Highly Oriented SrM Tape Hexaferrite

This paper reports on preparation of a magnetically anisotropic polycrystalline hexaferrite, Sr$_0.95$Ca$_0.05$Fe$_12$O$_19$, whose orientation degree is as high as 100%. The preparation uses standard ceramic techniques, including wet pressing and flowing-oxygen sintering. In this process, Ca$^+2$ions are substituted for Sr$^+2$ions in SrFe$_12$O$_19$by adding minimum impurities (Bi$_2$O$_3$and MnCO$_3$). Such a hexaferrite has a dielectric loss lower than$hbox2.3 times hbox10^-3$at 9.5 GHz, which is important for microwave applications and possibly for millimeter-wave applications. The paper describes the variation of specific saturation magnetization$sigma_ s$and magnetic crystal anisotropy field ($H_a$) with temperature ($T$), and compares the magnetic properties of the sintered hexaferrite with those of the strontium hexaferrite SrFe$_12$O$_19$.     

Magnetometric Demagnetizing Factors for Various Shapes of Rotational Symmetry

We have investigated the magnetometric demagnetizing factors of rotation-symmetrical magnetic bodies of relative permeability $mu$ placed in a uniform magnetic field parallel to the axis of rotation. The bodies we considered include a cylinder, a sphere, a bicone, and a rotating astroid. We numerically calculated the magnetometric demagnetizing factors from the surface magnetic charge densities $sigma$ obtained by the surface magnetic charge simulation method. We show that the magnetometric demagnetizing factors for various shapes of rotation-symmetrical bodies are significantly different from those of a sphere. We have determined the shape producing the largest demagnetizing field in ferromagnetic bodies with sufficiently large relative permeability $mu$ . We report the magnetometric demagnetizing factors for uniformly magnetized bodies ($mu=1$ , i.e., susceptibility $chi=0$ ) of various shapes.      

Anisotropic Fracture Behavior of Sintered Rare-Earth Permanent Magnets

Sintered rare-earth permanent magnets (for example, Sm$_2$Co$_17$, SmCo$_5$, and Nd$_2$Fe$_14$B magnets) are quite brittle and easily crack in the course of fabrication, machining, and application. Here, we report on an investigation of fracture behavior and mechanical characteristics of sintered rare-earth permanent magnets and discuss the origin of rare-earth magnet brittleness. We studied three groups of bending specimens with different orientations, cut from the same block of magnet. The bending strength was measured. The fracture surfaces were carefully observed by scanning electron microscopy and showed that the fracture behavior and bending strength of sintered rare-earth permanent magnets obviously exhibit anisotropy. Sintered Sm–Co magnets tend to cleavage fracture in the close-packed ($0001$) plane or in the ($10bar11$) plane. The fracture mechanism of sintered Nd$_2$Fe$_14$B magnet appears to be mainly intergranular fracture. Our analysis indicates that the anisotropy of fracture behavior and mechanical strength of sintered rare-earth magnets is caused by the strong crystal-structure anisotropy and grain texture created by magnetic field alignment during the fabrication process.     

Coercivity Variation in Ni$_80$Fe$_20$Ferromagnetic Nanowires

Deep ultraviolet lithography was used to fabricate highly ordered 80-nm-thick Ni$_80$Fe$_20$nanowire arrays over a large area (4$,times,$4 mm$^2$). The magnetic properties of the wire array display a marked increase in both the coercivity and switching field due to magnetic shape anisotropy compared with the bulk film. The angular variation of the coercivity for nanowire arrays has been investigated in order to probe the magnetization reversal process. Our systematic study of the coercivity variation, which is also aided by theoretical modeling, reveals that the curling mode of rotation is the dominant reversal mechanism.     

