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[en] Two series of Co-Sn doped M-type barium hexaferrites were fabricated using a co-precipitation method. Hexaferrites having the nominal composition BaFe12O19 were soaked at various temperatures (950, 1000, 1050, 1100. 1150 and 1200 .deg. C) for four h. The particle sizes of the resulting ferrites were seen to increase as the soaking temperatures were raised. A second series having the composition BaFe12-2xSnxCoxO19 (x = 0.0, 0.25, 0.50, 0.75 and 1.00) were also fabricated using one soaking temperature (1000 .deg. C). EDX (Energy dispersive X-ray) measurements were used to determine the actual chemical compositions of the specimens. The coercivities of the specimens of both series were measured with a vibrating samples magnetometer. We observed that the coercivities of the undoped series decreased as the soaking temperature increased and that the coercivities of the doped series increased as the impurity concentration increased.
[en] Magneto transport of carriers with a spin-dependent gap in a ferromagnetic-gated bilayer of graphene is investigated. We focus on the effect of an energy gap induced by the mismatch of the exchange fields in the top and bottom layers of an AB-stacked graphene bilayer. The interplay of the electric and exchange fields causes the electron to acquire a spin-dependent energy gap. We find that, only in the case of the anti-parallel configuration, the effect of a magnetic-induced gap will give rise to perfect spin filtering controlled by the electric field. The resolution of the spin filter may be enhanced by varying the bias voltage. Perfect switching of the spin polarization from + 100% to −100% by reversing the direction of electric field is predicted. Giant magnetoresistance is predicted to be easily realized when the applied electric field is smaller than the magnetic energy gap. It should be pointed out that the perfect spin filter is due to the layer-dependent exchange energy. This work points to the potential application of bilayer graphene in spintronics. (paper)
[en] The kinetics of the nanocrystallization of amorphous Fe81B13.5Si3.5C2 ribbon is studied. The changes in the microstructures and magnetic properties of ribbons annealed at 425 and 495 deg. C for 0.5-10 h were investigated using an X-ray diffractometer (XRD), Moessbauer spectroscopy (MS), differential scanning calorimeter (DSC) and vibrating sample magnetometer (VSM). The changes in the surface morphology were observed by a changed atomic force microscope (AFM). The XRD patterns and the Moessbauer spectrums show the formation of nanocrystallites of α-Fe(Si), Fe-B, Fe3C and Fe3Si of different grain sizes when annealed at different temperatures. The nanocrystallization kinetics of the Fe81B13.5Si3.5C2 ribbon are described by an Avrami growth curve with an exponent values of 1.34 and 1.01 for the isothermal annealing at 425 and 495 deg. C, respectively. AFM topography pictures and surface image show that the density of the microstructure and the size of the grain increase as higher annealing temperatures are used
[en] Biomimetic mineralization is a method to form natural materials. We have synthesized hydroxyapatite (HAP) by using an organic template of polyvinyl alcohol (PVA) and sodium dodecyl sulfate (SDS) as the surfactant. The X-ray diffractometer (XRD) patterns show the sizes of the HAP particles to be in the nanosize (15-30 nm) range. The chemical functional groups of the HAP were investigated by Fourier transform infrared (FT-IR) which shows the formation of the hydroxyl group (OH)-1 and the phosphate group (PO4)-3. The transmission electron microscopy (TEM) images show the particles to be in the range of 20-30 nm. The SDS looks like globular sphere, while the crystals of the HAP in the SDS/PVA appear as polyhedron crystal. The average size of each globular sphere is about 3-5 μm. As-prepared HAP samples were studied for their bioactive property by immersing them in a stimulated body fluid (SBF). After soaking them in SBF for one week, the SEM images show the accumulation of calcium carbonate or calcium phosphate SBF on surface of the HAP. This indicates that there might be an increased bioactive activity on the surface of the HAP prepared by this method
[en] The magnetic properties of the dilute magnetic semiconductor (DMS) are due the existence of two competing interactions, a direct ferromagnetic interaction and an indirect antiferromagnetic interaction. This is well established in the Zn1-xMnxO DMS, but is controversy in the Zn1-xCoxO DMS. To gain insights, a series of Co-substituted ZnO NRs (x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) have been fabricated using a low-temperature hydrothermal method. The magnetization of these Co-doped ZnO NPs was measured with a vibrating sample magneto-meter. Expressing the magnetization results in units of Bohr magnetons per Co ions, it is found that the ferromagnetic contributions become more dominant as more magnetic Co ions are doped into the ZnO NPs. X-ray diffraction, energy dispersive X-ray, scanning electron microscopy, and photoluminescence measurements were done to characterize the Co:ZnO NPs.
