Magnetic and Crystalline Microstructures of Fe-Pt-B Nanocomposite Ribbons

We investigate magnetic and crystalline microstructures of melt-spun (Fe0.675Pt0.325)100-xBx (x=12, 14, 16, 18,20) nanocomposite ribbons after optimal thermal treatment using a magnetic force microscope. The magnetic microstructures are characterized by darker spots adjacent to brighter ones in a sub-micro scale and in random distribution. It is found that the strength of the exchange coupling interaction between the crystals in the 10-100 nm scale, implied by the maximum value (δM)max of the Henkel plot, could be roughly described by the ratio of the average width of the magnetic spots w^- to the average crystal size D^- for the ribbons. Moreover, we find that the intrinsic coercivity jHc of the ribbons is sensitive to their crystal sizes, and the smaller D^-, the higher jHc. Finally, by using roughness analysis, the curve of the root mean square values (δФ)rms of the phase shift of the magnetic force images versus the boron content x is obtained, which is qualitatively consistent with that of the magnetization σ12 kOe of the ribbons versus x.     

Magnetic properties and coercivity mechanism of precipitation-hardened Gd－Co based ribbons

Gd(Co_{0.88-x}Cu_xFe_{0.09}Zr_{0.03})_z ribbons with x=0.075-0.200 and z=6.4－7.7 have been prepared by a melt-spinning technique. A cellular microstructure consisting of 2:17 cells surrounded by the 1:5 cell boundary phase is obtained after precipitation hardening. The dependence of room temperature coercivity on the heat treatment process suggests that the long-time isothermal aging is not helpful for the development of magnetic properties. Positive temperature coefficient of remanence from room temperature to about 673K is typical for all samples, while positive temperature coefficient of coercivity is obtained only in ribbons with low Cu content. The coercivity mechanism of the precipitation-hardened ribbons at different temperatures is also discussed.     

Magnetic domain structures of precipitation-hardened SmCo 2：17-type sintered magnets： Heat treatment effect

The typical magnetic domains of Sm（CObalFe0.25Cuo.07Zr0.02）7.4 magnets quenched through various heattreatment steps have been revealed by using magnetic force microscopy （MFM）. For the specimens in which the nominal c-axis is perpendicular to the imaging plane, the domain configurations change from plate-like for the as-sintered magnet to corrugation and spike-like for the homogenized one, and then to a coarse and finally to a finer domain structure when isothermally aged at 830℃ and then annealed at 400℃. However, only plate-like domains can be detected on the surfaces with the nominal c-axis parallel to the imaging plane. The finer domain （so-called interaction domain） is a characteristic magnetic domain pattern of the SmCo 2：IT-type magnets with high coercivities. Domain walls in a zigzag shape are revealed by means of MFM in final bulk SraCo 2：17-type sintered magnets.     

Effects of Al component content on optoelectronic properties of Al_xGa_(1-x)N

Using density functional theory, the electronic structures, lattice constants, formation energies, and optical properties of Al_xGa_(1-x)N are determined with Al component content x in a range from 0 to 1. As x increases, the lattice constants decrease in e-exponential form, and the band gap increases with a band bending parameter b=0.3954. The N–Al interaction force in the(0001) direction is greater than the N–Ga interaction force, while the N–Al interaction force is less than the N–Ga interaction force in the(10ī0) direction. The formation energies under different Al component content are negative and increase with Al component content increasing. The static dielectric function decreases, the absorption edge has a blue shift, and the energy loss spectrum moves to high energy with the Al component content increasing.     

Quantum Spectra of Hydrogen Atoms in Various Magnetic Fields with the Closed Orbit Theory

The quantum spectra of hydrogen atoms in various magnetic fields have been calculated with the closed orbit theory.The magnitude of the magnetic field decreases from 5.96 T to 0.56 T with a step of 0.6T.We demonstrate schematically that the closed orbits disappear with the decrease of the magnitude of the magnetic field when the corresponding finite resolution of experiment is fixed.This may give us a good way to control the shape and the number of the closed orbits in the system,and thus to control where a peak should exist in the Fourier transformation of the quantum spectra.     

