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Square arrays of circular pinning centers of various diameters were patterned in Nb thin films to explore their vortex pinning behavior. Periodic critical current matching peaks and some 'missing peaks' were observed in magnetotransport measurements. The larger the diameter of the pinning centers, the higher the index of the missing matching fields observed. This phenomenon is explained by molecular dynamics simulations and is caused by the interaction between interstitial vortices and vortices occupying the pinning centers. [ABSTRACT FROM AUTHOR]

The vortex propagation exhibits rectification effect in Nb superconductors with spacing-graded density of holes. A rectified dc voltage is obtained when the vortex lattice is driven by ac current. The asymmetric geometry of the pinning array produces a significant influence on the vortex motion. The rectified voltage depends considerably on the amplitude of the applied ac current and the magnetic field. The experimental results reveal a drastic change of the vortex rectification for magnetic field above/below the first matching field. The reason may be that the interstitial vortices are formed in the film above the first matching field. A reversible vortex motion is induced by the interstitial vortices for the field above the first matching field. [ABSTRACT FROM AUTHOR]

Utilizing a novel method with the resonance frequency of a LC circuit, we measured the superconducting anisotropy of single crystals of an Fe-based superconductor FeSe with applied magnetic field up to 16 T. We found that the temperature dependence of the upper critical field Hc2(T) of FeSe coincides with the Werthamer-Helfand-Hohenberg (WHH) model when taking the Maki parameter α into consideration, suggesting an important role played by spin-paramagnetic effect in suppressing the superconductivity. When temperature T → 0, the values of Hc2,∥c(0) and Hc2,∥ab(0) derived from the WHH fitting are close to and fall within the range of the Pauli limit, for field H0 applied parallel to the c-axis and to the ab-plane, respectively. As compared with other typical iron-based high-Tc superconductors, lower values of Hc2(0) and higher superconducting anisotropy Γ(0) were observed in FeSe. [ABSTRACT FROM AUTHOR]

Describes a treatment that lowers the threshold field for field emission and increases the emission site density from a nominally n-type diamondlike carbon film. Deposition of the film using an rf plasma of methane and nitrogen gases; Deposition of cesium followed by a low temperature anneal.

Superconducting films with composite pinning sites are made with electron-beam lithography and reaction dry etching techniques. The composite pinning array consists of large pinning sites and small pinning sites. The large pinning sites are positioned on a honeycomb grid and the small pinning sites are positioned at the center of every hexagonal cell formed by the large pinning sites. Magnetotransport measurements are carried out by a four-probe technique. Special critical currents matching peaks are observed for this film at different temperatures. The positions and structures of the matching peaks seem irregular and are very different from the structures of the matching peaks for previously explored films, such as films with triangular, square, or honeycomb pinning arrays. By considering the multiple-vortex filling of different pinning sites, we could give a reasonable explanation to this interesting phenomenon. [ABSTRACT FROM AUTHOR]

In this study, the effective spin-mixing conductance in Ni80Fe20/Pt was investigated by measuring the interface-induced enhancement of the Gilbert damping constant. Ferromagnetic resonance spectra were measured in coplanar waveguide geometry with different incident microwave powers. The nonlinear behavior of normal Gilbert damping G0 and effective spin-mixing conductance g↑↓ have been observed when the incident microwave power is above a critical ac field hrf of 1.6 Oe. Both phenomena are explained by considering the coupling between spin coherent precession and spin wave modes. This work demonstrates the nonlinear behavior of the effective spin-mixing conductance g↑↓. It suggests that the nonlinear spin wave modes excited at high incident microwave power are detrimental to the spin pumping effect and should be avoided in future spin battery designs. The capability of tuning G0 and g↑↓ through the microwave power is also useful for the fundamental study on the damping mechanism. [ABSTRACT FROM AUTHOR]