A method for simultaneously measuring resistivity and permeabilitywith a multilayer solenoid coil

A formula is given for a more accurate estimate of either the resistivity and permeability of a cylindrical magnetic material or the resistivity and penetration depth of a cylindrical superconductor, using the difference in the complex impedance between a circular multilayer solenoid coil having a conductor and a similar coil without a conductor. In comparison with the conventional method which uses a single-layer solenoid coil, it is shown experimentally that a multilayer solenoid coil is more effective for a short sample conductor because a larger difference in the complex impedance is obtained without increasing the width of the solenoid coil. The results of measurements of magnetic and superconductive materials are included     

Effect of Interface Roughness on Magnetoresistance and Magnetization Switching in Double-Barrier Magnetic Tunnel Junction

The three kinds of double-barrier magnetic tunnel junction (DMTJ) with or without amorphous ferromagnetic Co$_{70.5}$ Fe$_{4.5}$ Si$_{15}$ B$_{10}$ (in at. %) free-layer were investigated to understand the effect of the free-layer on the bias voltage dependence of tunneling magnetoresistance (TMR) ratio. The DMTJ structure consisted of Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO$_{rm x}$/free-layer 7/AlO$_{rm x}$/CoFe 7/IrMn 10/Ru 60 (thickness in nm). Various free layers, such as CoFe 7, CoFeSiB 7, and CoFe 1.5/CoFeSiB 4/CoFe 1.5 were prepared and compared. The roughness values of the interface between free-layer and tunnel barrier were confirmed by using the techniques of X-ray reflectivity and transmission electron microscopy. As a result, the amorphous free-layer made the interface roughness of DMTJ smoother, reducing the interlayer coupling field and suppressing the bias voltage dependence of TMR ratio at a given voltage.      

Microphotoluminescence and Microphotoreflectance Analyses of$hboxCO_2$Laser Rapid-Thermal-Annealed$hboxSiO_x$Surface With Buried Si Nanocrystals

The optical properties of a$hboxSiO_x$film rapid-thermal-annealed (RTA) by$hboxCO_2$laser are primarily investigated. The microphotoluminescence ($mu$-PL) and high-resolution transmission electron microscopy (HRTEM) analyses indicate that the precipitation of random-oriented Si nanocrystals can be initiated when laser intensity$(P_ laser)$is larger than 4.5$hboxkW/cm^2$. At$P_ laser$of 6$hboxkW/cm^2$, the Si nanocrystal exhibits a largest diameter of 8 nm and a highest density of$hbox4.5times hbox10^16 hbox cm^-3$, which emits strong PL at 790–825 nm. The microphotoreflectance of the$hboxCO_2$laser RTA$hboxSiO_x$film reveals a volume density product dependent refractive index increasing from 1.57 to 1.87 as$P_ laser$increases from 1.5 to 7.5$hboxkW/cm^2$. Nonetheless, the laser ablation of the$hboxSiO_x$film occurs with a linear ablation slope of 35$hboxnm/kW/cm^2$at beyond 7.5$hboxkW/cm^2$, which terminates the enlargement of Si nanocrystals, degrades the near-infrared PL, and slightly reduces the refractive index of the$hboxCO_2$laser RTA$hboxSiO_x$film.     

Induced Magnetic Anisotropy and Stress-Impedance Effect in Nanocrystalline$hboxFe_73.5hboxCu_1hboxNb_3hboxSi_13.5hboxB_9$Ribbons

The influence of direction and strength of induced magnetic anisotropy on stress-impedance (SI) effect was experimentally and theoretically studied in this paper. Experimentally, it was found that the magnetic anisotropy of the stress Joule heated$hboxFe_73.5hboxCu_1hboxNb_3hboxSi_13.5hboxB_9$nanocrystalline ribbons were determined by direction and strength of the induced anisotropy. Theoretical calculations of the direction and strength of the induced magnetic anisotropy suggest that transverse anisotropy and small anisotropic field result in an increase of the SI effect. To decrease the anisotropic field and increase the transverse anisotropy simultaneously, a complex annealing process was applied to$hboxFe_73.5hboxCu_1hboxNb_3hboxSi_13.5hboxB_9$ribbons, and it was found that the SI effect was drastically improved. A maximum change of 286% in the SI ratio of the complex annealed nanocrystalline$hboxFe_73.5hboxCu_1hboxNb_3hboxSi_13.5hboxB_9$ribbon was observed around 10 MHz frequencies.     

