J. R. Zhang

Relation between electroluminescence and photoluminescence of Si+-implanted SiO2

The electroluminescence (EL) from Si+ implanted SiO2 thin film prepared by thermal oxidation was compared with photoluminescence (PL) properties. Both EL and PL spectra indicate that the luminescence originate from the same three luminescence bands around 470, 600, and 730 nm. Annealing at temperatures below and above 1000 °C makes the 470 and the 730 nm bands dominate in PL spectra, respectively. The 600 nm band, which is weaker in PL, is usually the strongest in EL. The relative contributions from different luminescence bands to EL depend on annealing, but are independent of current density. The different excitation mechanisms of the 470, 600, and 730 nm luminescence bands give rise to the discrepancy between EL and PL.

The study of low-field positive and negative magnetic entropy changes in Ni43Mn46−xCuxSn11 alloys

A series of Ni43Mn46−xCuxSn11 (x=1, 2, and 3) alloys was prepared by the arc melting method. The martensitic transition shifts to a higher temperature with increasing Cu concentration. The isothermal magnetization curves around the martensitic transition temperature show a typical metamagnetic behavior. Under a low applied magnetic field of 10kOe, positive values of magnetic entropy change around the martensitic transition temperature are 14.1, 18.0, and 15.8J∕kgK for x=1, 2, and 3, respectively. While in the vicinity of the Curie temperature of the austenitic phase, these negative values are 1.1, 1.0, and 0.9J∕kgK for x=1, 2, and 3, respectively. The origin of the large entropy changes and the potential application for Ni43Mn46−xCuxSn11 alloys as a working substance for magnetic refrigeration are discussed.

Synthesis and magnetostriction of melt-spun Pr1−xTbx(Fe0.6Co0.4)2 alloys

Pseudobinary Pr1−xTbx(Fe0.6Co0.4)2 (0⩽x⩽0.4) cubic Laves single phases have been synthesized by melt spinning and subsequent annealing. Their structure, magnetic properties and stability have been investigated. The composition, at which the anisotropy of Pr1−xTbx(Fe0.6Co0.4)2 is compensated, is close to x=0.1. The spontaneous magnetostrictions λ111 of Pr0.9Tb0.1(Fe0.6Co0.4)2 and Pr0.8Tb0.2(Fe0.6Co0.4)2 are larger than 1500×10−6 and 1900×10−6, respectively. Pr1−xTbx(Fe0.6Co0.4)2 (0.1⩽x⩽0.4) ribbon-based materials with 3% epoxy resin combine high magnetostriction with significant magnetic coercivity. Pr0.9Tb0.1(Fe0.6Co0.4)2 is a promising magnetostrictive material.

Photoluminescence study of defects in Si+ ion implanted thermal SiO2 films

Point defects and structure damages in Si-implanted thermal SiO2 films were examined by photoluminescence (PL) spectra, electron spin resonance spectra, and infrared absorption spectra. Under ∼5 eV excitation, the as-implanted film had two PL bands peaked at 4.3 and 2.4 eV, respectively. After thermal annealing, the peak energy of the low-energy band shifted from 2.4 to 2.7 eV with the increase of the annealing temperature to 1100 °C, and its intensity and width, and the relative ratio between the intensities of the two PL bands changed also. The change of the ratio, and the width and peak position of the low-energy band via annealing temperature was consistent with the recovery of the implantation-induced structure damage such as densification and distortion of silicon tetrahedra. We propose that the two PL bands are due to neutral oxygen vacancies (NOVs), and the PL characteristics of the NOV defects are related to the structure damage of the SiO2 network.

Effect of Mo concentration on the phase composition and magnetic properties of Nd8(Fe,Mo)86B6 nanocomposite magnets

The phase composition and magnetic properties of Nd8Fe86−xMoxB6 (x=0, 1, 2, and 3) nanocomposite magnets prepared by melt spinning have been investigated by x-ray powder diffraction, transmission electron microscopy, thermomagnetic analysis, and vibrating sample magnetometer measurements. The optimal ribbons were found to be mainly composed of the α-Fe phase and the Nd2Fe14B phase. The average crystalline size of α-Fe is 14–18 nm. In the optimally quenched samples, the α-Fe phase content decreases with increasing Mo content; at the same time, an increase of coercivity and a decrease of energy products are observed. The hysteresis loops of the optimal ribbons show a single-phase hard magnetic behavior. The reduced remanence ratio is always above 0.66. The remanence enhancement is due to the exchange coupling between the soft and hard magnetic phases in these ribbons. The Curie temperature of the Nd2Fe14B phase in the samples decreases with increasing Mo content. The optimal hard magnetic properties of rema...