Microwave assisted magnetization switching was investigated using Fe30Co70/Al2O3/Ni80Fe20 magnetic tunnel junctions incorporated with a coplanar waveguide. Coercivity reduction (or switching field) of Ni80Fe20 layer was reduced with a microwave excitation. It was found that the coercivity reduction depends on both frequency and power of the microwave. The numerical simulation based on the Landau-Lifshitz-Gilbert equation reproduced the trend of the experimental data. The results indicate that microwave can be an efficient mean to switch the magnetization of a thin film. [ABSTRACT FROM AUTHOR]

Electron-beam lithography and reaction dry etching techniques have been used to fabricate superconductor thin films with regular square hole arrays to explore the vortex pinning behavior. Magnetotransport measurements were carried out by a four-probe technique. Periodic critical current matching peaks are observed. We found that the height of the third matching peak is close to that of the fourth matching peak for film with holes of diameter of 200 nm at certain temperatures, and it appears that the third matching peak is missing. Similar peculiar phenomena are observed for films with holes of different diameters, but the missing peaks are at the fourth or fifth matching field for larger holes. We made molecular dynamic simulations to study the ground state vortex distributions and the dynamics of the vortex motion. The simulation results offer good explanations to the features we found in the experiments. [ABSTRACT FROM AUTHOR]

The vortex propagation has been investigated in Nb superconductors with one dimension spacing-graded density of submicrometer-scaled holes in this research. A rectification effect can be discerned because the applied ac is transformed into a net dc voltage. These results demonstrate that the rectification is mainly characterized by the asymmetric pinning landscape and the rectified voltage depends considerably on temperature and the amplitude of applied ac. The dc voltage increases steeply up to the maximum value and then decreases to zero dc voltage with the increase of ac. The shape of peaks in the dc voltage signal become steeper and move to higher applied ac regime with the decrease of temperature. [ABSTRACT FROM AUTHOR]

We report the experimental observation of mutual synchronization of magnetic nano-oscillators driven by pure spin current generated by nonlocal spin injection. We show that the oscillators efficiently synchronize due to the direct spatial overlap of the dynamical modes excited by the spin current, which is facilitated by the large size of the auto-oscillation area inherent to these devices. The synchronization occurs within an interval of the driving current determined by the competition between the dynamic nonlinearity that facilitates synchronization and the short-wavelength magnetic fluctuations enhanced by the spin current that suppress synchronization. The demonstrated synchronization effects can be utilized to control the spatial and spectral characteristics of the dynamical states induced by the spin currents. [ABSTRACT FROM AUTHOR]

We present a micromagnetic study on the eigen excitations of an artificial skyrmion crystal, which has been experimentally confirmed to be stable at room temperature without the need of any Dzyaloshinsky-Moriya interaction (DMI). Three in-plane rotational modes and one breathing-type mode are identified. We find the intrinsic origin of the dynamics of skyrmion crystal is the nontrivial magnetic texture instead of DMI. And the rotational direction of a skyrmion is solely determined by the sign of the skyrmion number, irrespective of its circulation sense, evidencing the topological nature of the magnetic skyrmion. [ABSTRACT FROM AUTHOR]

Evaporative cooling technology utilizes phase-change heat transfer mode to achieve the cooling for heating equipment. The heat transfer capacity of evaporative cooling technology is far more than air or water cooling technology. The Electron Cyclotron Resonance ion source magnet is a typical super-high power density magnet, and the evaporative cooling technology is an ideal cooling method for the coils of magnet. In this paper we show the structure and process of coils and the special design of flow channels of coolant for an experiment magnet model. Additionally, the heat transfer circulation is presented and analyzed. By the finite element method, the flow channels are optimized to rationally allocate coolant and to reduce the temperature of coils. For the experiment model, the current density of copper wire of coils is 19 A/mm2² and the coil-windows current density is larger than 12 A/mm². The max temperature of coils is below 80 °C, and the total heat is about 200 kW. [ABSTRACT FROM AUTHOR]

