Search

The authors report on the excitation wavelength dependence of terahertz emission from n-InN and bulk p-InAs pumped with femtosecond pulses tunable from 800 to 1500 nm. The terahertz amplitude, normalized to pump and probe power, from both narrow band gap semiconductors remains relatively constant over the excitation wavelength range. In addition, terahertz radiation from In- and N-face InN samples with bulk carrier concentrations ranging from 10(17) to 10(19) cm(-3) is also investigated, showing a strong dependence of terahertz emission on bulk carrier concentration. The experimental results agree well with calculations based on drift-diffusion equations incorporating momentum conservation and relaxation. (c) 2006 American Institute of Physics.

The authors demonstrate the impact of growth kinetics on the surface and structural properties of N-face InN grown by molecular beam epitaxy. Superior surface morphology with step-flow growth features is achieved consistently under In-rich conditions in a low-temperature region of 500-540 degrees C. Remarkably, off-axis x-ray rocking curve (omega scans) widths are found to be independent of the growth conditions. The band gap determined from optical absorption measurements of optimized InN is 0.651 eV, while photoluminescence peak emission occurs at even lower energies of similar to 0.626 eV. Hall measurements show room temperature peak electron mobilities as high as 2370 cm(2)/V s at a carrier concentration in the low 10(17) cm(-3) region. Analysis of the thickness dependence of the carrier concentration demonstrates a n-type surface accumulation layer with a sheet carrier concentration of similar to 3x10(13) cm(-2). (c) 2006 American Institute of Physics.

We study the effect of different deposition conditions on the properties of In-polar InN grown by plasma-assisted molecular beam epitaxy. GaN buffer layers grown in the Ga-droplet regime prior to the InN deposition significantly improved the surface morphology of InN films grown with excess In flux. Using this approach, In-polar InN films have been realized with room temperature electron mobilities as high as 2250 cm(2)/V s. We correlate electron concentrations in our InN films with the unintentionally incorporated impurities, oxygen and hydrogen. A surface electron accumulation layer of 5.11x10(13) cm(-2) is measured for In-polar InN. Analysis of optical absorption data provides a band gap energy of similar to 0.65 eV for the thickest InN films. (c) 2006 American Institute of Physics.

Controllable remanent states have been studied on the microstructured magnetic tunnel junction (MTJ) rings through magnetoresistance measurements. These rings were designed accordingly with an outer/inner diameter of 2/1 and 1/0.5 [micro]m to reveal two and one metastable states, respectively, during the magnetization reversal process on the free layer. The distinct magnetoresistance levels based on the tunneling magnetoresistance effect are associated with the relative alignment of magnetization of free layer and pinned layer. As a result, four and three controllable remanent magnetic states on the free layer were manipulated by ramping external magnetic fields, applied in the biasing direction, with various field ranges, giving rise to four and three stable mangetoresistance values at zero field. These results may provide a great potential in magnetic multibit memory applications using ring-shaped cells. Index Terms--Magnetic tunneling junction (MTJ), magnetoresistance, multibit application, ring-shaped cells.

The structure and magnetic properties of a series of [Fe.sub.3-x][Mg.sub.x][O.sub.4] (0

Microstructured magnetic tunnel junction ring with outer/inner diameter of 2/1 [micro]m has been fabricated to investigate the angular dependence of magnetoresistance during magnetization reversal process. The minor loop of magnetoresistive curve reveals four distinct resistance levels associated with four magnetization configurations within the free layer throughout the magnetization reversal. The magnetoresistance decreases with increasing angle between applied external field and biasing direction, which is resulted from the relative alignment of total magnetization of pinned and free layer. An extra feature appeared in the minor loop when the external field is transverse to the biasing direction; this can be attributed to a vortex-pair formation/annihilation in the free layer. Furthermore, a series of schemes of magnetic configurations of pinned and free layers are illustrated to explain the routes of minor loops. Index Terms--Magnetization reversal, magnetoresistance, magnetoresistive devices.

Microstructured magnetic tunnel junction rings have been fabricated by a top-down technique combining electron beam lithography and ion milling process. Four-terminal magnetoresistance measurements and magnetic force microscopy were used to successfully explore a four-transition process within the free layer throughout the magnetization reversal. Various magnetization configurations were identified to be the onion state, vortex-pair state, vortex state, vortex-core state, and reverse onion state. In addition, the various durations of each magnetic state observed in the magnetoresistance curve can be utilized for the study of a coupling effect between the pinned layer and the free layer. Index Terms--Fabrication, magnetic force microscopy (MFM), magnetization reversal, magnetoresistance.

[Fe.sub.1-x][Co.sub.x][O.sub.y] (O [less than or equal to] x [less than or equal to] 1,1 [less than or equal to] y [less than or equal to] 4/3) mixture oxide films are grown, with epitaxial quality, on MgO(001) substrates by molecular beam epitaxy. The X-ray diffraction result along (001) plane is similar to [Fe.sub.3][O.sub.4] for the films with x < 0.5 (Fe rich) and is similar to CoO for the films with x > 0.5 (Co rich). An abrupt change, over 2%, on the lattice spacing is observed around Fe/Co ~ 1(x = 0.5) suggesting a phase transformation from spinel to rocksalt. A fast suppression of the saturation magnetization is found markedly different from the bulk materials as the Co ratio increases. Crystalline anisotropy and anomalous magnetic properties are affected substantially by Co ions. These unique magnetic behaviors of Fe-Co-O films are believed to be related to cation distribution in spinel structure originated by the low-temperature process of thin-film fabrication. Index Terms--Ferrite films, magnetic anisotropy, magnetic epitaxial layers.

