Yongbing Xu

Planar carbon nanotube–graphene hybrid films for high-performance broadband photodetectors

Graphene has emerged as a promising material for photonic applications fuelled by its superior electronic and optical properties. However, the photoresponsivity is limited by the low absorption cross-section and ultrafast recombination rates of photoexcited carriers. Here we demonstrate a photoconductive gain of ∼105 electrons per photon in a carbon nanotube–graphene hybrid due to efficient photocarriers generation and transport within the nanostructure. A broadband photodetector (covering 400–1,550 nm) based on such hybrid films is fabricated with a high photoresponsivity of >100 A W−1 and a fast response time of ∼100 μs. The combination of ultra-broad bandwidth, high responsivities and fast operating speeds affords new opportunities for facile and scalable fabrication of all-carbon optoelectronic devices.

Graphene Q-switched 2.78 μm Er3+-doped fluoride fiber laser

We report a diode-pumped 2.78 μm Er3+-doped ZBLAN fiber laser passively Q switched by a graphene saturable absorber, which was directly deposited onto a fiber dichroic mirror by the method of optically driven deposition. Stable Q-switched operation with a pulse duration of 2.9 μs and a pulse energy of 1.67 μJ was achieved in a 10 m long gain fiber. The pulse duration was reduced to 1 μs when the gain fiber length was shortened to 2 m. This Letter demonstrates that graphene is a promising and reliable saturable absorber for mid-infrared pulse generation at 3 μm.

Magnetization reversal and magnetoresistance in a lateral spin-injection device

We have investigated the magnetization reversal and magnetoresistance (MR) behavior of a lateral spin-injection device. The device consists of a two-dimensional electron gas (2DEG) system in an InAs quantum well and two ferromagnetic (Ni80Fe20) contacts: an injector (source) and a detector (drain). Spin-polarized electrons are injected from the first contact and propagating through InAs are collected by the second contact. By engineering the shape of the permalloy film distinct switching fields (Hc) from the injector and the collector have been observed by scanning Kerr microscopy and MR measurements. Magneto-optic Kerr effect (MOKE) hysteresis loops demonstrate that there is a range of magnetic field (20–60 Oe), at room temperature, over which magnetization in one contact is aligned antiparallel to that in the other. The MOKE results are consistent with the variation of the magnetoresistance in the spin-injection device.

Patterning ferromagnetism in Ni80Fe20 films via Ga+ ion irradiation

We demonstrate that focused Ga+ ion irradiation can comprehensively modify the ferromagnetic properties of Ni80Fe20 thin films. Magneto-optic Kerr effect measurements at room temperature and magnetoresistance measurements at temperatures between 1.5 and 270 K characterized the irradiation effects. Irradiation steadily reduced the films’ room temperature coercivity, and a dose of 1.0×1016 ions/cm2 at 30 keV was found sufficient to cause a loss of ferromagnetism at room temperature in films of thickness up to 15.5 nm. In situ end-point detection and postirradiation atomic force microscopy confirmed that the sputtering which accompanied doses up to 1.0×1016 ions/cm2 did not compromise the protective caps on these Ni80Fe20 films. We therefore conclude that the modification of ferromagnetic properties occurred primarily because of direct Ga+ ion implantation. From these results, we speculate that focused Ga+ ion irradiation could be a convenient tool for the nanoscale patterning of magnetic properties in 3d tr...

Single crystal Fe films grown on InAs(100) by molecular beam epitaxy

Thin Fe films have been grown on InAs(100) by molecular beam epitaxy, and studied using in situ magneto-optical Kerr effect (MOKE), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Despite the large lattice mismatch between Fe and InAs, the growth of Fe on InAs at 175 °C was found to be epitaxial with the orientation relationship Fe(100)〈001〉‖InAs(100)〈001〉, as evidenced by LEED. STM images indicate that growth proceeds via a 3D Volmer–Weber mode. The magnetic hysteresis loops measured using in situ MOKE show a distinct cubic anisotropy with the easy axis along 〈001〉, the easy axis of bulk bcc Fe, which further confirms that well ordered single crystal Fe films have been stabilized on the InAs(100) substrate. Current–voltage measurements in the temperature range of 2.5–304 K show that Fe forms an ohmic contact on InAs. We propose that Fe/InAs is a suitable heterostructure for magnetoelectronic devices as, unlike Fe/GaAs, there is no Schottky barrier to electron transport.

