Demagnetizing factors for rectangular prisms

For rectangular prisms of dimensions 2a/spl times/2b/spl times/2c with constant material susceptibility /spl chi/, we have calculated and tabulated the fluxmetric and magnetometric demagnetizing factors N/sub f/ and N/sub m/, defined along the 2c dimension as functions of c/(ab)/sup 1/2/(=1/spl sim/500), a/b(=1/spl sim/256), and /spl chi/(=0/spl sim/10/sup 9/). We introduce an interpolation technique for obtaining N/sub f,m/ with arbitrary values of c/(ab)/sup 1/2/, a/b, and /spl chi/.     

Inspection of intrinsic critical currents for spin-transfer magnetization switching

We examined the relationships between critical current, I/sub c/, and switching time, /spl tau//sub p/, for spin-transfer switching in two regions: (region I) /spl tau//sub p//spl Gt//spl tau//sub 0/, where thermal switching is accompanied and (region II) /spl tau//sub p/< several tens times /spl tau//sub 0/, where /spl tau//sub 0/ is the attempt time for thermal switching (/spl ap/1 ns). We estimated I/sub c0/, defined as the intrinsic I/sub c/ at 0 K, for both regions and confirmed experimentally that those I/sub c0/ coincided with each other at room temperature (RT). The value of I/sub c/ at /spl tau//sub p/=1 ns, measured with microwaves, was approximately 1.6 times the I/sub c0/. This suggested that we use at least two times I/sub c0/ as the writing currents of magnetic memory devices for nsec spin-transfer switching at RT. Although I/sub c0/ for both regions were defined as I/sub c/ at 0 K (I/sub c//sup 0K/) in theory, they showed temperature dependence at low temperatures; |I/sub c0/| for region I increased with decreasing temperature, and the estimated I/sub c//sup 0K/ was approximately three times I/sub c0/ for RT. This temperature dependence was quite different from that for region II.     

Structure, properties, and thermal stability of nanocrystallite Fe-Ti-N soft magnetic films

We deposited Fe-Ti-N magnetic films with a high sputtering power of 7 W/cm/sup 2/. When the composition of the films was in the range of Fe-Ti(3.9 at.%)-N(8.8 at.%) to Fe-Ti(3.3 at.%)-N(13.5 at.%), the films were composed of /spl alpha/' and Ti/sub 2/N precipitates. With the addition of nitrogen, 4/spl pi/M/sub s/ became higher than that of pure iron, reaching a maximum of 23.8 kG. At the same time, H/sub c/ was reduced to a minimum of 1.12 Oe. The best films can meet the needs of the recording head in dual-element giant magnetoresistive/inductive heads, yielding high storage density (10 Gb/in/sup 2/). The incorporation of N in /spl alpha/-Fe brought about the /spl alpha/' phase with its higher saturation magnetization. Ti additions inhibited the equilibrium decomposition /spl alpha/'/spl rarr//spl alpha/+/spl gamma/'. Because H/sub C//sup D//spl prop/D/sup 6/, where D is average grain diameter, grain size control is very important. The nitrogen induces severe distortion of the /spl alpha/' lattice, which can cause the grains to break into pieces and reduce the grain size. High sputtering power also led to the formation of fine grains, with diameter in the order of 14 nm. Probably Ti/sub 2/N is preferentially precipitated on the grain boundary, pinning the grain boundary and stabilizing the grain size during high-temperature heat treatment. The temperature limit for stability of the structure and its associated low coercivity was not less than 520/spl deg/C.     

The enhancement of Q factor for patterned ground shield inductors at high temperatures

We report on an effective way of using a patterned ground shield (PGS) to enhance the Q factor of on-chip spiral inductors. We fabricated PGS inductors using both 0.18 /spl mu/m and 0.35 /spl mu/m CMOS processes, with M1 and poly strip PGSs, respectively. The strip width and spacing of the PGSs are W/sub g/=0.8 /spl mu/m and S/sub g/=0.45 /spl mu/m, with metal thicknesses of t/sub p/={0.54,0.2} /spl mu/m in the 0.18 /spl mu/m process, and t/sub p/={0.6,0.3} /spl mu/m in the 0.35 /spl mu/m process. The separation distance D between PGS and top metal layer is different in both processes. We found that the Q factor degradation of inductors at high temperatures can be effectively compensated by using PGS. Among all geometric parameters of a PGS in the 0.18 /spl mu/m process, the parameter D is the critical factor for the shielding effectiveness, and M1 PGS is much more efficient than poly strip PGS in improving the inductor performance over the temperature range of 298 K to 358 K. However, in the 0.35 /spl mu/m process the latter is better than the former.     

