arXiv subjects

J. Lu

Publications and source records attributed to J. Lu.

85 records · Page 5Linked to original sources

arXiv2008

Propagation of Exchange Bias in CoFe/FeMn/CoFe Trilayers

CoFe/FeMn, FeMn/CoFe bilayers and CoFe/FeMn/CoFe trilayers were grown in magnetic field and at room temperature. The exchange bias field $H_{eb}$ depends strongly on the order of depositions and is much higher at CoFe/FeMn than at FeMn/CoFe interfaces. By combining the two bilayer structures into symmetric CoFe/FeMn($t_\mathrm{FeMn}$)/CoFe trilayers, $H_{eb}^t$ and $H_{eb}^b$ of the top and bottom CoFe layers, respectively, are both enhanced. Reducing $t_\mathrm{FeMn}$ of the trilayers also results in enhancements of both $H_{eb}^b$ and $H_{eb}^t$. These results evidence the propagation of exchange bias between the two CoFe/FeMn and FeMn/CoFe interfaces mediated by the FeMn antiferromagnetic order.

cond-mat.mtrl-sci↗

arXiv2007

Euler equation of the optimal trajectory for the fastest magnetization reversal of nano-magnetic structures

Based on the modified Landau-Lifshitz-Gilbert equation for an arbitrary Stoner particle under an external magnetic field and a spin-polarized electric current, differential equations for the optimal reversal trajectory, along which the magnetization reversal is the fastest one among all possible reversal routes, are obtained. We show that this is a Euler-Lagrange problem with constrains. The Euler equation of the optimal trajectory is useful in designing a magnetic field pulse and/or a polarized electric current pulse in magnetization reversal for two reasons. 1) It is straightforward to obtain the solution of the Euler equation, at least numerically, for a given magnetic nano-structure characterized by its magnetic anisotropy energy. 2) After obtaining the optimal reversal trajectory for a given magnetic nano-structure, finding a proper field/current pulse is an algebraic problem instead of the original nonlinear differential equation.

cond-mat.mes-hall↗

arXiv2007

Evidence for the intrinsic ferromagnetism of semiconductor (Ga,Cr)As revealed by magnetic circular dichroism

To clarify whether or not (Ga,Cr)As is an intrinsic diluted magnetic semiconductor, a systematic study of the magnetic circular dichroism (MCD) was carried out for a series of (Ga,Cr)As epilayers grown by the low-temperature molecular-beam epitaxy technique. The present work provides unambiguous evidence for the intrinsic ferromagnetism of the (Ga,Cr)As epilayers with all the necessary properties in accordance with that of a diluted magnetic semiconductor, especially the hysteresis characteristics, which is opened up in the magnetic field dependence of MCD.

cond-mat.mtrl-sci↗

arXiv2007

Giant magnetoelectric effect under high bias fields in cylindrical bilayered Ni-PZT composites

For conventional plate and disc layered magnetoelectric (ME) composites, the ME effect appeared under low bias magnetic fields, which caused by line magnetostriction. In this paper, we present a new structure ME composites, cylindrical bilayered composites. Unexpectedly, the ME effect also appeared under high magnetic fields in this cylindrical bilayered Ni-lead zirconate titanate (PZT) composite. We thought this ME effect caused by volume magnetostriction. The results showed that the ME voltage coefficient of the cylindrical bilayered composite of Ni-PZT was the sum of aE(l) caused by line magnetostriction under low bias fields and aE(w) induced by volume magnetostriction under high fields. At the resonance frequency, aE,A(w) increased linearly with bias magnetic field HDC to 30 V/cm Oe at HDC =8 kOe, and was much larger than aE,A(l) at Hm=0.6 kOe. The novel characteristic of ME voltage coefficient increased linearly with the rise of HDC after 1 kOe in axial mode makes the cylindrical composite a promising candidate for application in high magnetic field sensor.

