Effect of magnetic relaxation times on electromagnetic pulseshielding

The penetration of an electromagnetic pulse (EMP) through a ferromagnetic conducting thin film is investigated for the case in which the magnetization of the material has a characteristic time delay τ. Both saturation and hysteresis effects of he magnetization are considered and the susceptibility is defined in the nonlinear region as dM/dh. It is found that the response time τ has a significant effect on the shielding factor with optimum shielding when τ is on the order of twice the rise time of the pulse     

Effect of stress on the law of approach to saturation magnetizationin carbon steels

In high fields where the magnetization is approaching saturation, the resolved magnetic polarization I can be expressed by a power series of the inverse of external field H as: I=I_s-b/H ²-c/H³ . . ., or alternatively in terms of the reversible permeability μ_rev=1+(2b/H³+3c/H⁴+ . . .)/μ₀ where I_s is the spontaneous magnetic polarization and b, c, are constants. These equations express the law of approach to saturation magnetization. The coefficient b for a cubic crystal has been deduced as b=0.0762/I_s[K+1.5(λ₁₀₀-λ ₁₁₁)σ]², where σ is the applied stress and others are magnetic constants. The values of reversible permeability μ_rev under biasing field H were measured for carbon steels with applied stresses. The results showed that the square root of 2b changed linearly with the applied stress, The values of magnetic constant K andλ₁₀₀ -λ₁₁₁ were calculated, They agreed with the standard values, but were affected by chemical composition and heat treatment of materials, especially in λ₁₀₀-λ₁₁₁     

Magnetic properties of GdBa2Cu3O7crystals in high magnetic field

The magnetic properties of single crystals of GdBa₂Cu ₃O₇ were investigated in a magnetic field of up to 20 T applied parallel to the c axis in the 1.7-300 K temperature range. In the superconducting state, the field and temperature dependences of the critical current densities were deduced from the hysteresis of the half-cycle using Bean's critical state model. The Gd³⁺ paramagnetic moment was then studied. Above about 20 K, the M(H) isotherms were found to be given, at different temperatures, by the Brillouin function of the free Gd³⁺ ion. Below 20 K, the average magnetization does not obey the Brillouin law. The normal-state susceptibility was described by the free-ion Curie-Weiss law     

Properties of the W-type hexaferritePbZn2Fe16O27

W-type Pb-hexaferrites were prepared by standard ceramic methods. The lattice constants found by refinement were a=0.59140±0.00006 nm and c=3.29209±0.00041 nm. The X-ray density of a typical composition PbZn_1.9Fe_15.3O_25.8 was ρ=5.32 g/cm³ and the Vickers microhardness value h _v=6 kN/mm². A plot of the saturation magnetization versus temperature is given. The extrapolated value of the saturation magnetization (H→∞, T→0) was σ_s=108 emu×g^-1, and the Curie temperature was T_c=600±20 K     

Magnetic characteristics of ferromagnetic stainless steels

The magnetic characteristics of two representatives of martensitic and one of ferritic stainless steels, i.e. SUS 403, TAF, and SUS 405, were investigated in connection with development of high-temperature and nuclear resistive sensors. The test pieces were machined out from rod materials and heat-treated in an argon atmosphere. They were placed in an electric furnace, and their magnetic characteristics were measured at room temperature and elevated temperatures. The examined characteristics were of initial magnetization curve, hystersis loop, AC permeability, Curie point, and inverse magnetostriction. The martensitic stainless steels show somewhat semihard characteristics (H_c: 5 to 15 Oe) and the ferritic one is rather soft (H_c; 2 to 5 Oe). The Curie point ranges from 720 to 750°C. The magnetization is about 15000 G at a field strength of 100 Oe. A large inverse magnetostriction is observed in SUS 403 and SUS 405. Their magnetic characteristics were observed to change with heat treatment     

Analysis of temperature-dependent magnetization in Sm-Fe-Ccompounds using a two-sublattice model

The temperature dependence of magnetization in Sm₃Fe ₂₀C_x (x =0.3, 0.6, 0.8) and Sm₂Fe_16.5C_1.0 intermetallic compounds with rhombohedral structure has been analyzed using molecular field theory. On the basis of a two-sublattice model, the molecular field coefficients are calculated using a numerical fitting method. The Curie temperature and the Fe-Fe, Sm-Fe, Sm-Sm magnetic interaction energies for the compounds are determined from these coefficients. The results show that the increase of the Curie temperature (T_c) with the increase of carbon content x is attributed mainly to the enhancement of the Fe-Fe exchange interaction energy caused by carbon atoms added     

Global iterative solution of dielectric spectroscopy equations

A global method is presented for solving the permittivity equations for open- and short-circuited coaxial lines of general length for broadband measurements by iterating the recurrence schemes z_n+1=c cot z_z and z _n+1=c tan z_n, respectively, and their inverses. The global iteration theory of Fatou and Julia (see J. L. Howland and R. Vaillancourt, Num. Math., vol.46, 323-337, 1985), coupled with linear extrapolation and interpolation and Steffensen's acceleration procedure, supplies starting values and guarantees convergence even near double roots. When R_Z⩾0 for open, and R_Z⩽0 for short circuit terminations Newton's method, with appropriate starting values, converges to the desired roots. A combination of the three iterative schemes results in an almost global method of solution. Numerical results are quoted     

