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Weak Magnetic Fields (weak + magnetic_field)
Selected AbstractsMapping of periodic waveforms using the ghost reconstructed alternating current estimation (GRACE) magnetic resonance imaging techniqueMAGNETIC RESONANCE IN MEDICINE, Issue 3 2003Hua Yang Abstract A new method of estimating alternating currents using ghost images created when the magnetic field from a fluctuating current modulates the phase of the magnetic resonance (MR) signal between successive phase-encode views is described. The method, known as ghost reconstructed alternating current estimation (GRACE), may be useful for directly mapping fields, and hence current impulses produced by neuronal firing events when synchronized periodic modulation can be induced. Images were acquired on a 1.5 T MR system with small oil capsule phantoms and a single wire with an applied alternating current, placed perpendicular to the main field direction. Computer simulations of these experiments yielded ghost images that agreed with experimental results. A simulated ghost image resulting from an evoked neuronal waveform is also discussed. Weak magnetic fields were detected from both sinusoidal and square wave modulations. Magn Reson Med 50:633,637, 2003. © 2003 Wiley-Liss, Inc. [source] Luminescence spectra in metallic and ferromagnetic GaMnAs/GaAs multilayers: a self-consistent super-cell Kane k.p calculationPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2004G. M. Sipahi Abstract We present a self-consistent Kane k.p calculation for obtaining the luminescence spectra of GaMnAs/GaAs multilayers. Our model consists of substitutional Mn ions uniformly distributed in GaMnAs layers of width d1 at a concentration of 5%. The high Mn concentration allows to approximating the density of the magnetic moments by a continuous distribution when treating the magnetic interaction between holes and the localized moment on the Mn++ sites. These DMS layers are assumed to be ferromagnetic, at T = 0 K, metallic, and with a hole concentration equivalent, in bulk, to 1 × 1020 cm,3. The DMS layers are separated by non-magnetic GaAs layers. The magnetization is kept perpendicular-to-the-plane (parallel to the growth direction) by the application of a weak magnetic field, which does not affect the electronic structure directly. The luminescence spectrum is obtained for several numbers of DMS layers in the structure. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] New magnetic field measurements of , Cephei stars and slowly pulsating B stars,ASTRONOMISCHE NACHRICHTEN, Issue 4 2009S. Hubrig Abstract We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate , Cephei stars and Slowly Pulsating B (hereafter SPB) stars, along with a small number of normal B-type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three , Cephei stars and two stars suspected to be , Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3, has been diagnosed in two normal B-type stars, the nitrogen-rich early B-type star HD 52089 and in the B5 IV star HD 153716. Roughly one third of , Cephei stars have detected magnetic fields: Out of 13 , Cephei stars studied to date with FORS 1, four stars possess weak magnetic fields, and out of the sample of six suspected , Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: Roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non-magnetic pulsating stars in the H-R diagram. We find that their domains in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i -values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Light scattering study on hybrid structures of Zn1,x,yCdxMnySe quantum wells with ferromagnetic Co wiresPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2004M. Sakuma Abstract Sub-micron scale hybrid structures of Zn1,x,yCdxMnySe diluted magnetic semiconductor (DMS) quantum wells (QWs) with ferromagnetic Co wires have been fabricated for the purpose of applying local magnetic fields to the DMS-QW. The wire of DMS-QW with the width down to 100 nm was sandwiched between the Co wires. The magneto-optical properties are studied by spin-flip light scattering of paramagnetic Mn-ions in the DMS. The application of uniform magnetic fields higher than 0.25 T has been attained from the Co wires to the DMS-QWs. In addition, the field application from the Co to the DMS-QWs is switched on by weak magnetic fields of 0.03 T due to the magnetic shape anisotropy of the Co wires, which can realize the switching behavior of the spin alignment of Mn-ions in the DMS-QW. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electron transport properties in InAs four-terminal ballistic junctions under weak magnetic fieldsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2009M. Koyama Abstract We report on the electron transport properties based on ballistic electrons under magnetic fields in four-terminal ballistic junctions fabricated on an InAs/AlGaSb heterostructure. The four-terminal junction structure is composed of two longitudinal stems with two narrow wires slanted with 30 degree from the perpendicular axis. The electron focusing peak was obtained with the bend resistance measurement. Then it was investigated the nonlinear electron transport property of potential difference between longitudinal stems due to ballistic electrons with applying direct current from narrow wires. Observed nonlinearity showed clear rectification effects which have negative polarity regardless of input voltage polarity. Although this nonlinearity was qualitatively changed due to the Lorentz force under magnetic fields, the degradation of ballistic effects on nonlinear properties were observed when the current increased to higher strength. