A General Approach to First Order Phase Transitions and the Anomalous Behavior of Coexisting Phases in the Magnetic Case

First order phase transitions for materials with exotic properties are usually believed to happen at fixed values of the intensive parameters (such as pressure, temperature, etc.) characterizing their properties. It is also considered that the extensive properties of the phases (such as entropy, volume, etc.) have discontinuities at the transition point, but that for each phase the intensive parameters remain constant during the transition. These features are a hallmark for systems described by two thermodynamic degrees of freedom. In this work it is shown that first order phase transitions must be understood in the broader framework of thermodynamic systems described by three or more degrees of freedom. This means that the transitions occur along intervals of the intensive parameters, that the properties of the phases coexisting during the transition may show peculiar behaviors characteristic of each system, and that a generalized Clausius–Clapeyron equation must be obeyed. These features for the magnetic case are confirmed, and it is shown that experimental calorimetric data agree well with the magnetic Clausius–Clapeyron equation for MnAs. An estimate for the point in the temperature‐field plane where the first order magnetic transition turns to a second order one is obtained (the critical parameters) for MnAs and Gd₅Ge₂Si₂ compounds. Anomalous behavior of the volumes of the coexisting phases during the magnetic first order transition is measured, and it is shown that the anomalies for the individual phases are hidden in the behavior of the global properties as the volume.     

AlAs and InAs mode LO phonon emission assisted tunneling in (InGa)As/(AlIn)As double barrier structures

WE present a magnetospectral analysis of the different scattering processes involved in the tunneling current of an MBE grown (InGa)As/(AlIn)As double barrier diode. At zero magnetic field, the I(V) characteristics of the sample show a replica peak. We demonstrate that this peak is associated to the tunneling of electrons from the emitter to the quantum well, assisted by the emission of an AlAs mode LO phonon. The presence of InAs mode LO phonon emission processes is also revealed at high magnetic fields. At a field B where the crossing between InAs and AlAs mode phonon emission processes occurs, a change in the B-dependent behavior of the peak voltage positions is observed, that we attribute to a change in the dominant process. The higher probability for InAs mode LO phonon emission processes is explained by the stronger coupling of electrons to this mode in this system, in agreement with previous results on (InGa)As/(AlIn)As single heterojunctions.     

Excitation of Plasma Waves in an Unbounded Homogeneous Plasma by a Line Source

The radiation characteristics of a line source of magnetic current embedded in a homogeneous electron plasma of infinite extent are investigated for the case in which a uniform magnetic field is impressed externally throughout the medium in the direction of the source. The single-fluid theory of magnetohydrodynamics is employed. A very simple model is assumed for the plasma. Under this assumption, it is found that there are two modes of propagation of waves of small amplitude. By examining the behavior of these modes in the limiting cases of vanishing external magnetic field or infinite source frequency, they are identifiable as the modified forms of the usual plasma and optical modes which exist in an isotropic electron plasma. The dispersion relations for these two modes are discussed. The power radiated in each of the two modes is also evaluated. It is found that the power radiated in the optical mode is always lower than that due to the line source in free space, whereas the power radiated in the plasma mode is higher than that value for certain ranges of the source frequency.     

The spectrum of microwave radiation from InSb

The frequency spectrum of the microwave radiation emitted from InSb in the presence of applied electric and magnetic fields at 77°K was observed using a spectrum analyzer. In the frequency region from 7 to 26 Gc/s the spectrum showed oscillatory behavior with a period of 2.2 Gc/s, with the average value decreasing with increasing frequency. Such a structure could be due to a series of harmonics of a fundamental mode with frequency of 2.2 Gc/s. The frequencies of the peaks did not shift on changing the magnetic field and current. The spectrum near the frequency of 2.2 Gc/s was measured in detail in a coaxial-type cavity. The intensity of the radiation in this frequency region was much higher than that in the higher frequency region. The spectrum did not show clearly the expected peak at 2.2 Gc/s, but showed a further oscillatory behavior with a period of about 150 Mc/s. The physical mechanism which gives such a structure to the spectrum is not yet understood. It was also observed that the threshold conditions for the observation of radiation depended on the sample geometry, the thresholds being lower for thin samples, with the magnetic field perpendicular to both the current and the thin direction.     