Microstructure, Electrical, and Magnetic Properties of ZrO$_{2}$ Added MnZn Ferrites

We investigated the influence of ZrO$_{2}$ on the microstructure and electromagnetic properties of MnZn ferrites by characterizing fracture surface micrographs, magnetic properties, and dc resistivity. Powders of Mn $_{0.68}$Zn $_{0.25}$Fe $_{2.07}$O $_{4}$ composition were prepared by the conventional ceramic technique. Toroidal cores were sintered at 1350 $^{circ}$C for 4 h in N$_{2}$/O$_{2}$ atmosphere with 4% oxygen. The results show that the lattice constant and average grain size increase with ZrO$_{2}$ concentration, but excessive ZrO $_{2}$ concentration will result in exaggerated grain growth and porosity increase. The dc resistivity, activation energy, saturation magnetic flux density, and initial magnetic permeability increase monotonically when the ZrO$_{2}$ concentration is not more than 0.04 wt% and then decrease with further increase of ZrO$_{2}$ concentration. On the other hand, the porosity, drift mobility, resonance frequency, and core loss decrease initially and then increase with the increase of ZrO$_{2}$ concentration.      

A Fiber-Optic Sensor for Monitoring Trace Ammonia in High-Temperature Gas Samples With a ${hbox{CuCl}}_{2}$-Doped Porous Silica Optical Fiber as a Transducer

An optical fiber chemical sensor for detecting/monitoring trace ammonia in high-temperature gas streams has been developed. This sensor uses a ${hbox{CuCl}}_{2}$-doped porous silica optical fiber, prepared via a previously reported sol-gel process, as a transducer. Trace ammonia in a gas sample diffuses into the porous fiber to react with the doped agent to form a ${hbox{Cu}}^{2+}$-ammonia complex. The concentration of the ${hbox{Cu}}^{2+}$ -ammonia complex inside the porous silica optical fiber is proportional to ammonia concentration in the gas sample, to which the sensing porous silica fiber is exposed. Therefore, ammonia concentration in the gas sample can be detected through detecting the optical absorption signal of the formed ${hbox{Cu}}^{2+}$-ammonia complex inside the fiber by using a fiber-optic UV/Vis absorption spectrometric method. This sensor can be used to reversibly monitor trace ammonia in a gas sample at an elevated temperature up to 450 $^{circ}$C in the tested range. A detection limit of 0.24 ppmv ammonia in an air gas sample was achieved when the sensor was tested at a temperature of 450 $^{circ}$ C.      

A Study of Hydrogen Sensing Performance of Pt–GaN Schottky Diodes

The performance of hydrogen-gas detectors based on Pt–GaN Schottky diodes with 24-nm-thick Pt contact was investigated. Current–voltage ($I$–$V$) Characteristics were measured in two ambients (e.g., synthetic air (20%$hboxO_2$in$hboxN_2$) and 1-vol.%$hboxH_2$in synthetic air) at different temperatures. The forward current of the diodes is found to increase significantly upon introduction of$hboxH_2$into the synthetic air ambient. Analysis of the$I$–$V$characteristics as a function of temperature demonstrated that the observed current increase is due to a decrease in the effective barrier height (BH) through a decrease in the Pt work function upon absorption of hydrogen. The decrease in the BH was measured as high as 30 and 152 meV at 25$^circhboxC$and at 280$^circhboxC$, respectively, upon introduction of$hboxH_2$into the ambient. The changes in the BH were completely reversible upon restoration of the synthetic air ambient. The sensitivity to the hydrogen gas was investigated in dependence on the operating temperature for 1-vol.% hydrogen in synthetic air. The changes in the forward bias at a constant current density of 3.2$hboxA/cm^2$was 90 and 330 mV at 25$^circhboxC$and at 310$^circhboxC$, respectively, upon introduction of 1-vol.%$hboxH_2$into the ambient. Additionally, a significant increase in the sensitivity and a decrease in the response and recovery times have been observed after increasing the operating temperature up to$sim hbox310 ^circhboxC$.     

Modeling and Simulation of a Single Tin Dioxide Nanobelt FET for Chemical Sensors

This paper presents the modeling and simulation of a tin dioxide (${rm SnO}_{2}$) field-effect transistor (FET)-based nanobelt gas sensor. The model results are compared to numerical simulations and experimental data obtained from published results describing the fabrication of single crystal nanobelts grown through thermal evaporation techniques. The fabricated sensor shows good response when exposed to oxygen (${rm O} _{2}$) and hydrogen (${rm H} _{2}$) at room temperature. Gas adsorption causes changes in the electrical contacts due to oxygen vacancies in the bulk. As a result, the ${rm I}$ -${rm V}$ characteristics are very different when the device is exposed to (${rm O} _{2}$) versus (${rm H} _{2}$ ). In the presence of ${rm H} _{2}$, the behavior of the contacts is ohmic and saturation is caused by pinch-off of the channel at the drain contact. However, in the presence of ${rm O} _{2}$ , the behavior of the contacts is Schottky, and device saturation occurs at the source end of the device. Our model is based on a depletion mode MOSFET and it accounts for both ohmic and Schottky contacts when the device is exposed to oxygen or hydrogen. It also provides a possible explanation for the gate bias dependence of the saturation current seen in some published characterization data.      