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Magnetic-field-induced switching of spin injectionin Zn1,xMnxTe/ZnTe double quantum wellsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2004S. Shirotori Abstract Magnetic-field-induced switching of the spin configuration and the resultant spin injection direction have been studied in a Zn1,xMnxTe/ZnTe double quantum well. The up- and down-spin excitons are injected from the Zn1,xMnxTe layers to the ZnTe well in weak magnetic fields, where both exciton energies in the Zn1,xMnxTe layer are higher than those in the ZnTe. Above the level crossing field, the down-spin exciton energy in the Zn1,xMnxTe layers becomes lower due to the giant Zeeman effect. Therefore, the up- and down-spin excitons are spatially separated in each layer and the down-spin exciton in the ZnTe layer is injected to the Zn1,xMnxTe. It means that the direction of the excitonic spin injection can be switched by the external field. The injection time increases from 12 psec to 300 psec toward the level crossing field of 2.5 T, since the spin-polarized excitons in those layers are associated and can cause reverse spin injection processes. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] New magnetic field measurements of , Cephei stars and slowly pulsating B stars,ASTRONOMISCHE NACHRICHTEN, Issue 4 2009S. Hubrig Abstract We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate , Cephei stars and Slowly Pulsating B (hereafter SPB) stars, along with a small number of normal B-type stars. A weak mean longitudinal magnetic field of the order of a few hundred Gauss was detected in three , Cephei stars and two stars suspected to be , Cephei stars, in five SPB stars and eight stars suspected to be SPB stars. Additionally, a longitudinal magnetic field at a level larger than 3, has been diagnosed in two normal B-type stars, the nitrogen-rich early B-type star HD 52089 and in the B5 IV star HD 153716. Roughly one third of , Cephei stars have detected magnetic fields: Out of 13 , Cephei stars studied to date with FORS 1, four stars possess weak magnetic fields, and out of the sample of six suspected , Cephei stars two show a weak magnetic field. The fraction of magnetic SPBs and candidate SPBs is found to be higher: Roughly half of the 34 SPB stars have been found to be magnetic and among the 16 candidate SPBs eight stars possess magnetic fields. In an attempt to understand why only a fraction of pulsating stars exhibit magnetic fields, we studied the position of magnetic and non-magnetic pulsating stars in the H-R diagram. We find that their domains in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence. It is possible that stronger fields tend to be found in stars with lower pulsating frequencies and smaller pulsating amplitudes. A somewhat similar trend is found if we consider a correlation between the field strength and the v sin i -values, i.e. stronger magnetic fields tend to be found in more slowly rotating stars (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Evidence for weak magnetic fields in early-type emission starsASTRONOMISCHE NACHRICHTEN, Issue 10 2007S. Hubrig Abstract We report the results of our study of magnetic fields in a sample of 15 Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi-mode instrument FORS 1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four stars, HD56014, HD148184, HD155806, and HD181615. We note that for HD181615 the evolutionary status is not obvious due to the fact that it is a binary system currently observed in the initial rapid phase of mass exchange between the two components. Further, we notify the possible presence of distinct circular polarisation features in the circumstellar components of Ca II H&K in three stars, HD58011, HD117357, and HD181615, hinting at a probable presence of magnetic fields in the circumstellar mass loss disks of these stars. We emphasize the need for future spectropolarimetric observations of Be stars with detected magnetic fields to study the temporal evolution of their magnetic fields and the correlation of magnetic field properties with dynamical phenomena taking place in the gaseous circumstellar disks of these stars. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effects of weak static magnetic fields on endothelial cellsBIOELECTROMAGNETICS, Issue 4 2010Carlos F. Martino Abstract Pulsed electromagnetic fields (PEMFs) have been used extensively in bone fracture repairs and wound healing. It is accepted that the induced electric field is the dose metric. The mechanisms of interaction between weak magnetic fields and biological systems present more ambiguity than that of PEMFs since weak electric currents induced by PEMFs are believed to mediate the healing process, which are absent in magnetic fields. The present study examines the response of human umbilical vein endothelial cells to weak static magnetic fields. We investigated proliferation, viability, and the expression of functional parameters such as eNOS, NO, and also gene expression of VEGF under the influence of different doses of weak magnetic fields. Applications of weak magnetic fields in tissue engineering are also discussed. Static magnetic fields may open new venues of research in the field of vascular therapies by promoting endothelial cell growth and by enhancing the healing response of the endothelium. Bioelectromagnetics 31:296,301, 2010. © 2010 Wiley-Liss, Inc. [source] Affective response to 5 µT ELF magnetic field-induced physiological changesBIOELECTROMAGNETICS, Issue 2 2007Paul Stevens Abstract Research into effects of weak magnetic fields (MFs) at biologically relevant frequencies has produced ambiguous results. Although they do affect human physiology and behaviour, the direction of effects is inconsistent, with a range of complex and unrelated behaviours being susceptible. A possible explanation is that these effects, rather than being directly caused, are instead related to changes in affective state. A previous study showed that MFs altered the affective content of concurrent perceptions, but it was unclear whether the emotional response was direct or indirect. Here it is shown that exposure to a 0,5 µT MF (DC-offset sinudsoidal wave form) within EEG ,-band frequencies (8,12 Hz), results in a reported change in emotional state. This relates to a decrease global field power but lacks the frontal ,-asymmetry that would physiologically indicate a directly induced emotional state, suggesting that participant experiences are due to an interpretation of the effects of MF exposure. Bioelectromagnetics © 2006 Wiley-Liss, Inc. [source] | ||||||||||