Enhanced Initial Permeability and Dielectric Constant in a Double- Percolating Ni0.3Zn0.7Fe1.95O4–Ni– Polymer Composite

A novel, three-phase, double-percolating composite with NiZn-ferrite particles and nickel particles embedded in a poly(vinylidene fluoride) matrix is prepared by a simple hot-pressing method. Large ferrite particles in the composite not only act as a magnetic phase, thus endowing the composite with a high initial permeability, but also present (in a high volume fraction) a discrete (non-percolating) phase, confining polymer and metallic particles into a continuous double-percolating structure of low volume fraction. In particular, a large enhancement in both the initial permeability and the dielectric constant of the three-phase composites is observed, which is due mainly to the addition of a small number of nickel particles that act as both magnetic and percolative metallic phases. The dielectric and magnetic behavior observed in the three-phase composites can be explained by effective-medium and percolation theories.     

Magnetic Mesoporous Nanocarriers for Drug Delivery with Improved Therapeutic Efficacy

Mesoporous CoNi@Au core@shell nanorods are synthesized as magnetic drug nanocarriers by electrodeposition using ionic liquid‐in‐aqueous microemulsions. Mesoporous nanorods present a highly effective area (186 m² g^?1) and magnetic character that allows their manipulation, concentration, and retention by applying a magnetic field. The nanorods have been functionalized with thiol‐poly(ethyleneglycol) molecules, and molecules of Irinotecan, a drug used in chemotherapy, are retained in both the lattice of the linked thiol‐poly(ethyleneglycol) molecules and inside the interconnected nanorods pores. The nanorods' mesoporous character allows a high drug‐loading capability and magnetic behavior that allows the drug's controlled release. A high cellular viability of HeLa cells is obtained after their incubation with the nanorods functionalized with thiol‐poly(ethyleneglycol). However, when the nanorods function as carriers for Irinotecan, significant cell death is occurred when HeLa cells are incubated with the functionalized, drug‐loaded nanorods. Cell death is also produced by applying an alternating magnetic field due to both the effect of the release of Irinotecan from the carrier as to mechanical damage of cells by nanorods subjected to the effect of a magnetic field. The proposal to used mesoporous magnetic nanorods as drug carriers can thus dramatically reduce the amounts of both nanocarrier and drug needed to efficiently destroy cancer cells.     

Compressed-width leaky EH/sub 1/ mode PBG antenna

The recently developed electric-magnetic-electric (EME) microstrip is successfully used to design a compact leaky-mode antenna with fan beam radiation patterns. The EME microstrip consists of composite metals paralleling the electric and magnetic surfaces, where the magnetic surface is made of an array of coupled inductors. The frequency scanning behavior, experimentally and theoretically confirmed by matrix-pencil analyses, indicates that the line width can be reduced by 20%, compared to that of the conventional microstrip with the identical radiation angle of the first higher order (EH/sub 1/) mode at the same frequency.     

Nonlinear diffusion and internal voltages in conductingferromagnetic enclosures subjected to lightning currents

The voltage inside a conducting ferromagnetic enclosure, excited by direct attachment of lightning, is determined. A nonlinear approximate model is constructed and solved analytically to model the magnetic field diffusion through the enclosure wall excited by a filament current. This model gives the internal voltage waveform (at very early times it forms a rough approximation to the solution). The solution (including the early time behavior) for an attached current carrying conductor of large radius is also given. The model is constructed by use of known nonlinear saturation front theory. The paper also considers nonnegligible magnetic flux behind the saturation front (in the saturated region), and shows that this saturated flux results in a simple additive time shift to the penetration of the front     

