A First Principle Study of Co2CrZ (Z=In, Sn, Sb) Based on FP-LAPW Method

The structural and electronic properties of Heusler alloys Co₂CrZ (Z=In, Sn, Sb) have been studied using the generalized gradient approximation (GGA) and local spin density approximation (LSDA), respectively. Among the systems under investigation, Co₂CrSb gives 100 % spin polarization at E _F . Co₂CrSb is the most stable Half-Metallic Ferromagnets (HMFs) with an energy gap 0.25 eV at the Fermi level in the spin-down channel. We have also found that the increase in the total magnetic moment as Z goes from In to Sb. The calculated density of states (DOS) and band structures shows the half-metallic ferromagnets (HMF) character of Co₂CrSb.     

Optical and ESR investigation of borate glasses containing single and mixed transition metal oxides

The optical and electron spin resonance (ESR) spectra of barium borate glasses, containing the oxides of V, Fe and Cu separately and in mixed proportions, have been studied. The optical spectra of the single transition metal (TM) oxide glasses showed the usual features, while those for the mixed glasses showed single bands without showing individual features of the single TM oxide glasses. However, the linear plots of optical density against composition revealed the presence of two valence states for each TM element, and this was confirmed by ESR results as well. The ESR spectra of the mixed glasses showed a complicated interaction pattern for two different TM ions, in comparison with those of the glasses containing a single TM ion. For the Fe-V glasses, the progressively vanishing hyperfine structure of the VO²⁺ complex with increasing addition of iron oxide is discussed in terms of nuclear spin relaxation, cross-relaxation between two spin systems and spin diffusion within the vanadium spin system. The covalency of the VO²⁺ complex and the number of distorted Fe³⁺ ions were found to decrease with increasing addition of Fe₂O₃ replacing V₂O₅. Similar features were noted for the Cu-V glasses; the spectra of Cu-Fe glass also showed a strong interaction between two different TM ions. It has been suggested that all the possible four valence states (for a given mixed glass) from two different TM elements are present, and that pairing of two different TM ions from two dissimilar TM elements occurs, facilitating the formation of associates (e.g. V⁴⁺-O-Fe³⁺).     

Efficient Spin-Orbit Torque Switching of the Semiconducting Van Der Waals Ferromagnet Cr2Ge2Te6

Being able to electrically manipulate the magnetic properties in recently discovered van der Waals ferromagnets is essential for their integration in future spintronics devices. Here, the magnetization of a semiconducting 2D ferromagnet, i.e., Cr₂Ge₂Te₆, is studied using the anomalous Hall effect in Cr₂Ge₂Te₆/tantalum heterostructures. The thinner the flakes, hysteresis and remanence in the magnetization loop with out-of-plane magnetic fields become more prominent. In order to manipulate the magnetization in such thin flakes, a combination of an in-plane magnetic field and a charge current flowing through Ta—a heavy metal exhibiting giant spin Hall effect—is used. In the presence of in-plane fields of 20 mT, charge current densities as low as 5 × 10⁵ A cm^–2 are sufficient to switch the out-of-plane magnetization of Cr₂Ge₂Te₆. This finding highlights that current densities required for spin-orbit torque switching of Cr₂Ge₂Te₆ are about two orders of magnitude lower than those required for switching nonlayered metallic ferromagnets such as CoFeB. The results presented here show the potential of 2D ferromagnets for low-power memory and logic applications.     

From Local Moment EPR in Superconductors to Nanoscale Ferromagnets

Electron paramagnetic resonance (EPR) in metals has contributed a lot to the understanding of the electronic structure and magnetic properties in dilute alloys as well as in concentrated ferromagnets. We recall some pioneering work of the Kazan group and others, studying local moment EPR in superconductors. An SNS Josephson junction has been used as a microwave generator and as an EPR detector at once. EPR was also used to study the Kondo effect in the EPR g-shift and linewidth. Moreover, the high sensitivity of EPR (down to 10¹⁰ spins) allows to study single atomic layers of ferromagnets below and above the Curie temperature T _C as well as the spin fluctuations at T _C. The in situ ferromagnetic resonance (FMR) in ultrahigh vacuum (UHV) offers a unique possibility to study the interlayer exchange coupling (IEC) and spin dynamics of coupled ferromagnetic films. Furthermore, the magnetic resonance enables us to measure basic parameters of nanoscale magnets in absolute energy units (i.e., μeV/spin). The current status of the UHV-FMR in nanoscale ferromagnets will be discussed.     

