Magnetic Field Shift Research Articles

Magnetic properties of Nd-Ga-Febal-Nb-B alloy

Here, we have synthesized Nd-Ga-Febal-Nb-B alloy by strip casting method. The crystalline and magnetic properties of sample were investigated with x-ray diffractometer (XRD), vibrating sample magnetometer (VSM), and Mössbauer spectrometer. The XRD pattern was analyzed with the Rietveld refinement method, indicating a tetragonal structure and the space group of P42/mnm. The temperature dependence of zero-field cooled (ZFC) magnetization curve was measured under applied field at temperature ranging from 4.2 to 740 K. From the ZFC curve, Curie temperature and spin reorientation temperature are determined to be 615 K and 130 K, respectively. Also, Mössbauer spectra were measured at various temperatures ranging from 4.2 to 620 K. Each spectrum was fitted with 6-sextets for Fe site (8j1, 8j2, 16k1, 16k2, 4c, and 4e), and magnetic hyperfine field, Isomer shift, electric quadrupole shift, and area ratio values were obtained from the fit. We observed the change in slope of magnetic hyperfine field and electric quadrupole shift at 130 K while the Curie temperature was determined to be 615 K from the measurement of zero velocity counter, agreeing with the values obtained from VSM measurements.

Study of structural, electrical and magnetic properties of Cr doped Ni–Mg ferrite nanoparticle

We have reported the effect of Cr3+ ion doping on the structural, ac transport and Mössbauer spectral studies of ferrite nanoparticles Ni0.5Mg0.5Fe2−xCrxO4 (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) prepared by sol–gel auto combustion technique. The micro-structural studies of the samples were obtained by using scanning electron microscopy (SEM). The dependence of Mössbauer parameters, viz., line width, isomer shift, quadrupole splitting and hyperfine magnetic field on Cr3+ substitution have been analysed. Hyperfine magnetic field and isomer shift values decreases whereas line width increases with Cr3+ substitution. Rietveld refinement and Mössbauer spectroscopy studies reveal that the Ni0.5Mg0.5Fe2−xCrxO4 with various concentrations of x belongs to mixed spinel structure. Nèel’s molecular field model is used to explain the variation of hyperfine magnetic field with increasing Cr3+ substitution.

Diffusion weighted MRI by spatiotemporal encoding: Analytical description and in vivo validations

Diffusion-weighted (DW) MRI is a powerful modality for studying microstructure in normal and pathological tissues. The accuracy derived from DW MRI depends on the acquisition of quality images, and on a precise assessment of the b-values involved. Conventional DW MRI tends to be of limited use in regions suffering from large magnetic field or chemical shift heterogeneities, which severely distort the MR images. In this study we propose novel sequences based on SPatio-temporal ENcoding (SPEN), which overcome such shortcomings owing to SPEN’s inherent robustness to offsets. SPEN, however, relies on the simultaneous application of gradients and radiofrequency-swept pulses, which may impart different diffusion weightings along the spatial axes. These will be further complicated in DW measurements by the diffusion-sensitizing gradients, and will in general lead to complex, spatially-dependent b-values. This study presents a formalism for analyzing these diffusion-weighted SPEN (dSPEN) data, which takes into account the concomitant effects of adiabatic pulses, of the imaging as well as diffusion gradients, and of the cross-terms between them. These analytical b-values derivations are subject to experimental validations in phantom systems and ex vivo spinal cords. Excellent agreement is found between the theoretical predictions and these dSPEN experiments. The ensuing methodology is then demonstrated by in vivo mapping of diffusion in human breast – organs where conventional k-space DW acquisition methods are challenged by both field and chemical shift heterogeneities. These studies demonstrate the increased robustness of dSPEN vis-à-vis comparable DW echo planar imaging, and demonstrate the value of this new methodology for medium- or high-field diffusion measurements in heterogeneous systems.