Spatiotemporal Magnetocaloric Microenvironment for Guiding the Fate of Biodegradable Polymer Implants

The degradation behavior of implants is significantly important for bone repair. However, it is still unprocurable to spatiotemporally regulate the degradation of the implants to match bone ingrowth. In this paper, a magneto-controlled biodegradation model is established to explore the degradation behavior of magnetic scaffolds in a magnetothermal microenvironment generated by an alternating magnetic field (AMF). The results demonstrate that the scaffolds can be heated by magnetic nanoparticles (NPs) under AMF, which dramatically accelerated scaffold degradation. Especially, magnetic NPs modified by oleic acid with a better interface compatibility exhibit a greater heating efficiency to further facilitate the degradation. Furthermore, the molecular dynamics simulations reveal that the enhanced motion correlation between magnetic NPs and polymer matrix can accelerate the energy transfer. As a proof-of-concept, the feasibility of magneto-controlled degradation for implants is demonstrated, and an optimizing strategy for better heating efficiency of nanomaterials is provided, which may have great instructive significance for clinical medicine.     

Co-doped ZnO dilute magnetic semiconductor

This paper reports on recent findings in the Co-doped ZnO system where as-deposited samples with n-type semiconductor properties display magnetic ordering above room temperature. Detailed atomic scale structural characterization has eliminated clustering and second-phase formation as the source of the magnetic behavior. Upon high-temperature annealing in oxygen, the samples become insulating and the magnetization drops. This suggests that the observed magnetic behavior of the oxide is directly related to the presence of intrinsic defects, notably oxygen vacancies and Zn interstitials. These defects are believed to mediate exchange coupling of the Co spins through electron doping of the matrix, and perhaps also the formation of bound magnetic polarons.     

太赫兹二次谐波回旋振荡管

针对应用于动态核极化核磁共振成像的太赫兹回旋振荡器管展开研究,采用相对论电子回旋脉塞非线性理论分析0.46 THz 回旋振荡管互作用系统的动力学行为,研究系统模式竞争问题。优化器件系统参数以获得较高的注-波能量转化效率。利用粒子模拟(PIC)方法验证非线性动力学研究结果,模拟结果显示当工作电压为12 kV、工作电流为0.24 A 时,输出功率和效率分别为173 W 和6%。若采用降压收集极后,则整管效率可进一步提高至14.4%。     

Towards a generalized TLM algorithm for solving arbitraryreciprocal and nonreciprocal planar structures

A generalized transmission line matrix (TLM) algorithm is developed in the frequency domain to tackle arbitrary both reciprocal and nonreciprocal anisotropic waveguiding problems. In particular, the modeling issue for arbitrary planar structures is stressed in this work. A new three-dimensional (3-D) condensed node is used to consider the effect of both electric and magnetic constitutive tensors. Various results indicate how the modal dispersive behavior can be manipulated by changing not only the anisotropic characteristics of the substrate, but also the strip/slot geometry as well as the magnitude and orientation of the applied static magnetic field. The present algorithm is useful for CAD and simulation of a large class of gyrotropic waveguide-based microwave and millimeter-wave circuits     

Multilayer Microwires: Tailoring Magnetic Behavior by Sputtering and Electroplating

A novel technique for preparing multilayer microwires with controlled magnetic behavior has been developed. This technique involves combining sputtering and electroplating procedures to deposit (magnetic or non‐magnetic) metallic nano‐ and microlayers onto glass‐coated amorphous magnetic microwires. A suitable choice of magnetostrictive amorphous metallic nucleus, together with the specific stresses induced by the deposited layers, allows the tailoring of specific magnetic behavior. In this way, the preparation of multilayer microwires characterized either by square‐shaped hysteretic loops (typical of magnetically bistable microwires with longitudinal easy axes), or by nearly non‐hysteretic loops (for those microwires with a circumferential magnetization easy axes), can be achieved.     