First-Principles Investigation of Half-metallic Ferromagnetism in V-doped BeS, BeSe, and BeTe

We have investigated the electronic structure and half-metallic ferromagnetism in zinc blende phase of Be_1?x V _x M (M=S, Se, Te) at concentration x=0.125 by employing a first-principles calculations within the framework of density functional theory (DFT) based on the linearized augmented plane wave method (FP-LAPW), as implanted in the WIEN2k code with generalized gradient approximation functional proposed by Wu and Cohen (WC-GGA). The electronic properties exhibit half-metallic behavior. So the density of states shows the hybridization between the p (S, Se, Te) and 3d (V) states that creates the antibonding states in the gap, which stabilizes the ferromagnetic ground state associated with the double-exchange mechanism, whereas the spin polarized band structures depict half-metallic gap that increases from Be_0.875V_0.125S to Be_0.875V_0.125Se to Be_0.875V_0.125Te. These compounds are robust half-metallic ferromagnets with spin polarization of 100 % and predicted to be potential candidates for spin injection applications in spintronic devices. Therefore, our predictions require an experimental confirmation in the future.     

Coherent Dynamics of Localized Spins in an Inhomogeneous Magnetic Field

We studied the influence of the inhomogeneity of fringe fields originating from nanostructured ferromagnets on the coherent dynamics of localized Mn²⁺ spins in a (Zn,Cd,Mn)Se/ZnSe quantum well by time-resolved Kerr rotation. By studying hybrid systems with different geometry, we demonstrate that the spatially inhomogeneous fringe field results in two main consequences for the localized spin system: First, a temporal variation of the ensemble precession frequency is obtained, and second, a reduced ensemble spin dephasing time T ₂^* is observed in comparison to an unstructured reference area of the sample.     

Sol-gel preparation and properties of TiO2-P2O5 bulk glasses

Monolithic transparent and colorless, or Ti³⁺-free TiO₂-P₂O₅ glasses containing very large amounts of TiO₂ (up to 93 mol%) were successfully prepared by heat-treating the xerogels, which were made from titanium tetraisopropoxide and triethyl phosphate, through the sol-gel reaction. The density and refractive index n_632.8 nm of the sol-gel-derived glasses were higher than the melt-derived glasses of the corresponding compositions. The glasses of TiO₂ content of larger than 80 mol% seemed somewhat porous, but n_632.8 nm of these glasses was very high as 2.2-2.3. Higher density and higher n_632.8 nm than the melt-derived glasses were considered to be due to more abundance of six-fold coordinated Ti⁴⁺ ions.     

Ab-initio calculation of effective exchange interactions, spin waves, and Curie temperature in L21- and L12-type local moment ferromagnets

Employing first-principles electronic structure calculations in conjunction with the frozen-magnon method, we study the effective exchange interactions and spin waves in local moment ferromagnets. As prototypes we have chosen three L2₁-type full Heusler alloys Cu₂MnAl, Ni₂MnSn, and Pd₂MnSn, and the L1₂-type XPt₃ compounds with X = V, Cr, and Mn. We have also included CoPt₃ which is a usual ferromagnet. In all compounds due to the large spatial separation (～4 Å) of the magnetic transition-metal atoms, the 3d states belonging to different atoms overlap weakly and as a consequence the exchange coupling is indirect, mediated by the sp electrons. Calculated effective exchange parameters are long range and show RKKY-type oscillations. The spin-wave dispersion curves are in reasonable agreement with available experimental data. Using the calculated exchange parameters, we have estimated the Curie temperatures within both the mean-field and the random-phase approximations. In local moment ferromagents deviations of the estimated Curie temperature with respect to the available experimental data occur when the ground-state electronic structure calculations overestimate the values of the spin magnetic moments as in VPt₃.     

Magnetic properties for the transition-metal aluminides XAl2 (X = V,Cr, Mn,and Co): A first-principles study

We have investigated the magnetic properties of the transition-metal aluminides XAl₂ (X = V, Cr, Mn, and Co) in the MoSi₂ type structure by first-principles calculations. Considering the electron–electron correlations, we predict VAl₂ and CoAl₂ to be half-metallic ferromagnets. The transition-metal aluminides alloys, such as (CrFe)_0.5Al₂ and (MnFe)_0.5Al₂, obtained by substitution of half of the transition-metal atoms by other transition-metal atoms are nearly half-metals with high spin polarizations at the Fermi level. The density of states of these materials exhibit the features of a spin-up gap or a spin-down gap at the Fermi level. The total magnetic moments for these half-metals are integers and obey the Slater–Pauling rule of scaling linearly with the total number of the valence electrons in the unit cell in the form of M_t = N_t ? 28 (or M_t = 28 ? N_t). CrAl₂ and MnAl₂ are antiferromagnets, but their ferromagnetic spin configurations also show half-metallicity.     

Microstructure and properties of transparent glass-ceramics

Crystallization in glasses based on the ZnO-Al₂O₃-SiO₂ system nucleated with ZrO₂ have been studied by electron spin resonance. The glass crystallization has been monitored by observing the change in the coordination of Fe³⁺ ions present in the glasses as an impurity.     