A new room temperature ECR ion source, Lanzhou Electron Cyclotron Resonance ion source No. 4 (LECR4, previously named DRAGON), is under intense construction at Institute of Modern Physics. LECR4 is designed to operate with 18 GHz microwave frequency. The maximum axial magnetic fields are 2.3 T at injection and 1.3 T at extraction, and the radial field at the plasma chamber wall of 76 mm inner diameter is 1.0-1.2 T. One of the unique features for LECR4 is that its axial solenoids are winded with solid square copper wires which are immersed in a kind of special evaporative cooling medium for cooling purpose. Till now, a prototype of the cooling system has been successfully constructed and tested, which has demonstrated that the cooling efficiency of the designed system could meet the requirements of LECR4 under the routine operation conditions. All the main components of the ion source have been completed. Assembly and commissioning is ongoing. The latest developments and test results will be presented in this paper. [ABSTRACT FROM AUTHOR]

We present the width dependent study of edge distribution of graphene nanoribbons unzipped from multi-wall nanotubes. The partial unzipping of the carbon nanotubes yields a mixture of carbon nanotubes and nanoribbons. Comparing atomic resolution images of scanning tunneling microscopy with the lattice of graphene, the edge structures of nanoribbons are identified. Below 10 nm, the edges are closer to armchair type. Above 20 nm, the ribbons prefer to have edges close to zigzag type. In between, a more random distribution of the edges is found. The findings are of potential usages for the edge control in graphene nanoribbon based applications. [ABSTRACT FROM AUTHOR]

We investigated the tunneling magnetoresistance change in magnetic tunnel junctions in the presence of external microwaves. The changing relative angle between the free layer and the pinned layer results in a rectification of the average resistance change. Due to its miniature size and its sensitivity to the microwave magnetic field, the magnetic tunnel junction could be utilized as a microwave power sensor with the ability to detect microwave frequencies. Studying microwave power and bias current dependencies reveals desired sensor features with linear responses and enhanced signal levels. [ABSTRACT FROM AUTHOR]

We report the study of the real-time evolution of tunneling magnetoresistance (TMR) in CoFeB/MgO/CoFeB junctions during annealing at 380 °C. The TMR quickly developed at the early stage of the annealing, with 200% magnetoresistance observed in less than 10 min, followed by a slow approach to saturation. This evolution of TMR was correlated with the structural changes, including crystallization of amorphous CoFeB electrodes and improvement of barrier quality during the annealing. [ABSTRACT FROM AUTHOR]

Microwave-assisted magnetization switching was investigated using Fe30Co70/AlOx/Ni80Fe20 magnetic tunnel junctions incorporated with a coplanar waveguide. Coercivity field of Ni80Fe20 layer was dramatically reduced in a small amplitude microwave. The authors eliminated the thermal effect in coercivity reduction by comparing two types of measurements which are with and without spin precession in the presence of microwave. It was found that the coercivity reduction depends on both frequency and power of the microwave. The numerical simulation based on Landau-Lifshitz-Gilbert equation reproduced the trend of the experimental data. The results indicate that microwave can be an efficient means to switch the magnetization of a thin film. [ABSTRACT FROM AUTHOR]

Near-edge x-ray absorption fine structure (NEXAFS) measurements were performed on a variety of carbon materials, covering a range of hybrid bonding character from pure sp3 type to pure sp2 type. Diamond, chemical vapor deposited (CVD) diamond films of varying quality. Diamond-like carbon (DLC) films, and graphite were examined with this technique and these measurements were compared with Raman spectroscopy results and scanning electron microscopy images of carbon film morphology. For the mixed sp2 and sp3 bonded DLC materials, NEXAFS does not suffer from the large Raman cross-section difference between sp2 and sp3 type bonds, thus allowing unambiguous characterization of carbon thin films with a broader range of sp2/sp3 bonding ratios than possible with Raman spectroscopy alone. This capability was used to determine the transition point where the sequential-CVD carbon film growth technique produces predominately sp3 or sp2 bonded material. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the surfactant-assisted silicon (Si), germanium (Ge) epitaxial growth. Use of high resolution photoemission; Reduction of surface energy; Change in the position of antimony (Sb), Si, and Ge atoms; Segregation of the Sb atoms top the growth front.