The effect of space charge on shot noise in crossed-field electron guns

The two-dimensional electronic behavior of the crossed-field potential minimum is analyzed by means of a feedback network which provides a vehicle for understanding the complex phenomena, while allowing quantitatively accurate numerical calculations. The solutions, limited to the low-frequency range where transit time may be neglected, Show shot-noise smoothing as a function of magnetic field and cathode length. For low magnetic fields and short cathodes, the smoothing approaches the results of the North theory for zero magnetic field. It is also shown that if the cathode length is greater than about 11 normalized (Kino) units or 0.55 of the cycloid length for the existing field conditions, an initial perturbation of the emission current will lead to growing fluctuations of the beam current. This instability is considered to be responsible for the observation, in some crossed-field tubes, that the output noise increases when the cathode is heated sufficiently to form a potential minimum in front of the cathode. This criterion for the existence of excess noise is applied to all the experiments for which sufficient data have been published and no disagreement is found.     

Write Disturbance Modeling and Testing for MRAM

The magnetic random access memory (MRAM) is considered one of the potential candidates that will replace current on-chip memories (RAM, EEPROM, and flash memory) in the future. The MRAM is fast and does not need a high supply voltage for read/write operations, and is compatible with the CMOS technology. It can also endure almost unlimited read/write cycles. These combined advantages of RAM and flash memory make it a potential choice for SOC. In this paper, we present the write disturbance fault (WDF) model for MRAM, i.e., a fault that affects the data stored in the MRAM cells due to excessive magnetic field during the write operation. We also construct the SPICE macro model for the magnetic tunneling junction (MTJ) device of the toggle MRAM to obtain circuit simulation results. We then present an MRAM fault simulator called RAMSES-M, based on which we derive the shortest test for the proposed WDF model. The test is shown to be better and more robust as compared with the conventional March C-test algorithm. We also present a March 17 N diagnosis algorithm for identifying WDF. A 1 Mb MRAM chip has been designed and fabricated using a CMOS-based 0.18-mum technology. The proposed WDF model is justified by chip measurement results, with the march test results reported. Finally, specific MRAM fault behavior and test issues are discussed.     

Propagation in Longitudinally Magnetized Ferrite-Loaded Waveguide

Longitudinally magnetized reciprocal ferrite phase shifters have shown anomalous behavior in that some devices show increasing phase shift with increasing applied field, while others show decreasing phase shift with increasing applied field. This anomaly has been investigated theoretically by using a ferrite-filled parallel plane guide model. It is shown that for electrically thin guides the phase shift decreases with applied magnetic field, whereas with increased thickness, the phase shift becomes an increasing function of the applied field. The microwave electric and magnetic fields were calculated for various applied field values and reduced guide thicknesses. This showed that there are two competing mechanisms which govern the type of phase shift. These can be termed /spl mu/-effective" and "Faraday rotation." The latter sets in when the guide is thick enough to support a cross-polarized electric field of the same order of magnitude as the incident electric field. Similar analysis of the quasi-TE/sub 1/ and TM/sub 1/ modes were made, showing similar behavior at higher frequencies for a given guide thickness. Experimented verification of the quasi-TM/sub 0/ mode was made by observing transmission resonances versus applied field of resonated sections of ferrite-loaded reduced height guide.     

Design and Fabrication of Magnetically Functionalized Core/Shell Microspheres for Smart Drug Delivery

The fabrication of magnetically functionalized core/shell microspheres by using the microfluidic flow‐focusing (MFF) approach is reported. The shell of each microsphere is embedded with magnetic nanoparticles, thereby enabling the microspheres to deform under an applied magnetic field. By encapsulating a drug, for example, aspirin, inside the microspheres, the drug release of the microspheres is enhanced under the compression–extension oscillations that are induced by an AC magnetic field. This active pumping mode of drug release can be controlled by varying the frequency and magnitude of the applied magnetic field as well as the time profile of the magnetic field. UV absorption measurements of cumulative aspirin release are carried out to determine the influence of these factors. The drug release behavior is found to be significantly different depending on whether the applied field varies sinusoidally or in a step‐function manner with time.     

