtained by solid-state reaction method

Stoichiometric Ba_xSr_1−xTiO₃ ceramics (x=0.7, 0.8, and 0.9) have been prepared by solid-state reaction method. It has been observed in our experiments that the photoluminescence (PL) spectra in such samples at room temperature were centered around 800 nm; the results have been found to differ from observations in current literature. An explanation of this phenomenon was given, and the origin of such photoluminescence is ascribed to certain defects existing in these compounds, such as oxygen vacancies.     

ESR investigation of the spin dynamics in (Gd1−xYx)2PdSi3

We report electron-spin resonance (ESR) measurements in polycrystalline samples of (Gd_1−xY_x)₂PdSi₃. We observe the onset of a broadening of the linewidth and of a decrease of the resonance field at approximately twice the Néel temperature in the paramagnetic state. This characteristic temperature coincides with the electrical resistivity minimum. The high-temperature behavior of the linewidth is governed by a strong bottleneck effect.     

Ferromagnetism in MnxGe1−x films prepared by magnetron sputtering

Mn_xGe_1−x thin films were prepared by magnetron sputtering with a substrate temperature of 673 K and subsequently annealed at 873 K. The X-ray diffraction (XRD) measurements showed that all samples had a single Ge cubic structure. No films showed clear magnetic domain structure under a magnetic force microscope (MFM). Atom force microscope (AFM) measurements showed that the films had an uniform particle size distribution, and a columnar growth pattern. X-ray photoelectron spectroscopy (XPS) measurements indicated that the valences of both Mn and Ge atoms increase with the Mn concentration. The resistance decreased with increasing temperature, suggesting that the films were typical semiconductors. Magnetic measurements carried out using a Physical Property Measurement System (PPMS) showed that all samples exhibited ferromagnetism at room temperature. There was a small concentration of Mn₁₁Ge₈ in the films, but the ferromagnetism was mainly induced by Mn substitution for Ge site.     

Ferromagnetism of epitaxially grown CaRu1−xMxO3 (M=Ti, Mn) films

Epitaxial thin films of CaRu_1−xM_xO₃ (M=Ti, Mn) were fabricated on a (0 0 1)-SrTiO₃ substrate by spin-coat method using organometallic solutions (metal alkoxides). Results of X-ray diffraction and transmission electron microscopy indicate that the epitaxial films were grown pseudomorphically so as to align the [0 0 l] axis of the CaRu_1−xM_xO₃ films perpendicular to the (0 0 1) plane of the SrTiO₃ substrate. Ferromagnetism and metal-insulator transition are induced by the substitution of transition metal ions. The occurrence of ferromagnetism was explained qualitatively assuming a TiRu₆ cluster model for CaRu_1−xTi_xO₃ film and a mixed valence model for CaRu_1−xMn_xO₃ film. Ferromagnetism was also observed for layered CaRuO₃/CaMnO₃ film and CaRuO₃/CaMnO₃/CaRuO₃/CaMnO₃ multilayer film and the magnetism was explained by an interfacial exchange interaction model with magnetic Mn³⁺, Mn⁴⁺, and Ru⁵⁺ ions.     

Theoretical study on the composition dependence of Ga8−nAsn (n=0-8) clusters

Using full-potential linear-muffin-tin-orbital molecular-dynamics (FP-LMTO-MD) method, we have investigated the dependence of GaAs clusters with eight atoms on composition. It is found that the ground state structures for Ga-rich and As-rich clusters are cube structures. As the ratio between gallium atoms and arsenic atoms is close to one, structural distortion become increasingly severe, or even the clusters adopt other geometrical configurations as their ground state structures. The energy gap E_g between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO), and the vertical electron affinity show a certain degree of even/odd alternation with cluster composition. Among nine Ga_8−nAs_n (n=0-8) clusters, only a few of clusters have different energy orders between the ionic and neutral isomers with large binding energy. Some ionic structures would change into other configurations due to severe structural distortion.     

Dependence of electronic properties on nitrogen concentration in GaAs1−xNx dilute alloys

Based on the pseudopotential scheme, the effect of nitrogen concentration on electronic properties of zinc-blende GaAs_1−xN_x alloys has been investigated for small amounts of N. The agreement between our calculated electronic band parameters and the available experimental data is generally reasonable. In agreement with recent experiment, we find that the incorporation of a few percent of N in the material of interest reduces substantially the fundamental band-gap energy and narrows the full valence band width. The electron and heavy hole effective masses are found to decrease rapidly when adding a concentration of nitrogen less than 0.005 in GaAs. This may increase the mobility of electrons and heavy holes providing new opportunities regarding the transport properties. The information derived from the present study shows that GaAs_1−xN_x (0?x?0.05) properties may have an important optoelectronic applications in infrared and mid-infrared spectral regions.     