An algebraic method to reconstruct the relative phases andpolarization of a complex vector in N dimensions based on3(N-1) amplitude measurements

A real vector X in n dimensions whose components are sinusoidally varying with time can be represented by an n-dimensional complex factor Ze^jωt whose real part is X. In different real coordinate systems, the components of X transform linearly, but the amplitudes of the components of Z transform nonlinearly. The author describes a method to reconstruct the relative phases of the (complex) components of Z (and, therefore, of X) based on the amplitude measurements of its n components in an orthogonal coordinate system and at least 2n-3 additional amplitude measurements in different directions. The author shows the necessary and sufficient condition on these additional directions to ensure uniqueness of the phase and polarization reconstruction for any arbitrary vector X, and presents an algebraic method for the reconstruction which offers substantial reduction in computing time over the method of reconstruction by nonlinear optimization. The result of this phase of reconstruction is the complete characterization of the polarization of X except for chirality     

Hot-pressed Mn-Zn-Ni and Mn-Zn-Co ferrites for magnetic recordingheads

Ni²⁺ and Co²⁺ substituted Mn-Zn ferrites have been developed as high-density magnetic recording materials by hot pressing using optimum sintering parameters. Ferrite series of the composition Mn_0.6Zn_0.4-xNi_xFe₂ O₄ and Mn_0.6Zn_0.4-yCo_yFe₂O₄ were prepared by the hot-pressing technique wherein x and y varied from 0.0 to 0.4 in steps of 0.05. It yielded ferrites with improved magnetic properties having higher hardness (⩾650 Vickers units), low porosity (<0.1%), and small grain size (≈10 μm). The initial permeability increased, whereas the coercive field decreased for an Ni²⁺ concentration x or a Co ²⁺ concentration y equal to 0.05. Maximum values of saturation magnetization equal to 4850 and 5250 G were obtained for x and y equal to 0.225 and 0.275, respectively. The Curie temperature increased appreciably, whereas the DC resistivity decreased for a larger substitution of Ni²⁺ and Co²⁺ ions     

High-energy Co-Cr thin films sputtered on glass disks forperpendicular recording

The authors have developed high-energy Co-Cr thin-film perpendicular recording media for rigid disks. They obtained high perpendicular coercivity (Hc⊥) exceeding 2000 Oe with Co-Cr films sputtered on glass disks. They examined recording characteristics obtained with double-layered media and single-pole heads. Readout voltages were proportional to Hc⊥ up to 2000 Oe and not dependent on saturation magnetization. The authors explain the experimental results using the hysteresis curve of the Co-Cr film and the permeance factor determined by the magnetic reluctance of the head and medium. Using a magnetic circuit model, they clarify the effect of the difference in the operating point on the hysteresis curves of rigid- and flexible-disk systems     

Common mode rejection ratio for cascaded differential amplifierstages

A simple equation for calculating the total common mode rejection ratio, CMRR_T, for cascaded differential amplifier stages is given, and it is applied to several typical cases in instrumentation circuits. Two factors for each stage, C and D, are defined, and it is shown that, in general, CMRR_T can be calculated by adding the reciprocals of the equivalent CMRR for each stage, which is defined as the product of its C factor and the product of D factors for the preceding stages. The acceptability of the approximate formulas is verified, and application examples based on typical situations in instrumentation circuits are given. It is shown that the assumptions made to obtain such a simplified equation are acceptable     

Analysis of the noise sources in an optically pumped cesium beamresonator

The main noise sources that affect the clock signal detection in an optically pumped cesium beam resonator are determined and their influence on the clock S/N ratio is evaluated. The following noise sources are taken into account: atomic shot noise, fluorescence photon noise, laser frequency noise, and clock signal detection noise. A theoretical model that predicts the variations of the S/N ratio as a function of different parameters characterizing the atom-laser interaction is developed. The main conclusion concerns the saturation of S/N value for high atomic flux due to laser frequency fluctuations. All the theoretical predictions were experimentally verified     

Grain size dependence of coercivity and permeability innanocrystalline ferromagnets

Amorphous ribbons of composition Fe_74.5-xCu_xNb₃Si_13.5B₉ (x=0, 1 at.%) have been annealed between about 500°C and 900°C. This produced a series of crystallized samples with grain sizes between about 10 nm and 300 nm and with coercivities H _c and initial permeabilities μ_i varying over several orders of magnitude. The best soft magnetic properties (H _c≈0.01 A/cm and μ_i≈80×10³ ) were observed for the smallest grain sized of about 10 nm. With increasing grain size D, coercivity steeply increases following a D⁶-power law (up to D≈50 nm). H_c then runs through a maximum of H_c≈30 A/cm and decreases again for grain sizes above 150 nm according to the well-known 1/D law for polycrystalline magnets. The initial permeability was found to vary in a similar manner, essentially being inversely proportional to coercivity. The variation of the soft magnetic properties with the average grain size is discussed and compared with the predictions of the random anisotropy model and other theories for the magnetization reversal     