Understanding the enhanced microwave permeability of Fe–Si–Al particles by Mössbauer spectroscopy

The microwave permeability of milled Fe–Si–Al flaky particles has been found to be much larger than those of unmilled particles with irregular shape. Transmission Mössbauer spectra are found significantly different for particles with different shapes. More than 8 absorption peaks have been found for the unmilled particles while there are only 6 absorption peaks for the flaky particles. The unmilled particles are found with the ordered D03 type superlattice structure. Therefore the Mössbauer spectra can be fitted with 5 sextets representing 5 different Fe-site environments. However, the flaky particles are with the disorder α-Fe(Si,Al) structure, and their Mössbauer spectra can be fitted with a model with the distribution of hyperfine magnetic field and isomer shift. It also has been revealed by Mössbauer spectroscopy that the flaky particles have stronger tendency to possess the planar magnetic anisotropy, and their average magnetic moment is found to be about 1.63μB.

Microwave Permeability and Mössbauer Spectra of Flaky Fe–Si–Al Particles

Fe84.94Si9.68Al5.38 flaky particles have been obtained by ball attrition method. The volume fraction dependence of microwave permeability has been measured for composites filled with these particles. Effective medium theories have been employed to extract the apparent microwave permeability. It is found that the flaky Fe-Si-Al particles have large apparent microwave permeability and large microwave magnetic loss. For example, at 0.5 GHz, the real and imaginary parts of apparent permeability are 10 and 5, respectively. The extracted microwave permeability is found to be consistent with generalized Snoek's limit. Due to high microwave magnetic loss, the flaky Fe-Si-Al particles can be useful for technical applications in the field of electromagnetic compatibility. Mossbauer spectroscopy measurements show that the hyperfine magnetic field and isomer shift have wide distribution, indicating that Fe atoms in the flaky Fe-Si-Al alloys have complicated environments.

Frequency-dependent magnetotransport phenomena in a hybrid Fe/SiO2/p-Si structure

We report the large magnetoimpedance effect in a hybrid Fe/SiO2/p-Si structure with the Schottky barrier. The pronounced effect of magnetic field on the real and imaginary parts of the impedance has been found at temperatures 25–100 K in two relatively narrow frequency ranges around 1 kHz and 100 MHz. The observed frequency-dependent magnetotransport effect is related to the presence of localized “magnetic” states near the SiO2/p-Si interface. In these states, two different recharging processes with different relaxation times are implemented. One process is capture-emission of carriers that involves the interface levels and the valence band; the other is the electron tunneling between the ferromagnetic electrode and the interface states through SiO2 potential barrier. In the first case, the applied magnetic field shifts energy levels of the surface states relative to the valence band, which changes recharging characteristic times. In the second case, the magnetic field governs the spin-dependent tunneling of carriers through the potential barrier. The “magnetic” interface states originate, most likely, from the formation of the centers that contain Fe ions, which can easily diffuse through the SiO2 layer.

Drift Error Analysis of Atomic Spin Gyroscope

With the rapid development of quantum manipulation technique, atomic spin gyroscope has been demonstrated in recent years. Understanding the sources and the influence of error on the performance of atomic spin gyroscope is important during the design of such devices. The basic principle of atomic spin gyroscope was reviewed and the analysis of error sources presented began with the coupled Bloch equations describing the dynamics of atomic spin gyroscope including magnetic field, light shift and pump-probe non-orthogonality. It was shown analytically that the first order effects of the transverse magnetic fields and imperfections can be eliminated by turning the longitudinal magnetic field on the magnetic field compensation point. The effects of longitudinal magnetic field and light shift imperfections with other terms zeroed contributed only to scale factor of the atomic spin gyroscope. Furthermore, the effects magneto-optical crystal and magnetic shielding due to temperature drift were described and according approaches to suppress these errors were also proposed.

Magnetocaloric properties of La0.6Ca0.4MnO3

A theoretic work on magnetocaloric properties of the polycrystalline La0.6Ca0.4MnO3 system near a second-order phase transition from a ferromagnetic to a paramagnetic state is presented. The value of the magnetocaloric effect has been determined from the calculation of magnetization as a function of temperature under different external magnetic field shifts. The magnetic entropy change ∆SM reaches a peak of about 3 J kg−1 K−1 at 266 K upon 1.60 KA/m applied field variation. The ∆SM distribution is much more uniform than that of gadolinium, which is desirable for an Ericson-cycle magnetic refrigerator, which is beneficial for the household application of active magnetic refrigerant materials.