Crystallization dynamics and magnetoresistance of perovskite-like manganate synthesized by mechanical alloying

Nanocrystalline ferromagnetic La0.7 Ca0.3 MnO3 perovskites with magnetoresistance effect have been successfully prepared by mechanical alloying. Thermal transformation of the amorphous phase, resulting from ball milling, to perovskite structure was studied by differential scanning calorimetry. Following the law of mass action, we discuss the crystallization dynamics of the amorphous phase. The activation energy for crystallization transformation is calculated to be about + 1.25 kJ g-1 . The characterization of resistivity (T ) for La0.7 Ca0.3 MnO3 perovskites has also been investigated. At low temperature T , (T ) has a direct-proportion dependence on T 2 . With increasing annealing temperature, the slope of the -T 2 curve decreases. The temperature dependent magnetoresistance effect at temperature far below the Curie temperature can be well expressed as the equation / 0 = p 1 - p 2 T 3/2 - p 3 T 5/2 .

Structure and magnetic properties of melt-spinning Pr(Fe0.6Co0.4)2 alloys

Pr(Fe0.6Co0.4)2 ribbons were prepared by melt spinning with different wheel speeds from 35 to 45 m/s. Their structure, magnetic properties, and thermal stability are investigated. At a wheel speed of 35 m/s, the ribbon consists of a mixture of Pr(Fe,Co)2 cubic Laves phase and some noncubic phases. An almost Pr(Fe,Co)2 nanocrystalline single phase with a Curie temperature of 305 °C is obtained at a wheel speed of 40 m/s. Except for Pr(FeCo)2 phase a small amount of amorphous phase is observed with increasing wheel speeds to 45 m/s. Pr(Fe,Co)2 phase becomes unstable and decomposes above 770 °C. The resin-bonded Pr(Fe,Co)2 nanocrystalline phase which is obtained at a wheel speed of 40 m/s combines high magnetostriction (λ∥−λ⊥=140 ppm), with significant coercivity, iHc=5 kOe.

High-coercivity Sm2(Fe,Al,Mo)17C1.5 ribbons with addition of Mo

The effects of Mo additions on the structure and magnetic properties of Sm2Fe(15.5−x)MoxAl1.5C1.5 alloys have been investigated by means of x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and magnetic measurements. The XRD analyses show that as-cast alloys consist mainly of a 2/17-type carbide and a considerable amount of α-Fe. Rapidly quenched, the Mo-containing alloys form a nanocrystalline structure with an average grain size of 35–50 nm, while the amount of the α-Fe phase is greatly decreased. The magnetic hardening of the Mo-containing alloys can be achieved by direct quenching. A coercivity exceeding 1.35 T is obtained for the Sm2Fe14.9Mo0.6Al1.5C1.5 ribbon spun at 40 m/s. These results reveal that the addition of Mo is effective in improving the coercivity.

Structure and magnetic properties of melt-spun PrFe10Mo2Nx compounds

The structure and magnetic properties of PrFe10Mo2Nx compounds prepared by melt spinning have been investigated. A single-phase structure of ThMn12-like has been obtained in ribbons quenched at velocities vs below 40 m/s. The desired ThMn12 structure is considerably more stable for Pr(Fe, Mo)12 compounds than for Nd(Fe, Mo)12 compounds. Only at higher speeds than 50 m/s the PrFe10Mo2 ribbons quench into TbCu7-like compound of which Curie temperature is about 580 K. Annealing temperatures in excess of 1023 K are required to form ThMn12 crystal structure for the ribbons melt spun at 50 m/s. Upon nitrogenation at 723 K, the Curie temperatures increase to 663 and 580 K for TbCu7- and ThMn12-like nitrides, respectively. A coercivity of 3.3 kOe with a magnetization up to 118 emu/g at a field of 20 kOe has been obtained in the present ribbons.

Structure and magnetic properties of PrFe10VxMo2−x compounds and their nitrides

We have succeeded in synthesizing PrFe10VxMo2−x compounds (x=0–1.5) and their nitrides with ThMn12-type structure. The phase formation and magnetic properties have been investigated by x-ray diffraction, differential thermal analysis, and magnetic measurements. It is found that PrFe10V2 compound with ThMn12-type structure does not form and that PrFe10VxMo2−x compounds with ThMn12-type structure can be formed in the as-cast sample (x=0) or by annealing the as-cast samples in a lower temperature (x=0.5–1.5). The substitution of Mo for V in the compounds results in a decrease of the magnetization and Curie temperatures. The PrFe10VxMo2−xNy with x=0–1.5 has a Tc about 416–265 °C. The hard magnetic properties of PrFe10VMoNy magnets prepared by hydrogenation-disproportionation-desorption-recombination technique have been assessed.