Jinke Tang

Extraordinary Hall effect and ferromagnetism in Fe-doped reduced rutile

Room-temperature ferromagnetism is observed in reduced rutile TiO2−δ by Fe doping. The epitaxial films grown by pulsed-laser deposition are carefully examined by x-ray diffraction, transmission electron microscopy, and magnetic and transport measurements. The films exhibit the extraordinary Hall-effect and thin-film magnetic shape anisotropy. The magnetic moments and anticipated Curie temperatures of the films rule out Fe particles, iron oxides, and Ti–Fe oxides as possible sources for the observed magnetic signals. The carriers of the Fe-doped reduced rutile are p-type, with a carrier density of 1×1022/cm3. This room-temperature dilute magnetic semiconductor should find potential applications in spintronics.

An Intense Green/Yellow Dual-Chromatic Calcium Chlorosilicate Phosphor Ca3SiO4Cl2 : Eu2 + –Mn2 + for Yellow and White LED

A series of intense green/yellow phosphors Ca 3 SiO 4 Cl 2 :Eu 2+ , Mn 2+ was synthesized by a high-temperature solid-state reaction. Their luminescent properties were characterized by means of powder diffuse reflection, photoluminescence excitation and emission spectra, and lifetime and temperature-dependent emission spectra in the temperature range of 10-450 K. The phosphors Ca 3 SiO 4 Cl 2 :Eu 2+ , Mn 2+ show intense broad absorption bands between 250 and 450 nm, matching well with the near-ultraviolet (380-420 nm) emission band of InGaN-based chips, and exhibit two dominating bands situated at 512 and 570 nm, ascribed to the allowed 5d → 4f transition of the Eu 2+ ion and the 4 T 1g ( 4 G) → 6 A 1g ( 6 S) transition of the Mn 2+ ion, respectively. The lifetime of the Eu 2+ ion decreases with increasing the concentration of the Mn 2+ ion, strongly supporting an efficient energy transfer from Eu 2+ to Mn 2+ . By combining with near-ultraviolet (∼395 nm) InGaN chips, intense yellow light-emitting diodes (LEDs) with a much lower ultraviolet light leakage were successfully fabricated based on the Ca 3 SiO 4 Cl 2 :Eu 2+ , Mn 2+ phosphor, and intense white LEDs were made based on a blend of blue chlorophosphate phosphor and the green/yellow phosphor Ca 3 SiO 4 Cl 2 :Eu 2+ , Mn 2+ . The color coordinate, correlated color temperature T c , general color-rendering index R a , and luminous efficiency of the fabricated white LEDs are (0.3281, 0.3071), 6065 K, 84.5, and 11 lm/W, respectively.

Temperature- and magnetic-field-induced magnetization reversal in perovskite YFe0.5Cr0.5O3

Perovskite YFe0.5Cr0.5O3 exhibits magnetization reversal at low applied fields due to the competition between the single ion magnetic anisotropy and the antisymmetric Dzyaloshinsky–Moriya interaction. Below a compensation temperature (Tcomp), a tunable bipolar switching of magnetization is demonstrated by changing the magnitude of the field while keeping it in the same direction. The present compound also displays both normal and inverse magnetocaloric effects above and below 260 K, respectively. These phenomena coexisting in a single magnetic system can be tuned in a predictable manner and have potential applications in electromagnetic devices.

False heavy fermions

Abstract False indications of heavy fermion behavior can arise from non-magnetic atom disorder (NMAD) in a compound in which the Ce(U) atom occupies a periodic lattice. The NMAD introduces a varying electronic environment around the CE(U) ions which causes a spin glass behavior and leads to a large C/T peak near 1 K. The existence of low lying crystal field levels in Ce and U compounds can also lead to large heat capacities which can be interpreted, incorrectly, as heavy fermion behavior. Several examples of each kind of false heavy fermions are described.

Magnetic properties of nanocrystalline Fe{sub 3}O{sub 4} films

Nanocrystalline magnetite Fe3O4 films of about 180 nm thick have been deposited on Si(100) substrates by pulsed laser deposition. Zero-field-cooled magnetization shows clearly the Verwey transition near 120 K by an abrupt change, which is absent from the field-cooled magnetization. This is correlated to its hysteresis curves where the loops remain open until a high field of 2 T. The magnetization does not saturate in field 2 orders of magnitude higher than its coercive field. Such behaviors may result from the existence of antiphase domains. Antiphase boundaries inside the grains are clearly observed with transmission electron microscopy. Negative magnetoresistance of about 12% has been observed near 120 K in a field of 9 T.