Analytical Modeling of Open-Circuit Air-Gap Field Distributions in Multisegment and Multilayer Interior Permanent-Magnet Machines

We present a simple lumped magnetic circuit model for interior permanent-magnet (IPM) machines with multisegment and multilayer permanent magnets. We derived analytically the open-circuit air-gap field distribution, average air-gap flux density, and leakage fluxes. To verify the developed models and analytical method, we adopted finite-element analysis (FEA). We show that for prototype machines, the errors between the FEA and analytically predicted results are $≪$1% for multisegment IPM machines and $≪$ 2% for multilayer IPM machines. By utilizing the developed lumped magnetic circuit models, the IPM machines can be optimized for maximum fundamental and minimum total harmonic distortion of the air-gap flux density distribution.      

流动气体中应力焦耳热退火FeCo基薄带的GMI效应

采用单辊快淬法制备Fe36Co36Nb4Si4.8B19.2(FeCo基合金)薄带,在流动的气体中,施加不同的张应力并通以直流电进行退火,采用HP4294A型阻抗分析仪测量纵向驱动巨磁阴抗效应(LDGMI)曲线.对系列LDGMI曲线特征与外加张应力关系的分析结果表明,在流动气体中进行电流退火的FeCo基合金薄带,外加张...     

Room-Temperature Ferromagnetic Property in MnTe Semiconductor Thin Film Grown by Molecular Beam Epitaxy

MnTe layers of high crystalline quality were successfully grown on Si(111) and Al$_{2}$O$_{3}$(0001) substrates by molecular beam epitaxy. We have investigated the structure, magnetic and electric transport properties of MnTe layers by using X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometer, physical properties measurement system (PPMS), and X-ray photoelectron spectroscopy (XPS). Characterization of MnTe layers on Si(111) and Al $_{2}$O $_{3}$(0001) substrates by X-ray diffraction (XRD) revealed a hexagonal structure of polycrystalline growth for MnTe/Si(111) and epitaxial growth for MnTe/Al$_{2}$ O$_{3}$ (0001), respectively. Investigation of magnetic properties for MnTe layers showed ferromagnetic properties above room temperature unlike antiferromagnetic bulk MnTe materials. The great irreversibility between zero-field-cooling and field-cooling magnetization were observed. Apparent ferromagnetic hysteresis loops are measured at room temperature. In electro-transport measurements, the temperature dependence of resistivity revealed a noticeable semiconducting behaviors and showed a conduction via variable range hopping (VRH) at low temperature. From XPS results, we assume that the origin of ferromagnetism in samples may be due to the breaking of superexchange antiferromagnetic correlations between Mn spin moments arising from Tellurium vacancies.      

Electromagnetic Forces Acting on the Planar Armature of a Core-Type Synchronous Permanent-Magnet Planar Motor

The core-type synchronous permanent-magnet planar motor (SPMPM) discussed in this paper includes one or more planar armatures each of which contains two sets of three-phase windings named ${rm x}$-winding and ${rm y}$-winding. For each planar armature, a magnetic field energy equation is established first. This equation describes the mechanism of the coupling between the permanent-magnet array, the ${rm x}$-winding and the ${rm y}$ -winding in the core-type SPMPM. By using virtual work principle, ${rm x}$-direction thrust force, ${rm y}$ -direction thrust force and vertical force acting on the planar armature are modeling analytically. For eliminating the coupling in these force models, the excitation flux linkages and phase currents are all transformed into ${rm d}hbox{-}{rm q}$ synchronous reference frame. From the decoupling force equations, some characteristics of the vertical component of force on the planar armature are obtained. The electromagnetic force model is helpful for the design of the contactless planar bearing and the servo control system of the SPMPM.      

