Papers by Keyword: Ferromagnetism

Abstract: Nickel nanowires were synthesised using a template free chemical method. The phase purity and crystallinity of the synthesized nanowires were studied. Nickel nanowires with diameters of approximately 100-120 nm have exhibited a saturation magnetisation value of 51 emu/g and coercivity of 300 Oe which makes them potential candidates for magnetic sensor applications.

Abstract: The influence of the Co doping on the properties of zinc oxide films was investigated. The films were grown by oxygen and water assisted metal organic chemical vapor deposition (MOCVD). The large positive magnetoresistance (PMR) was observed in Co doped films at low temperatures while the negative magnetoresistance was observed in undoped ZnO films deposited under the same conditions. The relative variation of resistivity was comparable in the films grow by oxygen and water assisted MOCVD although the resistivity and its temperature dependence were significantly different. The magnitude of PMR increases with an increase of the Co content and with the decrease of temperature.

Abstract: Transport, magnetotransport and magnetic properties of structures GaAs/InAs/GaAs with a InAs quantum dot (QD) layer have been investigated in the temperature interval 4.2<T<300 K. The structures were delta-doped by Mn from a one side to provide magnetic properties and by carbon from the other side to enhance a p-type conductivity. The ferromagnetic phase up to 400 K was detected by SQUID magnetometer. Anomalous Hall-effect was observed at low T. The role of additional disorder in conducting channel due to the QD layer was investigated. It is shown a principal role of a fluctuation potential of Mn layer separated from conducting QD layer by a spacer in anomalous transport properties of structures. The negative magnetoresistance was observed at low T due to the reduction of the spin-flip scattering by aligning spins by magnetic field.

Abstract: Co-doped rod-like ZnO particles with nominal Co doping concentration of 1 at% were synthesized by hydrothermal method and characterized by X-ray diffraction, field-emission scanning electron microscopy, photoluminescence and superconducting quantum interference device. The results show that the as-synthesized samples are pure hexagonal wurtzite structure without metallic Co or other secondary phases and display rod-like shape with smooth surface. The room temperature PL spectrum of the Co-doped rod-like ZnO particles exhibits a strong blue emission at 440 nm, a shoulder violet emission at 410 nm and a weak green emission centered at 550 nm. The magnetization measurements reveal that the Co-doped rod-like ZnO particles show ferromagnetic behavior at room temperature. The saturation magnetization and coercive field are 0.0125 emu/g and 45 Oe, respectively.

Abstract: The room-temperature ferromagnetism of Cr-doped cerium oxide (Ce_0.96Cr_0.04O_2δ) nanopowders synthesized using a sol-gel process is reported in this paper. XRD and Raman spectroscopy confirm that the Cr atoms successfully displaced some of the Ce atoms in the CeO₂ lattice without forming any impure phases. The results also confirmed that all calcined samples exhibited a single-phase fluorite structure. The crystallite size (as confirmed by XRD) and the particle size (as confirmed by Raman spectroscopy) increased as the calcination temperature increased. Magnetic measurements indicated that the room-temperature ferromagnetism of the sample was sensitive to the calcination temperature. When the calcination temperature increased, the saturation magnetization decreased while the coercivity increased, which corresponds to less dense and larger particles. The calcined sample at 400°C exhibited superior magnetic properties with the highest saturation magnetization (M_s) of 2.5 × 10^-2 emu/g (H_c ~ 1.27 kOe). The results of the Raman and X-ray photoelectron spectroscopies suggest that the nature of the observed room temperature ferromagnetism in the samples are likely a result of the oxygen vacancies induced by Cr-doping in CeO₂.

Abstract: Co-doped ZnO nanoparticles were fabricated by an electrodeposition method. The XPS results show Co ions have doped into the ZnO crystal lattices successfully. The as-grown sample has no ferromagnetism at room temperature. But after an ammine plasma treatment the room temperature ferromagnetism were detected on Co_0.04Zn_0.96O nanoparticles. The Hall measurement reveals after the treatment the resistivity increase by three orders of magnitude. Although the aspect conductivity is n type, some holes generated by N doping play an important role to induce the ferromagnetic properties for Co doped ZnO sample.

Abstract: Pure and Fe-doped CuO nanostructures with different weight ratios (0.5, 1.0, 1.5, and 2.0 at wt% of Fe) were prepared by a facile and fast microwave combustion method. The structure and morphology of the samples were investigated by X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy and vibrating sample magnetometry (VSM). The XRD patterns refined by the Rietveld method indicated the formation of single-phase monoclinic structure and also confirm that the Fe ions are successfully incorporated into the CuO crystal lattice by occupying Cu ionic sites. Interestingly, the morphology was found to change considerably from nanoflowers to nanorod and disk-shaped then to nanoparticles with the variation of Fe content. The optical band gap calculated using DRS was found to be 2.8 eV for pure CuO and increases up to 3.4 eV with increasing Fe content. Photoluminescence measurements also confirm these results. The magnetic measurements indicated that the obtained nanostructures are found to show a room temperature ferromagnetism (RTF) with an optimum value of saturation magnetization at 2.0 wt.% of Fe-doped CuO, i.e.1.2960 x 10 ^-3 emu/g.

Abstract: Ni-doped rod-like ZnO particles with doping concentration of 1 at.% were synthesized at 200°C by hydrothermal method and characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and superconducting quantum interference device (SQUID). The results show that the as-synthesized samples are pure hexagonal wurtzite structure without metallic Ni or other secondary phases and display rod-like shape with smooth surface. The magnetization measurements reveal that the Ni-doped rod-like ZnO particles show ferromagnetic behavior at room temperature. The saturation magnetization and coercive field are 0.0046 emu/g and 15 Oe, respectively.

Abstract: Successful synthesis of room-temperature ferromagnetic semiconductors, (Cu, Co) co-doped ZnO film is obtained by sol-gel method. It is found that the essential ingredient in achieving room-temperature ferromagnetism is Cu co-doping. By Hall-effect measurement a p-type conductivity was observed for the Cu co-doped films, which induced the room-temperature ferromagnetism.

Abstract: In this investigations the systems of the nanoparticles with competing exchange interactions are considered. The critical concentrations and possible types of magnetic states of particles in the case of direct exchange and RKKY interaction in the framework of the random interaction field method are determined. It is observed that in magnetic materials with the competition of the direct and indirect exchanges changing the type of ordering is possible at the change in the intensity of the indirect exchange under the influence of external factors.