Critical Current in High-J$ _{\rm c}$ Nb$_{3}$Sn Rutherford Cables Affected Substantially by the Direction of the Applied Magnetic Field

Recently several Rutherford-type Nb Sn cables for possible use in new magnets for upgrades of the LHC have been measured at CERN. A summary is provided including a comparison of the critical current (Ic) of various cable types and strand materials versus their respective extracted strand measurements. The conductors were cabled at CERN and Lawrence Berkeley National Laboratory. The summary shows an unsurprising linear relation between the strand and the cable performance. However, assuming that the local peak magnetic field in the conductor is the relevant factor for determining the Ic(B) relation, a discrepancy is found when results from different directions of applied magnetic field on the cable are compared. In this paper it is shown that the critical current as a function of the peak magnetic field in the conductor can change in the range of 7-12% depending on the direction of the applied magnetic field. This effect originates from the superposition of the self field of the conductor and the applied magnetic field. When the direction of the applied magnetic field is changed, the position of the peak magnetic field in the conductor shifts. Mechanical degradation during the cabling process can create a local reduction in the critical current. When the position of the peak magnetic field shifts from a more degraded to a less degraded area, the measured Ic(B) curve of the conductor will change accordingly. The measured effect is in principle present in all cables exhibiting non-uniform cabling degradation but is substantially present in Nb3Sn conductors for the reasons mentioned.

Large magnetothermopower effect in Dirac materials (Sr/Ca)MnBi2

We report temperature and magnetic field dependence of the thermal transport properties in single crystals of (Sr/Ca)MnBi$_2$ with linear energy dispersion. In SrMnBi$_2$ thermopower is positive, indicating hole-type carriers and the magnetic field enhances the thermopower significantly. The maximum change of thermopower is about 1600% in 9 T field and at 10 K. A negative thermopower is observed in CaMnBi$_2$ with dominant electron-type carriers and, in contrast, the magnetic field suppresses the absolute value of thermopower. First-principle band structure shows that the chemical potential is close to the Dirac-cone-like points in linear bands. The magnetic field suppresses the apparent Hall carrier density of CaMnBi$_2$ below 50 K. The large magnetothermopower effect in (Sr/Ca)MnBi$_2$ is attributed to the magnetic field shift of chemical potential

Buffer layer-induced positive magnetoresistance in manganite-based heterojunctions

Effects of the LaMnO3 (LMO) buffer layer on the magnetoresistive behaviors of La0.67A0.33MnO3/LaMnO3/SrTiO3:0.05 wt% Nb (LAMO/LMO/STON, A = Ca, Sr) have been experimentally studied. In addition to an enhanced response to a magnetic field, the current-voltage relations show a downward shift in magnetic field, indicating an increase of the junction resistance. It is completely different from that observed in the junctions without buffer layer. The positive magnetoresistance (MR) is strongly dependent on the thickness of the LMO layer, increasing first then decreasing with the increase of layer thickness. Furthermore, it is significantly stronger in LCMO/LMO/STON than in LSMO/LMO/STON. The maximal MR at 50 K is ∼90% for LCMO/LMO/STON and ∼52% for LSMO/LMO/STON, occurring at the LMO thickness of 4 nm under the field of 5 T. The MR persists up to 350 K, and it is ∼30% and ∼24% for the LCMO and LSMO junctions, respectively. An analysis of the current-voltage characteristics indicates an increase in interfacial barrier in magnetic field, which is the origin for the positive MR.

A self-reference PRF-shift MR thermometry method utilizing the phase gradient

In magnetic resonance (MR) imaging, the most widely used and accurate method for measuring temperature is based on the shift in proton resonance frequency (PRF). However, inter-scan motion and bulk magnetic field shifts can lead to inaccurate temperature measurements in the PRF-shift MR thermometry method. The self-reference PRF-shift MR thermometry method was introduced to overcome such problems by deriving a reference image from the heated or treated image, and approximates the reference phase map with low-order polynomial functions. In this note, a new approach is presented to calculate the baseline phase map in self-reference PRF-shift MR thermometry. The proposed method utilizes the phase gradient to remove the phase unwrapping step inherent to other self-reference PRF-shift MR thermometry methods. The performance of the proposed method was evaluated using numerical simulations with temperature distributions following a two-dimensional Gaussian function as well as phantom and in vivo experimental data sets. The results from both the numerical simulations and experimental data show that the proposed method is a promising technique for measuring temperature.