Characterization of the natural barriers of intergranular tunnel junctions: Cr2O3 surface layers on CrO2 nanoparticles

Cold-pressed powder compacts of CrO2 show large negative magnetoresistance (MR) due to intergranular tunneling. Powder compacts made from needle-shaped nanoparticles exhibit MR of about 28% at 5 K. Temperature dependence of the resistivity indicates that the Coulomb blockade intergranular tunneling is responsible for the conductance at low temperature. In this letter we report direct observation and characterization of the microstructure of the intergranular tunnel barriers, using transmission electron microscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). A very thin native oxide layer with a thickness of 1–3 nm on the surface of CrO2 powders has been observed. The composition and crystal structure of this surface layer has been determined to be Cr2O3 by XPS and XRD. The dense and uniform Cr2O3 surface layers play an ideal role of tunnel barriers in the CrO2 powder compacts.

Room-temperature ferromagnetism in manganese doped reduced rutile titanium dioxide thin films

MnxTi1−xO2−δ (x=0.02–0.12) thin films grown on α-Al2O3 substrates by pulsed-laser deposition have been investigated. X-ray diffraction and transmission electron microscopy results indicate that the films are single phase and reduced rutile-type. Superconducting quantum interference device magnetometer measurements show the films are ferromagnetic at room temperature with nonzero coercivity up to 170 Oe. The saturation magnetization of the reduced films is as high as 0.83 μB per Mn atom at room temperature. The temperature dependence of the resistivity shows semiconducting behavior with p-type carriers. The nature of the p-type conduction and its significance to the ferromagnetism are discussed.

Ferromagnetism and transport properties of Fe-doped reduced-rutile TiO2−δ thin films

We have investigated FexTi1−xO2−δ (x=0.02, 0.06, and 0.08) thin films grown on α-Al2O3 substrates by pulsed-laser deposition. X-ray diffraction results indicate that the films are single phase and of reduced-rutile type. Detailed microstructural observations reveal no measurable magnetic impurities in the films. Vibrating sample magnetometer measurements show the films are ferromagnetic at room temperature with coercivity ranging from 340 to 770 Oe. The temperature dependence of the resistivity shows nearly metallic behavior at room temperature but semiconducting behavior at lower temperatures. The extraordinary Hall effect with coercivities similar to those in magnetic hysteresis curves was observed at room temperature. The carriers are p type with a carrier density of about 1022/cm3.

Origin of colossal dielectric permittivity of rutile Ti0.9In0.05Nb0.05O2: single crystal and polycrystalline

In this paper, we investigated the dielectric properties of (In + Nb) co-doped rutile TiO2 single crystal and polycrystalline ceramics. Both of them showed colossal, up to 104, dielectric permittivity at room temperature. The single crystal sample showed one dielectric relaxation process with a large dielectric loss. The voltage-dependence of dielectric permittivity and the impedance spectrum suggest that the high dielectric permittivity of single crystal originated from the surface barrier layer capacitor (SBLC). The impedance spectroscopy at different temperature confirmed that the (In + Nb) co-doped rutile TiO2 polycrystalline ceramic had semiconductor grains and insulating grain boundaries, and that the activation energies were calculated to be 0.052 eV and 0.35 eV for grain and grain boundary, respectively. The dielectric behavior and impedance spectrum of the polycrystalline ceramic sample indicated that the internal barrier layer capacitor (IBLC) mode made a major contribution to the high ceramic dielectric permittivity, instead of the electron-pinned defect-dipoles.

Pulsed laser deposition of CdSe Quantum dots on Zn2SnO4 nanowires and their photovoltaic applications.

In this work we report a physical deposition-based, one-step quantum dot (QD) synthesis and assembly on ternary metal oxide nanowires for photovoltaic applications. Typical solution-based synthesis of colloidal QDs for QD sensitized solar cells involves nontrivial ligand exchange processing and toxic wet chemicals, and the effect of the ligands on carrier transport has not been fully understood. In this research using pulsed laser deposition, CdSe QDs were coated on Zn(2)SnO(4) nanowires without ligand molecules, and the coverage could be controlled by adjusting the laser fluence. Growth of QDs in dense nanowire network structures was also achieved, and photovoltaic cells fabricated using this method exhibited promising device performance. This approach could be further applied for the assembly of QDs where ligand exchange is difficult and could possibly lead to reduced fabrication cost and improved device performance.