Attenuated Oscillation of the Tunneling Magnetoresistance in a Ferromagnet-Metal-Insulator-Ferromagnet Tunneling Junction

The tunneling magnetoresistance (TMR) ratio is investigated in a ferromagnet-metal-insulator-ferromagnet planar tunneling junction by use of the spin-polarized free-electron model. Due to the coherence multiple reflective scatterings within the metallic layer, the TMR ratio oscillates as the metallic thickness increases. Due to the same scatterings, not only electrons with the out-of-plane energy $E_{xi}$ close to the Fermi energy $E_{F}$ but also electrons with $E_{xi}$ below $E_{F}$ can tunnel sufficiently through the tunneling junction, which leads to the attenuated oscillation of the TMR ratio.      

Establishment of All Digital Closed-Loop Interferometric Fiber-Optic Gyroscope and Scale Factor Comparison for Open-Loop and All Digital Closed-Loop Configurations

This paper covers the design details of an all digital closed-loop interferometric fiber-optic gyroscope (ADCL-IFOG) prototype, constructed in TUBITAK UME, and scale factor comparison between open-loop and ADCL-IFOG prototypes with sine wave biasing modulation. The output of demodulation circuit, proportional to the applied rotation rate, was sampled by AD7714YN analog-to-digital converter (ADC), operated in 16 bit resolution. Error voltage, generated by microcomputer – controlled LTC 1667CG, 14 bit digital to analog converter (DAC), was sent to the phase modulator through a linear summing circuit to make Sagnac Phase Shift zero, depending on the rotation direction. For this implementation, the ultimate rotation rate of 1.84 ($^{circ}/{hbox{h}}$ ) was nullified. The averaged sensitivity of the proposed closed-loop IFOG in unit of error voltage applied to the phase modulator was determined as 132.65 $mu hbox{V}/(^{circ}/{hbox{h}}$ ). The scale factors of both the open-loop and ADCL-IFOG prototypes were compared in a range of 1–15270 ( $^{circ}/hbox{h}$) rotation rate, corresponding to Sagnac Phase Shifts varying from 0.00115 ( $^{circ}$) to 17.57448 ( $^{circ}$). The maximum peak to peak noise and the bias stability of ADCL-IFOG prototype were determined as 4.97 ($^{circ}/hbox{h}$ ) and 1.48 ($^{circ}/hbox{h}$ ) at 23.0$~^{circ}hbox{C}$ , respectively.      

Fabrication and Application of Ruthenium-Doped Titanium Dioxide Films as Electrode Material for Ion-Sensitive Extended-Gate FETs

A novel ruthenium-doped titanium dioxide (TiO $_{2}$: Ru) film for pH detection is based on an ion-sensitive extended gate field effect transistor (ISEGFET) sensor. For the preparation of the TiO$_{2}$ : Ru sensing film, a specific processing for metal modification of TiO$_{2}$ thin film is deposited by a co-sputtering system. After thermal annealing treatment, material analysis of the sensing layer is measured by SEM, Hall measurement system and electrical detection system. The average sensitivity of TiO$_{2}$: Ru for hydrogen ion detection is about 55.20 mV/pH (concentration range between pH1 and pH13). The effect of long-term drift for TiO$_{2}$ : Ru ISEGFET-based sensor is presented. Drift rate of the sensor for pH is 0.745 mV/h for 12 h. In order to prepare the calcium ion sensor, the sensing membrane of polymer materials is based on TiO $_{2}$: Ru ISEGFET-based sensor by physical adsorption. The average sensitivity of the calcium ion sensor in the concentration ranging between 1 M and 1$,times,$ 10$^{-3}$ M CaCl$_{2}$ is about 29.65 mV/pCa.      

Proposal of a Novel Pole Type Structures in Perpendicular MRAM for High Gb/Chip

This paper proposes a novel MRAM using perpendicular magnetic tunnel junction device for high capacity. Conventional MRAM has weak points to realize high capacity in the design structure of the cell, one of which is that using simple current injection system can generate only weak switching field. As a solution, we propose a novel MRAM that has two additional poles in this paper. Proposed novel MRAM has a strong switching field owing to two poles added on both sides of the free layer, just like perpendicular magnetic recording heads. In this paper, analysis of the switching field and useful designs for high Gb/Chip are presented. This research was done using three dimensional FEM with injected current density of $8times 10^{7}$ A/cm$^{2}$ –$6times 10^{8}$ A/cm$^{2}$ .