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[en] On the basis of the nonlinear equations for self-generated magnetic fields, it is numerically shown that the magnetic fields self-generated are instable and may collapse, resulting in spatially highly intermittent flux fragment. Numerical results show that the enhanced magnetic flux has a strength about up to 10-2 Gauss in range about around 250-350 km in auroral zones with kilometric radiation (AKR), which correspond to estimated values in both the strength and characteristic scale by Mckean et al. [J. Geophys. Res. [Oceans] 96, 21055 (1991)]
[en] Highlights: • The adsorption and dissociation behavior of oxygen molecule on Ag (111) supported χ3 borophene sheets are revealed. • The stability of Ag (111) supported χ3 borophene is found to be partly in accordance with experimental results. • The migration of O atom on Ag (111) supported χ3 borophene is discussed. - Abstract: The superstructure of χ3 borophene on Ag (111) has recently been synthesized in experiment. In this work, we investigate its structural, electronic properties and the oxidation mechanism through first-principles calculations. We find the superstructure of χ3 borophene on Ag (111) maintain the planar characteristics, like its free-standing form, owing to the weakly interaction between adsorbate and substrate. Moreover, oxygen molecule can be spontaneously adsorbed on its superstructure in a manner of chemical adsorption. Importantly, the energy barrier of ∼0.35 eV for oxygen dissociation indicates its relative stability in ambient conditions compared with the active silicene. Furthermore, the mobility of O2-dissociation-induced O atom is poor at room temperature, implying the difficult migration of O atom on borophene surface. On the other hand, due to the strong BO bonding, desorption of O2-dissociation-induced O atoms on superstructure of χ3 borophene becomes impossible, ultimately leading to form the boron oxides.
[en] The magnetization behaviors in Sm–Ni–Fe–Cu alloys at low temperatures have been investigated. It was found that the hysteresis loops show wasp-waisted character at low temperatures, which has been proved to be related to the existence of multi-phases, the Fe/Ni soft magnetic phases and the CaCu_5-type hard magnetic phase. A smooth-jump behavior of the magnetization is observed at T>5 K, whereas a step-like magnetization process appears at T<5 K. The CaCu_5-type phase is responsible for such abnormal magnetization behavior. The magnetic moment reversal model with thermal activation is used to explain the relation of the critical magnetic field (H_c_m) to the temperature (T>5 K). The reversal of the moment direction has to cross over an energy barrier of about 6.6×10"−"1"5 erg. The step-like jumps of the magnetization below 5 K is proposed to be resulted from a sharp increase of the sample temperature under the heat released by the irreversible domain wall motion. - Highlights: • Two different magnetization mechanisms, controlled by temperature, have been found in the Sm–Ni–Fe–Cu alloys. The smooth-jump behavior of the magnetization is observed at T>5 K and the step-like magnetization process appears at T<5 K. • The magnetic moment reversal model with thermal activation has been successfully used to explain the relation of the critical magnetic field (H_c_m) to the temperature (T>5 K). The energy barrier for the reversal of the moment direction has been found to be about 6.6×10"−"1"5 erg. • The transition field for the step-like jumps is very strict, independent from the magnetic sweep rate. This is remarkably different from the similar step-like jump behavior in reference . • According to the SEM images and EDX analysis, two kinds of regions are found in the alloys. The Fe–Ni–Cu regions are surrounded by the 1:5 Sm–Ni–Fe–Cu regions and shows fish-bone like structure. An interesting thing is that the Fe–Ni–Cu regions are arranged in two directions with an angle of 90°.
[en] Sm_7_._5Y_2_._5Fe_9_0_−_xSi_x (x=0.0, 2.5, 5.0, 7.5 and 10) alloys have been prepared by arc melting method and equilibrium disordered Th_2Zn_1_7-type phases, (Sm,Y)_2_−_y(Fe,Si)_1_7_+_2_y, with relative lower rare-earth content than the ordered Th_2Zn_1_7-type phase, have been obtained. Compared to the ordered Th_2Zn_1_7-type structure, the X-ray diffraction (XRD) intensity of the superstructure lines of the (Sm,Y)_2_−_y(Fe,Si)_1_7_+_2_y decreases with the increase of the Si content and becomes zero for x=10. According to the refinement with the disordered Th_2Zn_1_7-type structure, the occupation rates of the R atoms at (3a) and (6c) sites tend to reach the same value with the increase of the Si content, and the lattice parameter a decreases while the lattice parameter c increases, leading to an increase of c/a. It was found that the atomic ratio of Fe(Si)/Sm(Y) in the disordered Th_2Zn_1_7-type structure increases with the increase of Si content and reaches a maximum value of 9.07 with x=10. The XRD diagrams of the magnetic aligned samples indicate that the easy magnetization direction (EMD) of the (Sm,Y)_2_−_y(Fe,Si)_1_7_+_2_y is in the a-b plane, and the change of the EMD in a-b plane has also been observed due to the Si preferred site occupation. The remanence ratios along the easy direction are higher than that along hard direction; however, all the remanence ratios are less than 0.5. The magnetocrystalline anisotropy constant K increases first and then decreases with increasing the Si content. The Curie temperature of Sm_7_._5Y_2_._5Fe_9_0_−_xSi_x alloys increases by about 65 K per Si. The saturation magnetization increases first and then decreases with a maximum of 135.5 emu/g observed for x=2.5 at room temperature. - Highlights: • Equilibrium disordered Th_2Zn_1_7-type phases (Sm,Y)_2_-_y(Fe,Si)_1_7_+_2_y have been obtained. • The atomic ratio of Fe(Si)/Sm(Y) increases with increasing Si content and reaches a maximum value of 9.07 with x=10. • The change of the EMD in a-b plane has been observed due to the Si preferred site occupation. • The magnetocrystalline anisotropy constant K increases first and then decreases with increasing Si content. • The Curie temperature of Sm_7_._5Y_2_._5Fe_9_0_-_xSi_x alloys increases by about 65 K per Si.