Spin-transfer switching current distribution and reduction in magnetic tunneling junction-based structures

Spin transfer switching current distribution within a cell and switching current reduction were studied at room temperature for magnetic tunnel junction-based structures with resistance area product (RA) ranged from 10 to 30 /spl Omega/-/spl mu/m/sup 2/ and TMR of 15%-30%. These were patterned into current perpendicular to plane configured nanopillars having elliptical cross sections of area /spl sim/0.02 /spl mu/m/sup 2/. The width of the critical current distribution (sigma/average of distribution), measured using 30 ms current pulse, was found to be 3% for cells with thermal factor (KuV/k/sub B/T) of 65. An analytical expression for probability density function p(I/I/sub c0/) was derived considering a thermally activated spin transfer model, which supports the experimental observation that the thermal factor is the most significant parameter in determining the within-cell critical current distribution. Spin-transfer switching current reduction was investigated through enhancing effective spin polarization factor /spl eta//sub eff/ in magnetic tunnel junction-based dual spin filter (DSF) structures. The intrinsic switching current density (J/sub c0/) was estimated by extrapolating experimental data of critical current density (J/sub c/) versus pulse width (/spl tau/), to a pulse width of 1 ns. A reduction in intrinsic switching current density for a dual spin filter (DSF: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/spacer/CoFe/PtMn/Ta) was observed compared to single magnetic tunnel junctions (MTJ: Ta/PtMn/CoFe/Ru/CoFeB/Al2O3/CoFeB/Ta). J/sub c/ at /spl tau/ of 1 ns (/spl sim/J/sub c0/) for the MTJ and DSF samples were 7/spl times/10/sup 6/ and 2.2/spl times/10/sup 6/ A/cm/sup 2/, respectively, for identical free layers. Thus, a significant enhancement of the spin transfer switching efficiency is seen for DSF structure compared to the single MTJ case.     

Limiting sensitivity of a differential absorption spectrometer with direct detection in the 2/spl nu//sub 3/ and /spl nu//sub 2/+2/spl nu//sub 3/ vibration bands [methane detection applications]

A differential absorption spectrometer, for methane detection, using tunable laser diodes in the 1.33-/spl mu/m and 1.66-/spl mu/m bands, has been studied. The spectral scanning is carried out by current modulation of the laser diode. We analyzed the performance with a multimode Fabry-Perot laser diode in the /spl nu//sub 2/+2/spl nu//sub 3/ band of methane and a monomode laser diode in the 2/spl nu//sub 3/ band. The theoretical results are validated by several experiments. To determine the sensitivity limit of the sensor, we have examined the influence of the noise sources. A sensitivity of 10 ppm/spl middot/m was obtained in the 2/spl nu//sub 3/ band. The main limiting factors are the relative intensity noise of the laser, optical interferences, and quantization noise. We also analyzed the influence of the temperature on the laser diode emission spectra and the methane absorption spectra.     

50-nm fully depleted SOI CMOS technology with HfO/sub 2/ gate dielectric and TiN gate

In this paper, we demonstrate for the first time CMOS thin-film metal gate FDSOI devices using HfO/sub 2/ gate dielectric at the 50-nm physical gate length. Symmetric V/sub T/ is achieved for long-channel nMOS and pMOS devices using midgap TiN single metal gate with undoped channel and high-k dielectric. The devices show excellent performance with a I/sub on/=500 /spl mu/A//spl mu/m and I/sub off/=10 nA//spl mu/m at V/sub DD/=1.2 V for nMOSFET and I/sub on/=212 /spl mu/A//spl mu/m and I/sub off/=44 pA//spl mu/m at V/sub DD/=-1.2 V for pMOSFET, with a CET=30 /spl Aring/ and a gate length of 50 nm. DIBL and SS values as low as 70 mV/V nand 77 mV/dec, respectively, are obtained with a silicon film thickness of 14 nm. Ring oscillators with 15 ps stage delay at V/sub DD/=1.2 V are also realized.     

