Inhomogeneous Bias Field Research Articles

Effects of pump pulse propagation and spatial distribution of bias fields on terahertz generation from photoconductive antennas

Through theoretical analysis and numerical simulation, we show that bipolar terahertz pulses generated from photo-excited small-gap or mid-gap photoconductive (PC) antennas actually stem from a unique combination of spatially inhomogeneous bias field and propagation effects of pump pulses through the PC if the influence of fast carrier recombination on terahertz radiation is excluded. This finding provides new insights on the formation of bipolar terahertz pulses, instead of the traditional view based on the Drude-Lorentz model and the space-charge screening effect. We find large-aperture PC antennas always emit unipolar terahertz pulses rather than bipolar ones because the bias fields in this case can be considered homogeneous. We also show that the saturation of the peak amplitude of terahertz pulses at high pump fluence can be correctly predicted using our model without invoking space-charge screening.

New sensors based on the magnetoelastic resonance of metallic glasses

High magnetostriction Fe-rich metallic glasses in the form of ribbons show a magnetoelastic resonance characterized by the frequency of the longitudinal vibrational mode of the sample, ω r, at which the magnetic susceptibility χ shows a sharp maximum. In this work we propose to use the sensibility of ω r and χ to the external (bias) field in order to design a new class of sensors. Measurements were performed using two different configurations for detecting the magnetoelastic resonance: first, a typical set-up with two different coaxial coils to apply the excitation field and to pick up the signal, respectively. Secondly, with a more simplified set-up consisting only in a small excitation-detection coil. In both experiments the influence of applying an inhomogeneous bias field, provided by a small permanent magnet, has been tested. Results from these features are discussed.

Linearized magnetoacoustic sensor of angular displacements

The design of a quasilinear sensor of angular displacements based on acoustic resonance in an antiferromagnetic single crystal was considered. The frequency versus angle dependence of a crystalline alpha-Fe(2)O(3) acoustic resonator placed in a rotating inhomogeneous bias field was studied experimentally. A linearization of this dependence was obtained, respectively, for frequency and angular operation ranges equal to Deltaf/f(max)=14% and Deltatheta=140.

Virtual additive moments measured by a vibrating sample magnetometer on superconducting thin films

Virtual additive magnetic moment was observed on a series of YBaCuO thin films in an external field perpendicular to the film plane: magnetic hysteresis loops measured by a PAR 155 vibrating sample magnetometer up to fields of were symmetrical at high field sweep rates, whereas at low sweep rates they exhibited a large additive offset of the reversible magnetic moment. This effect increased particularly at high fields and it was especially noticeable at intermediate temperatures, typically between 40 and 65 K. We identified this phenomenon as an artefact originating from vibrations of a sample possessing a high differential susceptibility in a slightly inhomogeneous bias field. This undesirable effect is quite general and it can be generated in any type of VSM with even very slight inhomogeneity of magnetic field. It can however, be found only on laterally large, thin samples with a high differential susceptibility (typically thin films). Experimental results are consistently explained by the proposed model. We present expressions which enable us to estimate this effect from the differential susceptibility of the sample and known inhomogeneity of the bias magnetic field.

Novel approaches to low‐cost MRI

This paper presents a combination of speculative approaches, some related to earlier work and some apparently novel, which show great promise in providing a new class of MRI machines that would be considerably less expensive. This class would have advantages and disadvantages as compared to existing MRI, over and above that of low cost. The disadvantages include the apparent inability to perform classic spectroscopy, and limited flexibility in the area of selective excitation. The advantages include a fundamental immunity to inhomogeneity and susceptibility problems, the ability to create a wide class of machines that are designed for specific anatomy-related applications, the ability to design open machines for physician access, and improved capability for high speed imaging. Generic to all of the methods presented are a pulsed polarizing field and an oscillatory read-out bias field. The pulsed field initially polarizes the magnetic moments. Since it is not on during the readout operation it has negligible homogeneity requirements since changes in the field amplitude will merely shade the image intensity. During readout a relatively low bias field is used. To enable the use of a relatively inhomogeneous bias field, an oscillatory field is used that has a zero average value. This prevents any long-term buildup of phase errors due to a frequency error associated with inhomogeneity. Thus the average bias frequency will be determined solely by the frequency rather than the amplitude of the bias field. Three methods are described, all including the above features. The first two involve imaging in the laboratory frame, while the third involves imaging in the rotating frame. The second approach requires no RF excitation and the third approach uses RF bias and gradient signals. Some approaches to slice selection are described.

The kinetics of hopping motion of interstitials with chemical reactions in arbitrary time dependent inhomogeneous interactive fields

Atomic hopping motions of interstitial species with chemical reactions in an arbitrary time- and space-dependent field have been formulated in terms of a linear but coupled set of differential equations for a general lattice structure. The decoupled form of these equations has been solved exactly using the discrete Fourier k-space transformation supplemented by a Laplace transformation with respect to time. An explicit and compact expression for the Fourier transform of the partial concentration of interstitials is obtained for interactive fields which have absolutely convergent norms in the time domain. Some special cases of general interest such as the simple harmonic, almost periodic, or static inhomogeneous fields are also treated extensively. The power dissipation and the storage terms associated with interstitial hopping motion in harmonic or almost periodic fields are formulated rigorously. The effect of static inhomogeneous bias field due to other imperfections on the relaxation time spectrum is considered in terms of the first-order perturbation theory.

The effect of an inhomogeneous bias field on the delay characteristics of magnetostatic forward volume waves

The delay and loss characteristics of magnetostatic forward volume waves in a YIG slab have been studied experimentally where an inhomogeneous bias field is applied normal to the slab surface by using artificially designed magnetic pole pieces. The nondispersive group delay characteristics with a low propagation loss are observed.

Guided Wave Propagation in Gyromagnetic Media as Applied to the Theory of Exchange Spin-Wave Excitation

Source-excited propagation in a narrow, lossless, ferrite-filled parallel-plate waveguide is analyzed. The effect of an inhomogeneous bias field is considered. A new, linear theory of exchange spin-wave excitation is presented, and its relation to the theory of Schlömann is discussed. Several previously unpublished aspects of the dispersion relation and the solution of a corresponding fourth-order partial differential equation are presented.