Viscosity properties of sodium borophosphate glasses

The viscosity behavior of (1 − x)NaPO₃−xNa₂B₄O₇ glasses (x = 0.05-0.20) have been measured as a function of temperature using beam-bending and parallel-plate viscometry. The viscosity was found to shift to higher temperatures with increasing sodium borate content. The kinetic fragility parameter, m, estimated from the viscosity curve, decreases from 52 to 33 when x increases from 0.05 to 0.20 indicating that the glass network transforms from fragile to strong with the addition of Na₂B₄O₇. The decrease in fragility with increasing x is due to the progressive depolymerization of the phosphate network by the preferred four-coordinated boron atoms present in the low alkali borate glasses. As confirmed by Raman spectroscopy increasing alkali borate leads to enhanced B-O-P linkages realized with the accompanying transition from solely four-coordinated boron (in BO₄ units) to mixed BO₄/BO₃ structures. The glass viscosity characteristics of the investigated glasses were compared to those of P-SF67 and N-FK5 commercial glasses from SCHOTT. We showed that the dependence of the viscosity of P-SF67 was similar to the investigated glasses due to similar phosphate network organization confirmed by Raman spectroscopy, whereas N-FK5 exhibited a very different viscosity curve and fragility parameter due to its highly coordinated silicate network.     

First-Principle Investigations of Structural,Electronic, and Half-Metallic Ferromagnetic Properties in In1−xTM x P (TM = Cr,Mn)

First-principle calculations within the framework of density functional theory are employed to study the structural, electronic, and half-metallic ferromagnetic properties of In_1?x (TM) _x P (TM = Cr, Mn) at concentrations (x = 0.0625, 0.125, 0.25)of transition metal in zinc blende phase. The investigations of electronic and magnetic properties indicate that In_1?xTM _x P (TM = Cr, Mn) at x = 0.0625, 0.125, and 0.25 are half-metallic ferromagnets with 100 % magnetic spin polarization. On the one hand, the total magnetization is an integer Bohr magneton of 3 μ _B and 4 μ _B for In_1?xCr _x P and In_1?xMn _x P, respectively, which confirms the half-metallic feature of In_1?xTM _x P compounds. On the other hand, the densities of states of majority-spin states show that the large hybridization between 3p (P) and 3d (TM) partially filled states dominates the gap, which stabilizes the ferromagnetic state configuration associated with double-exchange mechanism. The band structures depict that half-metallic gap at x = 0.0625 is 0.404 eV for In_1?xCr _x P which is higher than 0.125 eV for In_1?xMn _x P. Therefore, the largest half-metallic gap in In_1?xCr _x P at low concentration x = 0.0625 reveals that Cr-doped InP seem to be a more potential candidate than that Mn-doped InP for spin injection applications in the field of spintronic devices.     

Half-Metallic Ferromagnetism in Strontium-Doped III–V: Ab Initio Calculations

First-principle calculations are employed by means of an all-electron full-potential linearized augmented plane wave to investigate the electronic structure and magnetic properties of AlP, AlAs, GaP, GaAs, InP, and InAs-based dilute magnetic semiconductors (DMS) with Sr impurities. It is shown that, in all the cases the ferromagnetic phase is energetically favored with respect to the paramagnetic one. In addition, these alloys are found to be half-metallic ferromagnets with a net total magnetic moment of 1.00 μ _B when they assume the zinc-blende structure at the equilibrium lattice constant. Ferromagnetism is induced by the spin polarization of the p shells of anions; the magnetic moment mainly comes from anion atoms surrounding the dopant atom, which is different from conventional DMS. These theoretical results make these materials interesting candidates for spin injection in spintronic devices.     

A relaxation trend in the electron spin resonance phenomena in copper tellurite glasses containing NiO,CoO and Lu2O3

Copper tellurite glasses containing NiO, CoO and Lu₂O₃ were prepared by the melt-quenching technique. The composition used was 65TeO₂-(35-x)CuO-xTMO (mol%), where TMO indicates NiO, CoO, Lu₂O₃, and for NiO- and CoO-doped glasses,x has the values 0, 0.5, 1 to 4, and for Lu₂O₃ doped glasses x=0 to 4. Electron spin resonance (ESR) spectra of all glasses were recorded at room temperature. The results on glasses doped with NiO, CoO and Lu₂O₃ are discussed in terms of oxidation-reduction, cross-relaxation and interelectronic repulsion processes, respectively. Cobalt oxide is found to be more effective in relaxing the ESR spectrum than nickel and lutetium oxides when substituted in copper tellurite glasses.     

