A model to design light emitting diodes matrix driven by constant current source

It is always desirable to operate LEDs within its designed maximum temperature for maximized life. For LEDs driven by a constant current source it is possible to improve LED life by putting more LEDs in parallel to share current and reduce temperature, but the cost would increase. This paper proposes a general LED model to estimate the relationship between temperature, operating current, luminous flux, life and reliability of light emitting diodes. This model can direct LED matrix design with respect to the required parameters. The model would help lighting designers to consider the driving current and number of LED in a lamp matrix in order to achieve the desired light flux, efficacy and life with desired cost. A method involved power electronic technique is proposed to form a dynamic matrix that can response to the changing ambient temperature. This allows the engineer to build the real desired matrix flexibly.published_or_final_versio

Shear-strain-induced Spatially Varying Super-lattice Structures on Graphite studied by STM

We report on the Scanning Tunneling Microscope (STM) observation of linear fringes together with spatially varying super-lattice structures on (0001) graphite (HOPG) surface. The structure, present in a region of a layer bounded by two straight carbon fibers, varies from a hexagonal lattice of 6nm periodicity to nearly a square lattice of 13nm periodicity. It then changes into a one-dimensional (1-D) fringe-like pattern before relaxing into a pattern-free region. We attribute this surface structure to a shear strain giving rise to a spatially varying rotation of the affected graphite layer relative to the bulk substrate. We propose a simple method to understand these moire patterns by looking at the fixed and rotated lattices in the Fourier transformed k-space. Using this approach we can reproduce the spatially varying 2-D lattice as well as the 1-D fringes by simulation. The 1-D fringes are found to result from a particular spatial dependence of the rotation angle.Comment: 14 pages, 6 figure

Edge effect on thermally excited mag-noise in magnetic tunnel junction sensors

Session AP - Magnetic Tunnel Junctions and Spin Valves (Poster Session): no. AP-14This journal issue contains selected papers from the 2012 International Magnetics (INTERMAG) ConferenceThermally excited magnetic noise (mag-noise) has gradually become a major concern in magnetic tunnel junction sensors. By conducting micromagnetic simulation, the spatial distribution of thermal mag-noise in the free layer (FL) was obtained under various hard bias (HB) field and applied field. It was demonstrated that the edges are the main contributor of thermal mag-noise in the FL. This result could be explained by the nonuniform distribution of the stiffness field around the edges. It was also found that both HB field and applied field could suppress the thermal mag-noise in edges. A relatively high applied field will decrease the influence of HB field on mag-noise in the edges. © 2012 IEEE.published_or_final_versionThe IEEE International Magnetics Conference (INTERMAG 2012), Vancouver, BC., 7-11 May 2012. In IEEE Transactions on Magnetics, 2012, v. 48 n. 11, p. 2831-283

Capacitance effect on the oscillation and switching characteristics of spin torque oscillators

We have studied the capacitance effect on the oscillation characteristics and the switching characteristics of the spin torque oscillators (STOs). We found that when the external field is applied, the STO oscillation frequency exhibits various dependences on the capacitance for injected current ranging from 8 to 20 mA. The switching characteristic is featured with the emerging of the canted region; the canted region increases with the capacitance. When the external field is absent, the STO free-layer switching time exhibits different dependences on the capacitance for different injected current. These results help to establish the foundation for capacitance-involved STO modeling.published_or_final_versio

Realization of ultra-wide resonance detection regime of spin-torque diode radio-frequency detector by utilizing tilted fixed-layer magnetization

A novel spin-torque diode radio-frequency detector structure based on tilted fixed-layer magnetic tunnel junction (MTJ) is proposed in this study. The MTJ sandwich structure is composed of NiFe free layer, FePt fixed layer, and MgO interlayer. In order to sense RF signals of different frequencies, both the spin-transfer-torque effect and the alternating-voltage-controlled magnetic anisotropy effect are integrated into the simulation. Results demonstrate that tilting the fixed-layer magnetization can drastically increase the resonance detection regime.postprin

Magnetic-field-sensing mechanism based on dual-vortex motion and magnetic noise

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Linear Phase Tuning of Spin Torque Oscillators Using In-Plane Microwave Fields

Session FA: Spin Dynamics and MicromagneticsWe demonstrate numerically and analytically that a nano-pillar spin torque oscillator (STO), operating either with in-plane or out-ofplane free-layer precession, locks to a microwave field ( ) having the same frequency as the STO. By varying the spatial direction of the microwave field, we further show the preferred phase shift ( ) between the STO and can be tuned in a linear fashion. We explain this phenomenon by using a magnetic-energy-based analysis. Our results provide a way to synchronize serially connected STOs by tuning the phase shift of each individual STO with external microwave field, which may enhance the locking efficiency, the locking range, and the output power of the serially connected STOs.published_or_final_versio

Thermally Excited Mag-Noise in Ferromagnetic Ring Structures

Session PE: Spin Dynamics and MicromagneticsAs the dimension of magnetic devices drastically decreases to nanometer range, thermally excited mag-noise gradually becomes the dominant noise source. Thermally excited mag-noise plays an important role in ferromagnetic ring structures. By conductingmicromagnetic simulation, the saturated state, triangle state, half triangle state, onion state, and vortex state are explored and studied, respectively. The mag-noise calculation shows that triangle state is the main reason for the mag-noise exhibiting tendency in both the low-frequency range and high-frequency range in relaxed state, while the onion state explains why a noise peak appears in high-frequency range in relaxed state. Meanwhile, it is proved that the area of the ferromagnetic rings is not the determining factor for the mag-noise distribution in saturated state. These results offer the theoretical framework for explaining the relation between domain structure and mag-noise, which is conducive to the future application of ferromagnetic ring structures as magnetic random access memory elements.published_or_final_versio