Pulse-echo single frequency acoustic nonlinearity parameter (B /A) measurement

A pulse-echo single-frequency method for B/A measurement is reported. The technique is based on excitation of the medium at a progressively increasing amplitude level and the computing of B/A from the deviation in linearity of the transducer input-output relationship. Starting from the nonlinear wave equation, a three-dimensional analytical formulation that predicts the approximate field distribution for finite-amplitude waves is derived. This provides the computation for the transducer output voltage for progressively increasing signal levels. It is shown that, by a suitable normalization, the effects of the diffraction can be excluded. From a nonlinear least-squares fit of the experimental data to the analytical equation, the B/A is computed. A number of simulations for the model together with preliminary experiments that were carried out at 3.5 GHz are presented     

Effect of annealing on magnetization process of Ni3Fealloy

The magnetocrystalline anisotropy, the magnetostriction, and the structure-sensitive properties in the magnetization process were studied in the ordered-disordered states of Ni₃Fe alloy. The single crystals were cooled very slowly at the rate of 1°C/day to obtain the fully ordered state. Magnetization curves and hysteresis loops were measured using picture frame specimens. The magnetocrystalline anisotropy and the magnetostriction were also measured in disk specimens. The magnetic domain was also observed. The experimental results obtained in the ordered state are considerably different from those obtained by previous investigators. The difference is attributed to the degree of order-the magnetic properties obtained by previous investigators are those of a partially ordered state     

Multirate polynomials prediction with unevenly spaced samples

A method for polynomial prediction with unevenly spaced samples is proposed for dynamic upsampling purposes, frequently needed in digital signal-processing applications. The procedure is based on Newton's divided-difference interpolation formula, which allows prediction along a polynomial of degree n if the samples are arbitrarily spaced. For this purpose, it is necessary to store n+1 latest values pairs (x_i, u_i) where x _i is the time instant when the sample y_i is available. Based on the (x_i, y_i ) pairs, the algorithm predicts y-samples for arbitrary values of x. Noise reduction techniques based on bandwidth limiting are discussed. As an application, a velocity measurement system is considered, based on pulse-width measuring, where the primary interest is in improving the accuracy at very low speeds     

High precision phase measurement using adaptive sampling

The conventional phase measurement techniques introduce error in the phase when the input signals are distorted by harmonics. A novel technique, known as adaptive sampling, for high-precision phase measurement is introduced. A digital signal-processing approach is used in this technique. The maximum sampling rate required for this technique is h+2 samples/cycle of the input signals, i.e. (h+2) f sampless, where h, is the highest harmonic present in the signals and f is the fundamental frequency of the signals. This sampling rate is way below the Nyquist sampling rate (more than 2hf samples/s) when h is a large number. In the adaptive sampling technique the sampling rate is started from three samples/cycle and then is gradually increased until the phase is correctly measured. This phase measurement technique has been verified using synthesized signals     

Automatic technique for measuring the Polder resonance in ferritespheres

An automatic technique for measuring the parameters of Polder resonance in polycrystalline ferrite spheres is proposed. The diagonal elements of the external susceptibility tensor versus DC magnetic field are calculated from the changes in resonance frequency and unloaded Q factor of a microwave cavity by perturbation theory. From these diagonal elements, all the elements of the intrinsic permeability tensor can be obtained. By fitting the theoretical curves to experimental data, the parameters of Polder resonance, ΔH M_s, and the g factor, are simultaneously calculated. The whole measurement procedure is controlled by a desktop computer. An accuracy of 5% is obtained in ΔH and M _s, and of 0.1% in the g factor     

STM control of noninteger residual charge Q0 onthe central electrode of a double-tunnel junction

In a voltage-biased mesoscopic double-tunnel junction, the Coulomb threshold V_C is an e-periodic function of Q ₀, the charge on the inner electrode. In the STM work described here, a large Au tip images 100-Å Ag particles deposited on a cleaved BSCCO crystal substrate. Stable double junctions between the tip, the imaged particle, and the substrate are identified by Coulomb staircase I(V)'s. The control of Q ₀ is demonstrated by the variation of tip-particle spacing. The parameterization of such results permits a discussion of Q ₀ variation mechanisms     

Solution of three dimensional eddy current problems by integral anddifferential methods

A differential T, Ω formulation using nonconformal elements is presented. It is an extension of the T integral formulation. The method has been used as the basis of a finite-element code that handles the open boundary problem of eddy-current analysis in fully 3-D conductors. The results obtained with this formulation for different treatments of the boundary conditions at infinity are shown and compared among themselves and with respect to a T integral formulation. The efficiencies are compared in terms of storage occupation, CPU time, and accuracy