Magnetic properties and magnetocaloric effects in amorphous and crystalline Gd55Co35Ni10 ribbons

The magnetic properties and magnetocaloric effects of amorphous and crystalline Gd55Co35Ni10 ribbons are investigated.A main phase with a Ho 12 Co 7-type monoclinic structure(space group P21/c) and a minor phase with a Ho4Co3-type hexagonal structure(space group P63/m) are obtained for crystalline ribbon after annealing.The amorphous ribbons order ferromagnetically and undergo a second-order transition at 192 K.For crystalline Gd55Co35Ni10 ribbons,two magnetic phase transitions occur at 158 and 214 K,respectively.The peak value of-△SM under a field change of 0-5 T is 6.5 J/kg K at 192 K for amorphous Gd55Co35Ni10 ribbons.A relatively large magnetic entropy change(～5.0 J/kg K) under a field change of 0-5 T for the crystalline Gd55Co35Ni10 ribbons is obtained in the temperature interval range of 154-214 K.The large platform of magnetic entropy change and the negligible thermal/magnetic hysteresis loss mean the crystalline Gd55Co35Ni10 compound can satisfy the requirement of the Ericsson-type refrigerator working in the temperature range from 154K to 214K.     

Effects of Slight Plastic Deformation on Magnetic Properties and Giant Magnetoimpedance of FeCoCrSiB Amorphous Ribbons

Slight plastic deformation of 0 to 1% by cold rolling is proposed as a treatment which may modify the responses of magnetoimpedance （MI） sensor with an amorphous ribbon used as a sensitive element. The dependence of the magnetic properties of melt spun Fe3Co67Cr3Si15B12 amorphous ribbons and their MI responses in the initial state and after slight plastic deformation on the value of the deformation were comparatively analysed. The shape of the hysteresis loops shows a clear correlation with the value of the deformation. The variations of the total impedance, the real and the imaginary components, are measured for the current intensity of 1.S mA for the frequency of lO MHz. Slight plastic deformation affects both real and imaginary components and allows a control of the shape of the MI curves in a small field in a range usually used in biomedical applications. The proposed deformation treatments can be useful for the construction of the MI sensitive elements with a new type of the responses.     

Magnetic properties and magnetocaloric effects of Mn5Ge3-xGax

We report on the magnetic properties and magnetocaloric effects of Mn5Ge3-xGax compounds with x=0.1,0.2,0.3,0.4,0.6 and 0.9. All samples crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm and order ferromagnetically.The Curie temperature of these compounds decreases with increasing x, from 306K (x=0.1) to 274K (x=0.9).The average Mn magnetic moments increases with increasing Ga content,reaching a maximum value at x=0.6.The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation.The Ga substitution has two kinds of influence on the magnetocaloric effect (MCE) of Mn5Ge3.One is that the magnitude of the magnetic entropy change decreases,the other is that the MCE peak becomes broadened.     

Magnetic properties of Nd0.5Pb0.5-xSrxMnO3 materials

The structure and magnetic properties of Nd0.5Pb0.5-xSrxMnO3 (0≤x≤0.4) manganites were systematically investigated. Significant changes in Curie temperature and metal-insulator (MI) transition temperature of the samples were observed. All samples exhibited a transition from paramagnetic semiconducting to ferromagnetic metallic state.Curie temperature Tc and the MI transition temperature Tp increased with increasing Sr content. We attributed these behaviours to the enhancing of both the double exchange mechanism and the Jahn-Teller electron-phonon coupling.     

Composition-Controlled Low Field Magnetostriction of TbFe Amorphous Films

Amorphous TbFe films are fabricated by dc magnetron sputtering, and their magnetostrictions at low field are examined over a wide range of terbium content （from 32at.% to 70at.%）. It is found that the terbium content plays an important role in the magnetic and magnetostrictive properties of TbFe films. TbFe film soft magnetic properties and low field magnetostriction can be efficiently improved by controlling the terbium at an optimum content. The magnetostriction at lower magnetic field is increased with the increase of terbium content up to 48.2at.%. After reaching the maximum value, further increase of terbium content would result in a great decrease of the low field magnetostriction. By contrast, at higher magnetic field, the magnetostriction is decreased monotonically with the increase of the terbium content.     