Local chemical distribution and electronic structure of Ge1−xTx DMS single crystals (T=Cr,Mn, Fe)

The spatial concentration distribution and local electronic structure of ferromagnetic Ge_1−xT_x (T=Cr, Mn, Fe) DMS single crystals have been investigated by using scanning photoelectron microscopy (SPEM), X-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). It is found that doped T ions in Ge_1−xT_x crystals are chemically phase-separated, suggesting that the observed ferromagnetism arises from the phase-separated T-rich phases in Ge_1−xT_x.     

Percolation threshold near the second phase transition in diluted system of Gd2−x(La, Y)xIn

In this paper it is demonstrated that the second phase transition of Gd₂In intermetallic compound gets eliminated by diluting Gd_2−x(LaY)_xIn at a critical composition of x=0.5. The exchange coupling for intra-cluster interactions is estimated in the correlation ranges of 3.3 Å<R_C<3.6 Å (anisotropic source) and for inter-cluster interactions in the ranges of R_C>4 Å where the correlation length is defined as . The sign and strength of the exchange coupling are identified by the eigenvalues λ(k) and are obtained from zeros of the 4×4 matrix of J_ij^RR′ along the three directions of the reciprocal lattice for each dilution (x=0.25, 0.5, 0.75, 1). The transition temperature is calculated using the minimum eigenvalue λ_min (k=0, π) which agrees with the experiment. Magnetic field and temperature dependence of the magnetization and electrical resistivity measurements show that: (i) Elimination of the AFM phase is caused by breaking of some FM short-range exchange couplings, and (ii) Conduction electrons order antiferromagnetically at low temperatures and ferromagnetically at high temperatures.     

Dielectric properties of SrBi2−xPrxNb2O9 ceramics (x=0, 0.04 and 0.2)

SrBi_2−xPr_xNb₂O₉ (x=0, 0.04 and 0.2) ceramics were prepared by a solid state reaction method. X-ray diffraction analysis indicated that single-phase layered perovskite structure ferroelectrics were obtained. A relaxor behavior of frequency dispersion was observed among Pr-doped SrBi₂Nb₂O₉. The degree of frequency dispersion ΔT increased from 0 for x=0-7 °C for x=0.2, and the extent of relaxor behavior γ increased from 0.94 for x=0-1.45 for x=0.2. The substitution of Pr ions for Bi³⁺ ions in the Bi₂O₂ layers resulted in a shift of the Curie point to lower temperatures and a decrease in remanent polarization. In addition, the coercive field 2E_c reduced from 110 kV/cm for an undoped specimen to 90 kV/cm for x=0.2.     

Theoretical investigations on KClxBr1−x, KClxI1−x and KBrxI1−x: A first-principles study

Using first-principles total energy calculations within the full-potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic and thermodynamic properties of potassium halides (KCl_xBr_1−x, KCl_xI_1−x and KBr_xI_1−x), with x concentrations varying from 0% up to 100%. The effect of composition on lattice constants, bulk modulus, band gap and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the three alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and coworkers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram.     

The influence of substitution of Co for Mn on magnetism of the HoMn6−xCoxSn6 compounds (0⩽x⩽0.25)

We have studied the effects of Co substitution for Mn on the structure and magnetic properties of the HoMn_6−xCo_xSn₆ compounds (0?x?0.25) with HfFe₆Ge₆-type structure (space group P6/mmm) by X-ray powder diffraction and magnetization measurements. A monotonic decrease of the lattice parameters a and c is observed with increasing Co content. While the compounds with x=0 and 0.05 exhibit ferrimagnetism in the whole temperature range, the compounds with 0.1?x?0.15 show ferrimagnetism, helimagnetism and re-entrant ferrimagnetism with decreasing temperature. For the compounds with x=0 and 0.05, the spin reorientation temperature is observed. A metamagnetic transition from helimagnetic magnetic ordering to ferrimagnetism is observed for the compounds with x=0.1 and 0.2. The results are summarized in the HoMn_6−xCo_xSn₆ magnetic phase diagram.     

Proton activity and spontaneous strain of Cs3H(SeO4)2 in the phase transition at 369 K

We have investigated the origin of the change in proton activity in the phase transition at T_II-III (=369 K) in Cs₃H(SeO₄)₂ from the viewpoint of its ferroelasticity by using ¹H NMR and X-ray measurements. It is found that the second moment of the ¹H NMR absorption line rapidly decreases at T_II-III with increasing temperature. From this result, we conclude that the hopping motion of a proton, which is the precursor motion in the superprotonic phase, becomes more active above T_II-III. This result is consistent with the fact that the electrical conductivity in phase II is larger than that in phase III. Furthermore, it is also found that the spontaneous strain decreases abruptly at T_II-III. From these results, it is deduced that the decrease in the spontaneous strain at T_II-III causes the increase in the proton activity at T_II-III. In addition, it is deduced that the increase in proton activity and the decrease in the spontaneous strain at T_II-III are closely related with the appearance of the superprotonic phase transition at T_I-II (=456 K).