Recently, there has been a great deal of interest in Na2S·9H2O as a passivating chemical treatment for GaAs surfaces. It has been shown that it reduces the high surface recombination velocity characteristic of GaAs surfaces, and may offer hope for ‘‘unpinning’’ the surface Fermi level. We have used photoemission spectroscopy to study the band bending and chemistry of these overlayers on n-type GaAs (100). Identically prepared samples show the characteristic increase in the photoluminescence signal, and have also been characterized using surface conductivity measurements. We find using photoemission that the surface Fermi level of the treated wafer is still near midgap. We also observe the chemistry at the interface, and offer a possible explanation of the photoluminescence and surface conductivity data in terms of it and the advanced unified defect model. [ABSTRACT FROM AUTHOR]

We performed a series of molecular dynamics simulations on monodisperse polymer melts to investigate the formation of shear banding. Under high shear rates, shear banding occurs, which is intimately accompanied by the entanglement heterogeneity. Interestingly, the same linear relationship between the end-to-end distance Ree and entanglement density Z is observed at homogeneous flow before the onset of shear banding and at the shear banding state, where Ree ∼ ln(Wi0.87)-ξ0Z is proposed as the criterion to describe the dynamic force balance of the molecular chain in flow with a high rate. Deviating from this relation leads to a force imbalance and results in the emergence of shear banding. We establish a scaling relation between the disentanglement rate Vd and the Weissenberg number Wi as Vd∼Wi0.87 for stable flow in homogeneous shear and shear banding states. The formation of shear banding prevents chains from further stretching and disentanglement. The transition from homogeneous shear to shear banding partially dissipates the increased free energy from shear and reduces the free energy of the system., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)

We describe a high-temperature, uniaxial creep apparatus designed to investigate nonlinear attenuation of materials over a wide range of temperatures (25-1300 °C) using forced oscillations combined with a bias stress. This apparatus is primarily designed for investigation of minerals and rocks with high melting temperatures. An oscillatory compressional stress is used to determine attenuation and Young's modulus at frequencies of 10 -1 -10 2  Hz and high stress amplitudes (>0.1 MPa). Large bias stresses are applied in addition to the oscillatory stresses such that attenuation tests are conducted simultaneously with the ongoing creep. The complex compliance of the apparatus was characterized by conducting calibration tests on orientated crystals of sapphire. The real part of the apparatus compliance exhibits a dependence on sample length and frequency, whereas the imaginary part is only dependent on frequency. The complex compliance is not dependent on the oscillation amplitude or the bias stress. We assess the accuracy and precision of this calibration by comparing measurements of the attenuation and Young's modulus of aluminum and acrylic to previously published values. We outline a set of criteria defining the conditions over which this apparatus can precisely determine the attenuation and Young's modulus of a sample based on the sample length and expected values of attenuation and Young's modulus.

A passive broadband source localization approach in the deep ocean is proposed based on the coherent matched-field processing combined with a Riemannian distance. The spatial coherence between hydrophones over frequency bandwidth is utilized for eliminating the unknown source spectrum. Three Riemannian distances are introduced for the measurement of the similarity between the data and replica matrices. The experimental results indicate that the proposed processors with a short array can effectively suppress the sidelobe and improve the localization performance. The source localization influenced by the factors such as the aperture, number of hydrophones, and desired bandwidth is also demonstrated.

The Hall effect and its varieties such as quantum, anomalous, and spin Hall effects provide indispensable tools for the characterization of electronic and magnetic properties of materials, metrology, and spintronics. The conventional four-probe Hall configuration is generally not amenable to measurements at nanoscale due to current shunting by the Hall electrodes. We demonstrate that Hall measurements on the nanoscale can be facilitated by the three-probe Hall configuration that avoids the shunting problem. We illustrate the efficiency of the proposed approach with anomalous Hall effect-based measurements of individual activation events during domain wall motion in magnetic films with perpendicular anisotropy.