Identification of the Amplification Mechanism in the First Free-Electron Laser as Net Stimulated Free-Electron Two-Quantum Stark Emission

We find that the electron phase with respect to the incident laser radiation must be random in the first freeelectron laser （FEL） and, hence, the incident laser radiation works as a relaxation force to keep a Maxwellian distribution. We formulate the threshold laser intensity for amplification which agrees with the measured value in the order of magnitude in the first FEL. The magnetic wiggler must produce an electric wiggler whose period is the same as that of the magnetic wiggler. We find that net stimulated free-electron two-quantum Stark （FETQS） emission driven by this electric wiggler is the mechanism responsible for the measured gain and the measured laser intensity at the plateau in the first FEL.     

Microstructure and Magnetic Domains of Iron Films on Liquid Surfaces

Iron （Fe） films with a thickness ranging from 1.Onto to 80.Onto are deposited on silicone oil surfaces by a vapor phase deposition method. The films with a thickness old 〈 2.0 nm do not exhibit planar morphology but ramified aggregates instead. Magnetic force microscopy studies for the Fe films （10.0nm ≤d ≤ 80.0nto） show that the domain wall structure is widespread and irregularly shaped and the oscillation phase shift △θ, which records as the magnetic force image, changes from 0.29°to 0.81°. Correspondingly, the magnetic force gradient varies from 1.4 ×10^-3 to 4.0× 10^-3 N/m, respectively. In our measurement, the characteristic domain walls, such as Bloch walls, Neel walls and cross-tie walls, are not observed in the film system clearly.     

Performance study of aluminum shielded room for ultra-low-field magnetic resonance imaging based on SQUID:Simulations and experiments

The aluminum shielded room has been an important part of ultra-low-field magnetic resonance imaging(ULF MRI)based on the superconducting quantum interference device(SQUID). The shielded room is effective to attenuate the external radio-frequency field and keep the extremely sensitive detector, SQUID, working properly. A high-performance shielded room can increase the signal-to-noise ratio(SNR) and improve image quality. In this study, a circular coil with a diameter of 50 cm and a square coil with a side length of 2.0 m was used to simulate the magnetic fields from the nearby electric apparatuses and the distant environmental noise sources. The shielding effectivenesses(SE) of the shielded room with different thicknesses of aluminum sheets were calculated and simulated. A room using 6-mm-thick aluminum plates with a dimension of 1.5 m×1.5 m×2.0 m was then constructed. The SE was experimentally measured by using three-axis SQUID magnetometers, with tranisent magnetic field induced in the aluminum plates by the strong pre-polarization pulses. The results of the measured SE agreed with that from the simulation. In addition, the introduction of a 0.5-mm gap caused the obvious reduction of SE indicating the importance of door design. The nuclear magnetic resonance(NMR) signals of water at 5.9 kHz were measured in free space and in a shielded room, and the SNR was improved from 3 to 15. The simulation and experimental results will help us design an aluminum shielded room which satisfies the requirements for future ULF human brain imaging. Finally, the cancellation technique of the transient eddy current was tried, the simulation of the cancellation technique will lead us to finding an appropriate way to suppress the eddy current fields.     

Measurements of total absolute collision cross section of ultracold Rb atom using magneto-optic and pure magnetic traps

The implementation of a technique to measure total collision cross sections using laser-cooled rubidium atoms along with the introduction of room-temperature background gases, confined in both magnetooptic and magnetic traps, is proposed. Atom loss from the trap and total collision cross section can be inferred from the knowledge of the density of background gases. The measured cross sections from the magneto-optic and magnetic traps are represented, compared, and found to be significantly different. The measurements using this technique have a very small error in the range of approximately 2%-7%.     