Core-current-enhanced domain-wall pinning in nanocrystalline Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 15.5/B/sub 7/ ribbon

We have studied the influence of surface fields H/sub p/ (generated by either direct or alternating core current) on soft magnetic properties of amorphous and nanocrystalline Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 15.5/B/sub 7/ ribbon. While in an amorphous ribbon the coercive field H/sub c/ decreases with H/sub p/, in the same optimally annealed ribbon (H/sub c/=1.3 A/m, M/sub m//spl ap/M/sub s/) H/sub c/ increases with H/sub p/ for all the explored types of H/sub p/ (static and dynamic with different phases with respect to that of the magnetizing field H). The unexpected increase of H/sub c/ in nanocrystalline ribbon is associated with the influence of H/sub p/ on the surface and main (inner) domain structure. Here, we develop a model that takes into account this influence and explains the experimental results.     

Recent developments in high curie temperature perovskite single crystals

The temperature behavior of various relaxor-PT piezoelectric single crystals was investigated. Owing to a strongly-curved morphotropic phase boundary, the usage temperature of these perovskite single crystals is limited by T/sub R-T/- the rhombohedral to tetragonal phase transformation temperature - which occurs at the significantly lower temperatures than the Curie temperature T/sub c/. Attempts to modify the temperature usage range of Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PZNT) and Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PMNT) rhombohedral crystals (T/sub c/ /spl sim/ 150-170/spl deg/C, T/sub R-T/ /spl sim/ 60-120/spl deg/C) using minor dopant modifications were limited, with little success. Of significant potential are crystals near the morphotropic phase boundary in the Pb(Yb/sub 1/2/Nb/sub 1/2/)O/sub 3/-PbTiO/sub 3/ (PYNT) system, with a T/sub c/ > 330/spl deg/C, even though T/sub R-T/ was found to be only half the value at /spl sim/160/spl deg/C. Single crystals in the novel BiScO/sub 3/-PbTiO/sub 3/ system offer significantly higher T/sub c/s > 400/spl deg/C, while exhibiting electromechanical coupling coefficients k/sub 33/ > 90% being nearly constant till the T/sub R-T/ temperature around 350/spl deg/C, which greatly increases the temperature range for transducer applications.     

Modeling plastic deformation effects in steel on hysteresis loops with the same maximum flux density

Plastic deformation affects the hysteretic magnetic properties of steels because it changes the dislocation density, which affects domain-wall movement and pinning, and also because it places the specimen under residual strain. An earlier paper proposed a model for computing hysteresis loops on the basis of the effect of grain size d and dislocation density /spl zeta//sub d/. In that paper, hysteresis loops were compared that all had the same maximum flux density B/sub max/. The result was that coercivity H/sub c/ exhibited a linear relationship with inverse grain size (1/d) and /spl zeta//sub d//sup 1/2/. The same was true of hysteresis loss W/sub H/. If one compared hysteresis loops all with the same H/sub max/, these linear dependences were only approximately found. Because the relationships are simpler for loops of constant B/sub max/, core loss experimenters compare loops that all have the same B/sub max/. In this paper, we modify the model to study the effect of plastic tensile deformation on hysteresis loops with the same B/sub max/. We found linear relationships between H/sub c/ and residual plastic strain /spl epsiv//sub r/ and between W/sub H/ and /spl epsiv//sub r/. With increasing residual tensile strain, H/sub c/ increases (whereas with increasing elastic tensile strain, H/sub c/ decreases). Also, with increasing residual tensile strain, the slope of the hysteresis loop decreases (whereas with increasing elastic tensile strain, the slope increases). We also consider the effect of compressive plastic deformation.     