Critical analysis on the magnetic properties of synthetic ferrimagnet (SyF), Ta/MgO/CoFeB/Ru/ CoFeB/MgO/Ta, is demonstrated via both static and dynamic techniques. With the Ru thickness being 2.3 nm, the coupling between two CoFeB layers becomes orthogonal, which can be used for spin-transfer-torque nano-oscillator (STNO). The fitting of angular dependent ferromagnetic resonance (FMR) allows the precise determination of magnetic anisotropy of each CoFeB layer, the relative magnetizations and the exchange field near the frequency of STNO applications. In addition, the mechanism of resonance broadening at out-of-plane direction is identified to be magnetic inhomogeneity by fitting the angular dependent linewidth of FMR spectra, which provides indispensable information for the future design of STNO devices. [ABSTRACT FROM AUTHOR]

A study was conducted to analyze the microscopic interfacial structure of Co/Cu/Co/NiO multilayers on lattice matched substrates grown by molecular beam epitaxy. Growths were carried out continuously by introducing or terminating oxygen plasma. Samples supporting various crystalline orientations were examined using angular dependent magnetoresistance measurements. Experimental results indicated the existence of some intermediate antiparallel spin states.

Magnetization of a series of Epitaxial [Fe.sub.3][O.sub.4]/MgO multilayers grown on MgO(100) substrates by oxygen-plasma-assisted molecular-beam epitaxy has been studied. The Curie temperature decreases with decreasing the thickness of each [Fe.sub.3][O.sub.4] layer. The saturation magnetization of all the multilayer samples seems to be lower than that of the 200 nm thick [Fe.sub.3][O.sub.4] film.

We demonstrate here a double-shifted exchange bias which spontaneously exists in the as-deposited [Pt/Co]n/IrMn multilayers without resorting to any 'external forces,' such as deposition with field, field annealing, (zero) field cooling with specific procedures. The observed results indicate that the exchange bias might occur spontaneously when a ferromagnetic layer is in contact with an antiferromagnetic layer; herein it is tentatively attributed to interactions among perpendicular anisotropy of [Pt/Co]n multilayers, its intrinsic demagnetization field, and magnetic anisotropy of the IrMn layer. The thickness and the magnitude of magnetic anisotropy of the ferromagnetic multilayer are crucial for the adjustment of asymmetric double-shifted exchange bias which can also be tuned to exist in the in-plane or out-of-plane directions. [ABSTRACT FROM AUTHOR]

The perpendicular magnetic anisotropy of a series of top MgO/CoFeB/Ta layers is studied. Similar to the bottom Ta/CoFeB/MgO structure, the critical thickness of CoFeB is limited in a range of 1.1-1.7 nm. However, the cap layer shows much sensitive effect. Not only the type of material is crucial, but the thickness of the cap layer also affects the magnetic anisotropy. The perpendicular anisotropy of a 1.2 nm-thick CoFeB can only exist with the capping Ta thickness less than 2 nm. The magnetic characterizations, including the magnetic remanence and coercivity, also show strong dependence on the Ta thickness. The diffusion of Ta into CoFeB layer is considered to play an important role, which could explain changes in perpendicular anisotropy and related magnetic responses. In addition, the asymmetric role of Ta layer in the top structure and bottom structures is also discussed. [ABSTRACT FROM AUTHOR]

The complex dielectric permittivities ([variant_greek_epsilon]′,[variant_greek_epsilon]″) of RMn2O5 (R=Tb, Dy, and Er) are measured in the temperature range of 78–328 K with frequencies ranging from 100 Hz to 1 MHz. A large real part of the dielectric constant [variant_greek_epsilon]′ (40–140) is observed at room temperature in all three samples studied. These values are higher than the peak value found at the ferroelectric transition of RMn2O5 single crystals. Our results reveal that high dielectric permittivities are related to thermally activated dipolar motions with activation energies of ∼300 meV and a relaxation frequency of ∼3×1012 Hz. The possible relation between the high dielectric permittivity and the size effect of R ions based on the spin-phonon interaction is suggested. [ABSTRACT FROM AUTHOR]

Several x-ray diffraction and magnetization measurements are conducted to explain the synthesis and the structural and magnetic properties of the [Mn.sub.2.5][Co.sub.0.5][O.sub.4] tetragonal spinel thin films. The magnetic short ordering exhibited by the films is also studied.

The complex dielectric permittivity in the paraelectric phase of polycrystalline R[Mn.sub.2][O.sub.5] (R = Tb, Dy and Er) with a few R of different ionic radii is measured. The possible relation between the high dielectric permittivity and the size effect of R ions based on the spin-phonon interaction is described.

The structure of the so-called 'vortex' domain walls in soft magnetic nanoparticles is discussed. A qualitative model accounting for the energies of such a domain wall is provided.

The structure and energetics of domain walls in the limit dominated by the magnetostatic interaction, achieved in strips whose width and thickness substantially exceed the exchange length lambda, are discussed. The simplest domain wall in the regime dominated by magnetostatic forces is found to be composed of two -1/2 edge defects and a vortex, in accordance with observations and numerics.

A series of Fe3O4/Mn3O4 superlattices with various modulation wavelengths in which the thickness of the Fe3O4 layer is fixed to be 4 nm and varies the thickness of Mn3O4 from 1 to 16 nm are discussed. When the layer thickness of Mn3O4 is less than 4 nm the domain-wall-like spin configuration no longer exists and a parallel magnetic state at the interface of F3O4/Mn3O4 is observed.

The effect of anisotropy on compensation point in Fe3O4/Mn3O4 superlattice grown on MgO(110) and its in-plane angular-dependent magnetic response is investigated. The study shows that the compensation point and spin-flop phase are orientationally dependent in an antiparallel-coupled magnetic layer system, which has at least one constituent component with strong anisotropy.