A dynamic macromodel for distributed parameter magnetic microactuators

This paper presents a reduced-order model to describe the mechanical behaviour of microbeam-based magnetic devices. The integration for magnetic force is calculated by dividing the microbeam into several segments, and the nonlinear equation set has been developed based on the magnetic circuit principle. In comparison with previous models, the present macromodel accounts for both the micro-magnetic-core reluctance and the coupling between the beam deflection and magnetic force. This macromodel is validated by comparing with the experimental results available in some papers and finite-element solutions.     

Effects of the Mn/Co ratio on the magnetic transition and magnetocaloric properties of Mn1+xCo1-xGe alloys

We have investigated the magnetic transition and magnetocaloric effects of Mn 1+x Co 1 x Ge alloys by tuning the ratio of Mn/Co.With increasing Mn content,a series of first-order magnetostructural transitions from ferromagnetic to paramagnetic states with large changes of magnetization are observed at room temperature.Further increasing the content of Mn (x=0.11) gives rise to a single second-order magnetic transition.Interestingly,large low-field magnetic entropy changes with almost zero magnetic hysteresis are observed in these alloys.The effects of Mn/Co ratio on magnetic transition and magnetocaloric effects are discussed in this paper.     

Giant Magnetoimpedance Effect in Surface Modified CoFeMoSiB Amorphous Ribbons

Thin magnetic Fe layers in thickness of 10-240 nm were deposited onto a wheel surface of CoFeMoSiB amorphous ribbons to check our concept of a new type of heterogeneous magnetoimpedance materials formed by two different magnetic parts. The presence of an additional iron layer modifies the magnetoimpedance response of the composite material and leads to increase of the magnetoimpedance ratio from 330 to 345% at a frequency of 3.5 MHz. Two possible mechanisms are discussed for explanation to the observed behaviour. Modification of the surface properties of the amorphous ribbons may have certain potential for technological applications.     

Electron Acceleration in Collisionless Magnetic Reconnection

A 21/2-dimensional electromagnetic particle-in-cell (PIC) simulation code is used to investigate the electron acceleration in collisionless magnetic reconnection. The results show that the electrons are accelerated in the diffusion region near the X point, and the acceleration process can be roughly divided into two procedures: firstly the electrons are accelerated in the z direction due to the electric field in the negative z direction. Then the electrons gyrate surrounding the magnetic field with the action of the Lorentz force, through this procedure the electrons reach higher velocity in the x direction and then flow out of the diffusion region. After being accelerated away from the diffusion region, part of electrons is trapped near the O point, and the other part of electrons flows into plasma sheet boundary layer along the magnetic field.     

Interaction of Two Parallel Cracks in REBCO Bulk Superconductors under Applied Magnetic Field

Interactions of two collinear and parallel a/b-plane cracks in REBCO(where RE is a rare-earth element and usually Y is adopted) bulk superconductors under the Lorentz force resulted from the applied magnetic field are studied.By using the derived boundary integral equation for the crack problem of a cylindrical bulk superconductor under the applied magnetic field,we comprehensively investigate the stress intensity factor(SIF) of modes Ⅰ and Ⅱ at the crack tips of the two collinear and parallel cracks with their sizes,relative positions and the applied magnetic field.The calculated results show that in most cases,the SIF of mode I is found to be about tens of times of the one of mode Ⅱ,and all the SIFs are always proportional to increase in the applied magnetic field,and the cracks near the center are more dangerous due to the larger Lorentz force.     

Current sensor based on an atomic magnetometer for DC application

A DC current sensor based on an optically pumped atomic magnetometer is proposed.It has a high linearity in a wide operation range,since the magnetometer measures the absolute magnitude of the magnetic field produced by the current to be measured.The current sensor exhibits a high accuracy with a non-moment solenoid and magnetic shielding to suppress the influence from the environment.The absolute error of the measured current is below 0.08 m A when the range is from 7.5 m A to 750 m A.The relative error is 5.54×10^-5 at 750 m A.