Synthetic ferrimagnetic media: effects of thermally assisted writing

Thermally assisted writing on high-coercivity synthetic ferrimagnetic media (SFM) was demonstrated using a conventional spin stand equipped with an optical head for commercial magnetooptical drives. The laser light (/spl lambda/ = 685 nm) was focused through a glass substrate onto a recording layer. The optical spot size was 1.1 /spl mu/m and a commercial magnetic head had a writer width of /spl sim/0.25 /spl mu/m. The recording properties were measured as a function of the writing current (I/sub w/) and the laser power (P/sub w/). For the thermally stable medium with H/sub c/ = 6 kOe, a laser irradiation with an optimum power significantly improved the overwrite performance and the signal-to-noise ratio (SNR) values. The SNR values were improved by optimizing P/sub w/ over a wide writing current range. The improvements with the assist were found in both the signal and the noise. The media with a large dynamic coercivity value or with thick magnetic layers clearly showed the advantages with thermal assist.     

Electric fields within cells as a function of membrane resistivity--a model study

Externally applied electric fields play an important role in many therapeutic modalities, but the fields they produce inside cells remain largely unknown. This study makes use of a three-dimensional model to determine the electric field that exists in the intracellular domain of a 10-/spl mu/m spherical cell exposed to an applied field of 100 V/cm. The transmembrane potential resulting from the applied field was also determined and its change was compared to those of the intracellular field. The intracellular field increased as the membrane resistance decreased over a wide range of values. The results showed that the intracellular electric field was about 1.1 mV/cm for R/sub m/ of 10 000 /spl Omega//spl middot/cm/sup 2/, increasing to about 111 mV/cm as R/sub m/ decreased to 100 /spl Omega//spl middot/cm/sup 2/. Over this range of R/sub m/ the transmembrane potential was nearly constant. The transmembrane potential declined only as R/sub m/ decreased below 1 /spl Omega//spl middot/cm/sup 2/. The simulation results suggest that intracellular electric field depends on R/sub m/ in its physiologic range, and may not be negligible in understanding some mechanisms of electric field-mediated therapies.     

Magnetic and structural properties of Co ion-implanted GaN

A GaN epilayer was grown on Al/sub 2/O/sub 3/ substrate by metal-organic chemical vapor deposition, and Co/sup -/ ions with a dose of 3/spl times/10/sup 16/ cm/sup -2/ were implanted into GaN at 350/spl deg/C. The implanted samples were postannealed at 700/spl deg/C-900/spl deg/C to recrystallize the samples and to remove implantation damage. We have investigated the magnetic and structural properties of Co ion-implanted GaN by using X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometer, and X-ray photoelectron spectroscopy (XPS). XRD results did not show any peaks associated with the second phase formation, and only the diffraction from the GaN layer and substrate structure were observed. The temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions showed the features of superparamagnetic system in films annealed at 700/spl deg/C-900/spl deg/C. The magnetization curves at 5 K for samples annealed at 700/spl deg/C-900/spl deg/C exhibits ferromagnetic hysteresis loops, and the highest residual magnetization (M/sub R/) and coercivity (H/sub c/) of M/sub R/=1.5/spl times/10/sup -4/ emu/g and H/sub c/=107 Oe were found in the 800/spl deg/C annealed sample. XPS measurement showed the metallic Co 2p core levels and the metallic valence band spectra for as-implanted and 700/spl deg/C-900/spl deg/C annealed samples. From these, it could be explained that the magnetic property of our films originated from Co and CoGa magnetic clusters.     

Determination of the shear impedance of viscoelastic liquids using cylindrical piezoceramic resonators