[en] Highlights: • The mechanical properties of the W_1_-_xTi_x alloys are calculated from DFT. • Ti alloying enhances the ductility of W metal substantially. • The mechanical strength of W-Ti alloys is slightly weaker than W while stronger than Ti. - Abstract: The effect of Ti concentration on the fundamental mechanical properties of W-Ti alloys has been studied from first principles calculations. The lattice constants, the cell volumes and the formation energies of the W_1_-_xTi_x (x = 0.0625, 0.125, 0.1875, 0.25, 0.5) alloys were calculated. It is shown that Ti alloying in bcc W lattice is thermodynamically favorable when the Ti concentration is lower than 25% and the W_0_._8_1_2_5Ti_0_._1_8_7_5 have the lowest formation energy. With the optimized geometry and lattice, the elastic constants are calculated and then the elastic moduli and other mechanical parameters are derived. Results show that although the mechanical strength of the W-Ti alloys is lower than that of pure W metal, it is much higher than that of pure Ti metal. On the other hand, the B/G ratio and the Poisson's ratio of the W-Ti alloys is much higher than that of pure W, and even higher than that of pure Ti, indicating that Ti alloying can improve the ductility of bcc W substantially.
[en] Highlights: • The interaction structure, solution energies, energy barriers and mechanical properties between C and the W–Ti lattice are calculated from DFT. • The effect of titanium doping on the carbon migration behavior is small, while the migration energy barrier is quite high, and the highest energy barrier is up to 1.60 eV. • Titanium doping tungsten can compensate the degradation of mechanical strength induced by carbon impurities. • The ductility of pure W metal and W–Ti alloys is improved substantially by carbon impurities from the B/G ratio and the Poisson's ratio ν. The effect of the improved ductility is more obvious in pure W than that in the W–Ti alloys. • In addition, substitutional C atom is better than interstitial C atom in terms of enhancing the ductility of pure W. - Abstract: We investigated the structural and mechanical properties, the solution and diffusion behavior of carbon atom in the W–Ti lattice from first-principles calculations. The single C atom is energetically most favorable sitting at the octahedral interstitial site (OIS) nearest neighboring to the Ti atom in the W–Ti lattice. The minimum solution energy is about 0.874 eV. It is shown that the effect of titanium doping on the carbon migration behavior is small, while the migration energy barrier is quite high, and the highest energy barrier is up to 1.60 eV. Based on the elastic constants analysis, titanium doping tungsten can compensate the degradation of mechanical strength induced by carbon impurities. On the other hand, it is found that the ductility of pure W metal and W–Ti alloys is improved substantially by carbon impurities from the B/G ratio and the Poisson's ratio ν. The effect of the improved ductility is more obvious in pure W than that in the W–Ti alloys. In addition, substitutional C atom is better than interstitial C atom in terms of enhancing the ductility of pure W.
[en] A comprehensive study of the structural and magnetic properties of the Ni_2Mn_1_._4_8Sb_0_._5_2 compound is presented using neutron diffraction and magnetic measurements. It is found that Mn atoms on 4a and 4b sites are ferrimagnetic coupled in the austenitic phase. At 300 K, the magnetic moments of Mn (4a) and Mn (4b) are determined to be 2.72(3) μ_B and − 2.67(5) μ_B, respectively. From 260 K to 4 K, the compound is in martensite phase and the magnetic moments of Mn (2a), Mn (2f), Mn (2b), Mn (2e) at 4 K are 2.2(5) μ_B, 2.3(5) μ_B, − 2.1(5) μ_B, and − 2.6(5) μ_B, respectively.
[en] We report on an efficient near-infrared downshifting in Eu"3"+/Yb"3"+ codoped glass ceramics containing Ba_4La_6(SiO_4)_6O crystals. The structural and luminescence properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), static and dynamic photoemission and excitation spectroscopy. After crystallization, the doping ions are found to be selectively incorporated into the precipitated oxyapatite crystals on La"3"+ sites, contributing to the remarkably enhanced visible emission of Eu"3"+ under 394 nm excitation. The Eu"3"+/Yb"3"+ codoped glass ceramics additionally exhibits efficient near-infrared luminescence of Yb"3"+ around 1000 nm upon photon excitation of Eu"3"+ within the 300–550 nm range. The reduced visible emission and decay time of "5D_0 state with Yb"3"+ codoping further confirm the energy transfer from Eu"3"+ to Yb"3"+, which is promoted due to the shortened distance between Eu"3"+ and Yb"3"+ within crystals. The maximum energy transfer efficiency is evaluated to be 61%. It is revealed that energy transfer process occurs predominantly through the cross relaxation of Eu"3"+("5D_0) + Yb"3"+("2F_7_/_2) → Eu"3"+("7F_6) + Yb"3"+("7F_5_/_2).