Preparation of transparent oxyfluoroborate glass-ceramics containing CaF2〈Eu〉 crystals

The glass-forming region in the CaF₂-Bi₂O₃-B₂O₃ system has been determined for the first time. We have prepared glasses of various compositions, investigated their physicochemical properties, and examined the effect of rare-earth (La, Nd, Eu, Dy, Er, Tm, Yb) doping on the crystallization behavior of the glasses. Transparent glass-ceramics containing ～40-nm Ca_{1 ? x} Eu _x F₂ and Ca_{1 ? x} La _x F₂ crystallites have been obtained. The luminescence properties of the glasses and glass-ceramics have been studied.     

Thermoanalytical study of some fluoroberyllate glasses

Some glasses in the BeF₂-KF-CaF₂-AlF₃ (BeF₂ = 50 mol%) system were examined by thermoanalytical methods (DSC, TG) and powder X-ray diffraction to obtain glass transition and crystallization temperatures. Values of T_g varied from 165 to 265°C as the KF content decreased from 40 to 12.5 mol%. Those glasses with KF contents ? 30 mol% exhibited crystallization exotherms in the region of 280–300°C. The ability to clearly establish T_c decreased markedly for the better glass forming composition. Various phases crystallizing in these glasses as a function of temperature and time of heat treatment were identified.     

Electrical conductivity and ESR of lithium borate glasses containing mixed transition metal oxides

Electron spin resonance, optical spectra and d.c. electrical conductivity of 2Li₂O · 3B₂O₃ glasses doped with V₂O₅ and CuO were investigated. Spin Hamiltonian parameters of transition metal (TM) ions have been determined. The theoretical optical basicity _th of these glasses has been calculated and related to the spin-Hamiltonian parameters of TM ions. The variation of electrical conductivity with composition has been explained.     

Mechanochemical synthesis of SnO-B2O3 glassy anode materials for rechargeable lithium batteries

The xSnO·(100 ? x)B₂O₃ (0 ≦ x ≦ 80) glasses were successfully prepared by a mechanical milling technique. The glass with 40 mol% SnO showed the maximum glass transition temperature of 347°C. The SnO-B₂O₃ milled glasses consisted of both BO₃ and BO₄ units, and the fraction of BO₄ units was maximized at the composition of 50 mol% SnO. The electrochemical properties of the milled glasses were examined using a simple three electrodes cell with a conventional liquid electrolyte. The glasses with high SnO content exhibited high charge capacities more than 1100 mAh g^?1 and discharge capacities more than 700 mAh g^?1 at the first cycle. The SnO-B₂O₃ milled glasses proved to work as anode materials for rechargeable lithium batteries.     

Graphene/MoS2 organic glasses: Fabrication and enhanced reverse saturable absorption properties

Graphene/MoS₂ (G/MoS₂) composites were synthesized for the first time by a hydrothermal method, and then were dispersed in methyl methacrylate (MMA) to fabricate organic glass by a casting method. Here, the poly(methylmethacrylate) (PMMA) host can be obtained by the polymerization of MMA. The nonlinear absorption (NLA) properties of the G/MoS₂/PMMA organic glasses were investigated by using an open-aperture Z-scan technique. The experimental results showed that the G/MoS₂/PMMA organic glasses (the NLA coefficient β, was up to 2110 cm/GW) exhibited enhanced reverse saturable absorption (RSA) properties compared to G/PMMA (β = 242 cm/GW) and MoS₂/PMMA (β = 365 cm/GW) organic glasses. The enhanced mechanism of RSA was analyzed in terms of the energy-level diagram of graphene and MoS₂. The G/MoS₂/PMMA organic glasses have very promising applications in optical devices such as optical limiters and optical shutters.     

Formation and thermo-assisted stabilization of luminescent silver clusters in photosensitive glasses

Various photo-induced silver luminescent centres were obtained in photosensitive zinc and phosphate glasses containing silver ions after exposure to gamma or ultraviolet nanosecond pulsed-laser radiation. Gamma-irradiation of the glasses results mainly in the formation within the glass of electron-trapped and hole-trapped silver centres as evidenced by optical absorption, luminescence and electron spin resonance spectroscopies. For the highest irradiation doses silver clusters are obtained. Under ultraviolet nanosecond pulsed-laser exposure similar species are generated along the beam propagation direction as proven by the analogous optical and luminescence signatures. In this case for high irradiation doses few silver clusters are created. The evolution of the luminescence spectra with respect to the temperature and to the duration of the heat treatment after ultraviolet nanosecond pulsed-laser irradiation evidences the presence of potential barriers determining the stability limits of some species such as the Ag²⁺ hole-trapped centres or the Ag_m^x+ clusters composed of silver ions and silver atoms. A heat treatment of several hundreds of degrees is identified as a the key parameter for tailoring the optical properties and controlling the formation of Ag_m^x+ clusters in the photosensitive glasses.