In this paper, a new method for determining the rheological parameters of viscoelastic liquids is presented. To this end, we used the perturbation method applied to shear vibrations of cylindrical piezoceramic resonators. The resonator was viscoelastically loaded on the outer cylindrical surface. Due to this loading, the resonant frequency and quality factor of the resonator changed. According to the perturbation method, the change in the complex resonant frequency /spl Delta/~/spl omega/ = /spl Delta/w/sup re/ + j/spl Delta//spl omega//sup im/ is directly proportional to the specific acoustic impedance for cylindrical waves Zc of a viscoelastic liquid surrounding the resonator, i.e., /spl Delta/~w /spl sim/ jZ/sub c/, where j = (-1)/sup 1/2/. Hence, the measurement of the real and imaginary parts of the complex resonant frequency determines the real part, R/sub c/, and imaginary part, X/sub c/, of the complex acoustic impedance for cylindrical waves Z/sub c/ of an investigated liquid. Further-more, the specific impedance Z/sub L/ for plane waves was related to the specific impedance Z/sub c/ for cylindrical waves. Using theoretical formulas established and the results of the experiments performed, the shear storage modulus /spl mu/ and the viscosity /spl eta/ for various liquids (e.g., epoxy resins) were determined. Moreover, the authors derived for cylindrical resonators a formula that relates the shift in resonant frequency to the viscosity of the liquid. This formula is analogous to the Kanazawa-Gordon formula that was derived for planar resonators and Newtonian liquids.     

Sputtered FeCoN soft magnetic thin films with high resistivity

Fe/sub 70/Co/sub 30/N thin films with thickness from 20 to 1100 /spl Aring/ were prepared by radio- frequency reactive sputtering in an N/sub 2/--Ar mixture. The FeCoN films prepared in a low nitrogen flow rate percentage (<6%) and sputtering pressure (<8 mTorr) have a high B/sub s/ of about 24.0 kG, but a moderate hard-axis coercivity H/sub ch/ of 5-30 Oe. With further increase in N/sub 2/ percentage or sputtering pressure, films become significantly softer, with H/sub ch/ of about 0.1-0.6 Oe, and have a higher resistivity of up to about 160 /spl mu//spl Omega//spl middot/cm. The change in the magnetic properties with nitrogen flow rate percentage and sputtering pressure can be attributed to the formation of an ultrafine grain size nanocrystalline FeCoN thin film as observed by high-resolution transmission electron microscope. The soft properties of FeCoN films with nano-sized crystallites remain stable even after being annealed at 270/spl deg/C.     

Influence of AC magnetic field amplitude on the surface magnetoimpedance tensor in amorphous wire with helical magnetic anisotropy

We have performed experimental and theoretical studies on the influence of ac magnetic field amplitude on the magnetoimpedance tensor in an amorphous wire with helical magnetic anisotropy. For the experimental measurements, we used an amorphous wire of composition (Co/sub 0.94/Fe/sub 0.06/)/sub 72.5/Si/sub 12.5/B/sub 15/ with negative, nearly zero magnetostriction constant, excited either by an ac circular h/sub /spl phi// or by an axial h/sub z/ magnetic field created by an ac electric current. We changed the ac current amplitude from 7.5 to 40 mA and the current frequency f from 1.5 to 20 MHz. The values of the asymmetric giant magnetoimpedance ratio associated with the sweeping direction of the dc field H/sub ex/ and the corresponding sensitivity were 211% and 0.64 V/Oe, respectively, for an ac current of 37.5 mA at 3 MHz. For the theoretical study based on the magnetization rotation, we obtained the second-order harmonic of the ac magnetization m/spl I.oarr//sup (2)/ induced by the relatively high ac magnetic field by solving the Landau-Lifshitz-Gilbert (LLG) equation. We also considered a second-order surface impedance tensor /spl sigmav//spl circ//sup (2)/, which allowed us to analyze quantitatively the influence of the ac magnetic field amplitude on the impedance tensor of the wire. We obtained the domain model of the wire with helical magnetic anisotropy having multidomains and the magnetization vector /spl plusmn/M/sub 0/ directed in the easy direction, and the corresponding static magnetic configurations, by solving the static LLG equation. For the given magnetic configurations, we calculated the second-order impedance tensor /spl sigmav//spl circ//sup (2)/. The results can well explain the irregular field characteristics of the voltage responses at low dc field value, when the wire was excited at high frequency and at large ac magnetic field.     