Spatially Resolved Electric‐Field Manipulation of Magnetism for CoFeB Mesoscopic Discs on Ferroelectrics

Electric‐field control of magnetism in ferromagnetic/ferroelectric multiferroic heterostructures is a promising way to realize fast and nonvolatile random‐access memory with high density and low‐power consumption. An important issue that has not been solved is the magnetic responses to different types of ferroelectric‐domain switching. Here, for the first time three types of magnetic responses are reported induced by different types of ferroelectric domain switching with in situ electric fields in the CoFeB mesoscopic discs grown on PMN‐PT(001), including type I and type II attributed to 109°, 71°/180° ferroelectric domain switching, respectively, and type III attributed to a combined behavior of multiferroelectric domain switching. Rotation of the magnetic easy axis by 90° induced by 109° ferroelectric domain switching is also found. In addition, the unique variations of effective magnetic anisotropy field with electric field are explained by the different ferroelectric domain switching paths. The spatially resolved study of electric‐field control of magnetism on the mesoscale not only enhances the understanding of the distinct magnetic responses to different ferroelectric domain switching and sheds light on the path of ferroelectric domain switching, but is also important for the realization of low‐power consumption and high‐speed magnetic random‐access memory utilizing these materials.     

Radiation propagating transverse to the external magnetic field from an electromagnetic source in an unbounded plasma

The radiation characteristics of a line source of magnetic current embedded in an unbounded plasma are investigated for the case in which a uniform magnetic field is impressed externally throughout the medium in the direction of the source. The plasma is assumed to be a homogeneous and macroscopically neutral mixture of compressible gas of electrons and ions. A two-fluid continuum theory of plasma dynamics is employed. It is shown that it is possible to define three suitable wave functions which satisfy separately simple wave equations whose solutions are written down by inspection. These wave functions specify the three possible modes which are identified, respectively, to be the modified forms of the electromagnetic, the electron plasma and the ion plasma modes. The limiting behavior of these modes are discussed for the following two cases: 1) infinite source frequency and 2) vanishing external magnetic field. The dispersion relations for the three modes are examined in detail for the general case using a perturbation procedure. It is shown that the modified ion plasma (MIP) mode propagates for all frequencies whereas both the modified electron plasma (MEP) mode and the modified electromagnetic (MEM) mode have a low-frequency cutoff. Explicit expressions for the cutoff frequencies are obtained. The power radiated in each of the three modes is also evaluated. It is found that the power radiated in the MEM mode is always lower than that due to the line source in free space, whereas the power radiated in the two plasma modes is higher than that value for certain ranges of frequency.     

Integrated electroplated micromachined magnetic devices using lowtemperature fabrication processes

Micromachining techniques are used to realize inductors and transformers integrated with a multichip package, allowing compact integration with chips, sensors, and other components. The processing steps chosen are all low-temperature, which allows the use of low cost substrates such as MCM-L compatible materials. A variety of micromachined inductors and transformers with different geometries and magnetic core materials are designed, fabricated, tested, and compared. Integrated permalloy and orthonol core inductors (15 μm thick) with nominally identical geometries of 4 mm×1.0 mm×0.13 mm and 30 turns of multilevel copper coils (40 μm thick) show differences in performance due to differences in core behavior. The permalloy core inductor has a slightly higher inductance, but it has much lower dc saturation current than the orthonol core inductor. The effect of insertion of a core air gap was also studied, Although inductors with no air gap having dimensions of 4 mm×4 mm×0.145 mm and 156 turns of multilevel electroplated copper coils (40 μm thick) and electroplated permalloy magnetic core (35 μm thick) have slightly higher inductance (about 1.5 μH), air gap inductors have much higher saturation current (180=250 mA). These devices have high current capability (up to 3 A steady dc current) and are suitable for low power converter applications