cond-mat.mtrl-sci↗

arXiv2007

Shape demagnetization effect on layered magnetoelectric composites

Magnetoelectric (ME) voltage coefficient was always considered independent on in-plane shape of plate layered magnetostrictive-piezoelectric composites due to the oversimplification in ME theoretical models. In this article, we present that in fact the ME voltage coefficient is largely dependent on the in-plane shape owing to shape demagnetization effect on magnetostrictivity. Theoretical analysis and experimental results both indicate that ME voltage coefficient changes notably with the variation of in-plane sizes (length and width) of layered ME properties. ME coefficient increases with the rise of in-plane sizes, and different aspect ratio will also result in different ME coefficient. Proper design of in-plane shape will greatly promote the development of ME devices.

cond-mat.mtrl-sci↗

arXiv2007

Preliminary Results on Vibration Damping Properties of Nanoscale-Reinforced Composite Materials

The focus in this paper is an analysis of existing state of the arts directed toward the development of the next generation of vibration damping systems. The research work concentrates on an investigation related to nanoparticles/fibres/tubes-reinforced materials and coatings dynamic characterization and modeling of the fundamental phenomena that control relationships between structure and damping/mechanical properties of the materials. We simulated composite materials using finite element and mesh free methods, using a hollow shell representation of the individual nanotube/fiber. Results of the research work will provide a platform for the development of nanoparticle-reinforced damping materials that are light-weight, vibration and shock resistant. The outcome of the research work is expected to have wide-ranging technical benefits with direct relevance to industry in areas of transportation (aerospace, automotive, rail), electronics and civil infrastructure development.

cond-mat.mtrl-sci↗

arXiv2006

Comparison of room-temperature multiferroics in Bi4Fe2TiO12 film and bulk

It is reported that both dielectricity and magnetism at room temperature have been appreciably improved in Bi4Fe2TiO12 film compared with that of Bi4Fe2TiO12 bulk. X-ray diffraction profiles reveal similar crystalline nature and random orientation of the two but X-ray photoelectron spectroscopy (XPS) experiments indicate 1.4 eV lower binding energy of core-state O 1s in film relative to those of bulk, so the improvement of multiferroics in film are attributed to the oxygen vacancies and high fraction of interface. The results have promising applications in multifunctional integrated devices.

cond-mat.mtrl-sci↗

arXiv2006

Magnetodielectric effect of Bi6Fe2Ti3O18 film under an ultra-low magnetic field

Good quality and fine grain Bi6Fe2Ti3O18 magnetic ferroelectric films with single-phase layered perovskite structure have been successfully prepared via metal organic decomposition (MOD) method. Results of low-temperature magnetocapacitance measurements reveal that an ultra-low magnetic field of 10 Oe can produce a nontrivial magnetodielectric (MD) response in zero-field-cooling condition, and the relative variation of dielectric constants in magnetic field is positive, i.e., MD=0.05, when T<55K, but negative with a maximum of MD=-0.14 when 55K<T<190K. The magnetodielectric effect appears a sign change at 55K, which is due to transition from antiferromagnetic to weak ferromagnetic; and vanishes abruptly around 190K, which is thought to be associated with order-disorder transition of iron ion at B site of perovskite structures. The ultra-low-field magnetodielectric behaviour of Bi6Fe2Ti3O18 film has been discussed in the light of quasi-two-dimension unique nature of local spin order in ferroelectric film. Our results allow expectation on low-cost applications of detectors and switches for extremely weak magnetic fields in a wide temperature range 55K-190K.

cond-mat.mtrl-sci↗

arXiv2005

The First Laser Guide Star Adaptive Optics Observations of the Galactic Center: Sgr A*'s Infrared Color and the Extended Red Emission in its Vicinity

(Abridged) We present the first Laser Guide Star Adaptive Optics (LGS-AO) observations of the Galactic center. LGS-AO has dramatically improved the quality, robustness, and versatility with which high angular resolution infrared images of the Galactic center can be obtained with the W. M. Keck II 10-meter telescope. Specifically, Strehl ratios of 0.7 and 0.3 at L'[3.8 micron] and K'[2.1 micron], respectively, are achieved in these LGS-AO images. During our observations, the infrared counterpart to the central supermassive black hole, Sgr A*-IR, showed significant infrared intensity variations, with observed L' magnitudes ranging from 12.6 to 14.5 mag. The faintest end of our L' detections, 1.3 mJy (dereddened), is the lowest level of emission yet observed for this source by a factor of 3. No significant variation in the location of SgrA*-IR is detected as a function of either wavelength or intensity. Near a peak in its intensity, we obtained the first measurement of SgrA*-IR's K'-L' color (3.0 +- 0.2 mag, observed), which corresponds to an intrinsic spectral index of -0.5 +- 0.3. This is significantly bluer than other recent infrared measurements. Because our measurement was taken at a time when Sgr A* was ~6 times brighter in the infrared than the other measurements, we posit that the spectral index of the emission arising from the vicinity of our Galaxy's central black hole may depend on the strength of the flare, with stronger flares giving rise to a higher fraction of high energy electrons in the emitting region.