Influence of spinel substrate and over-layer for enhanced SAW and AO properties with KNbO3 thin film

Surface acoustic wave (SAW) propagation characteristics have been studied using modeling calculations for a potassium niobate (KNbO/sub 3/) thin film-layered structure with [001] and [110] orientation on a single crystal spinel (MgAl/sub 2/O/sub 4/) substrate, and a spinel buffer layer on silicon. Variation in the electromechanical coupling and acoustic attenuation has been compared. A significantly high value of coupling factor (k/sub max//sup 2/=23%) is obtained for the [001]KNbO/sub 3//spinel structure by introducing an optimum thickness of spinel over-layer for potential wide bandwidth SAW device applications. The dispersion characteristics with the [110] KNbO/sub 3/ orientation indicate an initial peak in the coupling coefficient value (k/sub max//sup 2/=8.8%) at a relatively low KNbO/sub 3/ film thickness that appears attractive for fabricating devices with thinner films. The KNbO/sub 3/ film with [001] orientation is found attractive for efficient acousto-optic (AO) device application with the formation of a symmetric waveguide structure (spinel(0.5 /spl mu/m)/KNbO/sub 3/(1.0 /spl mu/m)/spinel). A high value of k/sup 2/=23.5% with 50% diffraction efficiency has been obtained for the spinel(0.5 /spl mu/m)/KNbO/sub 3/(1.0 /spl mu/m)/spinel structure at 1 GHz SAW frequency and 633 nm optical wavelength at a very low input drive power of 15.4 mW.     

Stochastic assembly of sublithographic nanoscale interfaces

We describe a technique for addressing individual nanoscale wires with microscale control wires without using lithographic-scale processing to define nanoscale dimensions. Such a scheme is necessary to exploit sublithographic nanoscale storage and computational devices. Our technique uses modulation doping to address individual nanowires and self-assembly to organize them into nanoscale-pitch decoder arrays. We show that if coded nanowires are chosen at random from a sufficiently large population, we can ensure that a large fraction of the selected nanowires have unique addresses. For example, we show that N lines can be uniquely addressed over 99% of the time using no more than /spl lceil/2.2log/sub 2/(N)/spl rceil/+11 address wires. We further show a hybrid decoder scheme that only needs to address N=O(W/sub litho-pitch//W/sub nano-pitch/) wires at a time through this stochastic scheme; as a result, the number of unique codes required for the nanowires does not grow with decoder size. We give an O(N/sup 2/) procedure to discover the addresses which are present. We also demonstrate schemes that tolerate the misalignment of nanowires which can occur during the self-assembly process.     

Amorphous silicon two-color microbolometer for uncooled IR detection

This paper describes the modeling and design of two-color microbolometers for uncooled infrared (IR) detection. The goal is to develop a high resolution IR detector array that can measure the actual temperature and color of an object based on two spectral wavelength regions. The microbolometer consists of high temperature amorphous silicon (a-Si:H) thin film layer held above the substrate by Si/sub 3/N/sub 4/ bridge. A thin NiCr absorber with sheet resistance of 377 /spl Omega//sqr is used to enhance the optical absorption in the medium and long IR wavelength windows. A tunable micromachined Al-mirror was suspended underneath the detector. The mirror is switched between two positions by the application of an electrostatic voltage. The switching of the mirror between the two positions enables the creation of two wavelength response windows, 3-5 and 8-12 /spl mu/m. A comparison of the two response wavelength windows enables the determination of the actual temperature of a viewed scene obtained by an IR camera. The microbolometer is designed with a low thermal mass of 1.65/spl times/10/sup -9/ J/K and a low thermal conductance of 2.94/spl times/10/sup -7/ W/K to maximize the responsivity R/sub v/ to a value as high as 5.91/spl times/10/sup 4/ W/K and detectivity D/sup */ to a value as high as 2.34/spl times/10/sup 9/ cm Hz/sup 1/2//W at 30 Hz. The corresponding thermal time constant is equal to 5.62 ms. Hence, these detectors could be used for 30-Hz frame rate applications. The extrapolated noise equivalent temperature difference is 2.34 mK for the 8-12 /spl mu/m window and 23 mK for the 3-5 /spl mu/m window. The calculated absorption coefficients in the medium and long IR wavelength windows before color mixing are 66.7% and 83.7%. However, when the color signals are summed at the output channel, the average achieved absorption was 75%.