Effect of oxygen deficiency on room temperature ferromagnetism in Co doped ZnO

We investigated the correlation between magnetization and oxygen vacancies in Zn0.95Co0.05O nanoparticles. Enhanced magnetizations were found in SiO2 nanopowders and carbon nanotubes (CNTS) treated Zn0.95Co0.05O, which are attributed to minimizing nanoparticle size and increasing oxygen vacancy concentration. After oxygen annealing, the magnetization of both non-treated Zn0.95Co0.05O and CNTS treated Zn0.95Co0.05O decreased sharply with the filling of the oxygen vacancies, while the SiO2 treated Zn0.95Co0.05O was influenced little as the amorphous SiO2 shell prevents the diffusion of oxygen into magnetic particles. It demonstrated that the ferromagnetism comes from the interfacial oxygen deficiency and is tunable by changing the oxygen vacancies.

Origin of room temperature ferromagnetism in MgO films

We report a systematic study of the crystallinity dependence of room-temperature ferromagnetism (RTFM) in pure MgO thin films prepared by pulsed laser deposition. A sequential transition from ferromagnetism to diamagnetism as a function of deposition temperature is observed. All the samples deposited from room temperature (RT) to 200 °C show clear RTFM, and the magnetization decreases monotonically with the increase of the substrate temperature, whereas the MgO film grown at 300 °C shows diamagnetism behavior like bulk MgO sample. The maximum saturation magnetization of 8 emu/cm3 is obtained for the MgO film deposited at RT, which degrades dramatically after crystallization under the annealing in both vacuum and air atmosphere. Further photoluminescence and X-ray photoelectron spectroscopies reveal that the ferromagnetism in the MgO thin films is correlated directly with the Mg vacancies.

Magnetic nanoscale dots on colloid crystal surfaces

We demonstrate that uniform, ordered, single-domain magnetic nanoscale dots can be fabricated on concentrated colloid surfaces. The substrate consists of compact silica nanosphere arrays grown on a glass wafer. Through the subsequent deposition and oxidation treatment of a Co film, monodisperse magnetic Co nanoscale dot arrays with controlled magnetic properties and size were obtained. We suggest that magnetic dots deposited on colloidal surfaces might open a way of developing artificially nanostructured materials for fundamental studies in nanomagnetism and for applications such as patterned magnetic recording media.

Spintronic materials and technology

Spintronic Materials and Characterizations Magneto-Optical Studies of Magnetic Oxide Semiconductors Synthesis and Characterization of Wide Band-Gap Semiconductor Spintronic Materials Magnetic Properties of (Ga,Mn)As-Based Magnetic Semiconductors Soft X-Ray Resonant Magnetic Scattering from Magnetic Nanostructures The Effect of Ru on Magnetization Switching and CPP-GMR Enhancement The Spin-Dependent Interfacial Transparency Spin Torque and Domain Wall Magneto Resistance Current-Driven Switching of Magnetization: Theory and Experiment Domain Wall Scattering and Current-Induced Switching in Ferromagnetic Wires-Experiment Domain Wall Magnetoresistance in Magnetic Nanowires-Theory Introduction to a Theory of Current-Driven Domain Wall Motion Spin-Injection and Spin Devices Silicon-Based Spin Electronic Devices: Toward a Spin Transistor Spin-LEDs: Fundamentals and Applications Spin Photoelectronic Devices Based on Fe and the Heusler Alloy Co2MnGa Electron Spin Filtering across a Ferromagnetic Metal/Semiconductor Interface Measured by Photoexcitation Ferromagnetic Metal/III-V Semiconductor Hybrid Spintronic Devices The Spin-Valve Transistor INDEX

Evidence of the defect-induced ferromagnetism in Na and Co codoped ZnO

The effect of Na concentration on the room-temperature ferromagnetism in Na and Co codoped ZnO diluted magnetic semiconductor (DMSs) was investigated. The ferromagnetic state was found to be stable below 5% doping of Na due to the exchange interaction via electron trapped oxygen vacancies (F-center) coupled with the magnetic Co ions. With large Na doping of up to 10%, a sharp reduction in the magnetization was observed, showing that the oxygen vacancy mediated antiferromagnetic state becomes predominant. The observed correlation between the Na concentration, the carrier concentration, and the magnetization demonstrated the effect of the defect in controlling the ferromagnetism in the ZnO-based DMS system.