astro-ph↗

arXiv2004

An Overabundance of Transient X-ray Binaries within 1 pc of the Galactic Center

During five years of Chandra observations, we have identified seven X-ray transients located within 23 pc of Sgr A*. These sources each vary in luminosity by more than a factor of 10, and have peak X-ray luminosities greater than 5e33 erg/s, which strongly suggests that they are accreting black holes or neutron stars. The peak luminosities of the transients are intermediate between those typically considered outburst and quiescence for X-ray binaries. Remarkably four of these transients lie within only 1 pc of Sgr A*. This implies that, compared to the numbers of similar systems located between 1 and 23 pc, transients are over-abundant by a factor of 20 per unit stellar mass within 1 pc of Sgr A*. It is likely that the excess transient X-ray sources are low-mass X-ray binaries that were produced, as in the cores of globular clusters, by three-body interactions between binary star systems and either black holes or neutron stars that have been concentrated in the central parsec through dynamical friction. Alternatively, they could be high-mass X-ray binaries that formed among the young stars that are present in the central parsec.

astro-ph↗

arXiv2001

Interlayer exchange coupling and giant magnetoresistance in Fe/V (001) superlattices

Magnetization and magnetoresistivity studies of Fe/V (001) superlattices are reported. The first giant magnetoresistance peak with respect to the vanadium and iron layer thicknesses is investigated. The interlayer antiferromagnetic coupling strength is found to show a peak at a vanadium layer thickness of 13 atomic monolayers ($\approx$ 20 Å) with a full width at half maximum of about 2 monolayers. The antiferromagnetic coupling shows a maximum at an iron layer thickness of about 6 monolayers ($\approx$ 9 Å) for series of superlattices with vanadium thicknesses around 13 monolayers. The magnitude of the giant magnetoresistance shows a similar variation as the antiferromagnetic coupling strength.

cond-mat.mtrl-sci↗

arXiv2000

CTMC calculations of electron capture and ionization in collisions of multiply charged ions with elliptical Rydberg atoms

We have performed classical trajectory Monte Carlo (CTMC) studies of electron capture and ionization in multiply charged (Q=8) ion-Rydberg atom collisions at intermediate impact velocities. Impact parallel to the minor and to the major axis, respectively, of the initial Kepler electron ellipse has been investigated. The important role of the initial electron momentum distribution found for singly charged ion impact is strongly disminished for higher projectile charge, while the initial spatial distribution remains important for all values of Q studied.

physics.atm-clus↗

arXiv1998

Luttinger Liquid Behavior in Carbon Nanotubes

An interacting one-dimensional (1D) electron system is predicted to behave very differently than its higher-dimensional counterparts. Coulomb interactions strongly modify the properties away from those of a Fermi liquid, resulting in a Luttinger liquid (LL) characterized by a power-law vanishing of the density of states at the Fermi level. Experiments on one-dimensional semiconductor wires and fractional quantum Hall conductors have been interpreted using this picture, but questions remain about the connection between theory and experiment. Recently, single-walled carbon nanotubes (SWNTs) have emerged as a new type of 1D conductor that may exhibit LL behavior. Here we present measurements of the conductance of individual ropes of such SWNTs as a function of temperature and voltage. Power law behavior as a function of temperature or bias voltage is observed: G~ T^a and dI/dV ~ V^a. Both the power-law functional forms and the inferred exponents are in good agreement with theoretical predictions for tunneling into a LL.

cond-mat.mes-hall↗