[en] The bioceramics, hydroxyapatite (HAP), is a material which is biocompatible to the human body and is well suited to be used in hyperthermia applications for the treatment of bone cancer. We investigate the substitution of iron and manganese into the hydroxyapatite to yield ceramics having the empirical formula Ca9.4Fe0.4Mn0.2(PO4)6(OH)2. The samples were prepared by the co-precipitation method. The formation of the nanocrystallites in the HAP structure as the heating temperatures were raised to obtain a glass-ceramic system are confirmed by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction (ED) and electron spin resonance (ESR). TEM images show the core/shell structure of the nanoparticles, with the core being formed by the ferrites and the shell by the hydroxyapatite. The ED patterns indicate the nanoparticles formed at 500 deg. C have an amorphous structure while the nanoparticles formed at 1000 deg. C are crystalline. ESR spectroscopy indicated that the Fe3+ ions have a g-factor of 4.23 and the Mn2+ ions have a g-factor of 2.01. The values of the parameters in the spin Hamiltonian which describes the interaction between the transition metal ions and the Ca2+ ions, indicate that the Mn2+ ion substitute into the Ca2+ sites which are ninefold coordinated, i.e., the Ca(1) sites
[en] The exchange bias fields in FeCo nanoparticles are investigated. A series of Fe1−xCox ( x =0.3, 0.5, and 0.7) NPs were fabricated using the hydrothermal method. XRD studies determined that the structures of the first two NPs were body-centered cubic (BCC) while that of the third was of a disordered structure having a few small disordered BCC crystallites. SEM images of the first two showed that these nanocrystals have well-structured pyramid shapes with planar surfaces on them. FC and ZFC magnetization curves showed the temperature dependences of the magnetization of the first two FeCo NPs were different from that of the third FeCo NPs. The hysteresis loops measured at 300, 150, and 10 K showed horizontal shifts of several of the loops. The exchange bias fields at 10 K of the three FeCo NPs were small but in two of them, the Fe0.7Co0.3 and the Fe0.3Co0.7 NPs, the fields were large at 150 K. The exchange bas field in the first NPs remained large as the temperature was raised to 300 K. HE for the last FeCo NPs became small again at 300 K.
[en] Iron-rich permalloy FexNi1−x nanoparticles (NPs) were fabricated using a low-temperature wet chemistry method involving the hydrothermal growth of the NPs. Three Fe–Ni NPs (Fe0.5Ni0.5, Fe0.625Ni0.375, and Fe0.83Ni0.17) were fabricated and studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). After the initial formations, the three Fe–Ni NPs were annealed at 200, 400, and 600 °C; the XRD showed that all three FeNi NPs had a bcc structure (not the fcc structure of the standard Fe20Ni80 permalloy). The SEM images showed that the NPs had spherical shapes. The VSM measurements showed that none of the Fe–Ni nanoparticles were superparamagnets but were ferromagnets. The heat treatments cause the work needed to the flip the spin (coercivity (Hc)) to increase and the values of the saturation magnetizations to fluctuate.
[en] The conductance in a N-I-MgB2 tunnel junction is studied for the case where the c-axis of the MgB2 is orientated perpendicular to the interface. The thick barrier limit is taken. It is assumed that the electric current is composed of two currents, one which describes the tunneling in the π-band in the MgB2 and the other, the tunneling into the σ-band. The Bogoliubov-de Gennes equations are used to determine the nature of the quasi-particles in the different regions of the junctions. An exact expression for the normalized conductance in the c-axis direction is obtained by applying the usual boundary conditions at the two interfaces of the junction. The numerical evaluations lead to results which are in excellent agreement with the published result of Cucolo et al. [A.M. Cucolo, F. Giubileo, D. Roditchev, Physica A 339 (2004) 112]