Quantification and compensation of eddy-current-induced magnetic-field gradients

Two robust techniques for quantification and compensation of eddy-current-induced magnetic-field gradients and static magnetic-field shifts (Δ B 0) in MRI systems are described. Purpose-built 1-D or six-point phantoms are employed. Both procedures involve measuring the effects of a prior magnetic-field-gradient test pulse on the phantom’s free induction decay (FID). Phantom-specific analysis of the resulting FID data produces estimates of the time-dependent, eddy-current-induced magnetic field gradient(s) and Δ B 0 shift. Using Bayesian methods, the time dependencies of the eddy-current-induced decays are modeled as sums of exponentially decaying components, each defined by an amplitude and time constant. These amplitudes and time constants are employed to adjust the scanner’s gradient pre-emphasis unit and eliminate undesirable eddy-current effects. Measurement with the six-point sample phantom allows for simultaneous, direct estimation of both on-axis and cross-term eddy-current-induced gradients. The two methods are demonstrated and validated on several MRI systems with actively-shielded gradient coil sets.

Room temperature long range ferromagnetic ordering in (BiFeO3)1−x (PbTiO3)x nanocrystallites

Room temperature Mössbauer spectra of (BiFeO3)1−x (PbTiO3)x (x = 0, 0.1, 0.2, 0.3) nano multiferroic systems prepared by solution combustion technique reveal the long range ferromagnetic ordering, supported by M-H loop measurements. Magnetic hyperfine interaction field and isomer shift for these nanocrystallite indicate the presence of Fe3 + ion only, likewise confirmed by from X-ray photoelectron spectroscopy study, which is essential for enhancement of multiferroic properties. A small residual magnetic moment is observed for these nanoparticles, which may be attributed to size confinement at nanoscale and to spin canting. These nanomaterials are of great importance in basic as well as applied research.

Effects of air susceptibility on proton resonance frequency MR thermometry

The temperature dependence of the proton resonance frequency (PRF) is often used in MR thermometry. However, this method is prone to even very small changes in local magnetic field strength. Here, we report on the effects of susceptibility changes of surrounding air on the magnetic field inside an object and their inferred effect on the measured MR temperature. MR phase thermometry was performed on spherical agar phantoms enclosed in cylindrical containers at 7 T. The air susceptibility inside the cylindrical container was changed by both heating the air and changing the gas composition. Changing the temperature of surrounding air from 23 to 69°C caused an apparent MR temperature error of 2 K. When ambient air was displaced by 100% oxygen, the MR temperature error increased to 40 K. The magnetic field shift and therefore error in inferred MR temperature scales linearly with volume susceptibility change and has a strong and nontrivial dependence on the experimental configuration. Air susceptibility changes associated with oxygen concentration changes greatly affect PRF MR thermometry measurements. Air temperature changes can also affect these measurements, but to a smaller degree. For uncalibrated MR thermometry, air susceptibility changes may be a significant source of error.

Magnetic field sensor based on magnetic fluid clad etched fiber Bragg grating

A novel fiber optical fiber-magnetic sensor based on magnetic fluid is proposed in this paper. The stable nanoparticles Fe 3O 4 magnetic fluid was synthesized firstly; the Fe 3O 4 magnetic fluid was injected in capillaries containing etched fiber Bragg grating (FBG) as sensing element. The reflected Bragg wavelength was changed by varying the magnetic field which is perpendicular to the axial of FBG. Experimental results show that the FBG with small diameter has more sensitive wavelength shift in magnetic field. When the magnetic field increases to 25 mT, the wavelength shift of the most sensitive FBG is 86 pm, and the etched FBG shows reversible response on magnetic fields under 16 mT.

Modulation of Insulator-Metal Transition Temperature by Visible Light in La7/8Sr1/8MnO3 Thin Film

Effects of magnetic field and light illumination on the electrical transport properties of La7/8Sr1/8MnO3 thin film grown on a Si substrate are investigated. The film shows an insulator-metal transition at TP ≈ 191.9 K and a low-temperature resistance minimum at Tmin ≈ 48 K in darkness. Both magnetic field and light illumination shift the insulator-metal transition temperature T P to be higher, while the low-temperature transport properties of the film induced by them show different trends. That is, the magnetic field and light illumination make the Tmin shift to lower and higher temperatures, respectively. The enhancement of both TP and Tmin under light illumination could be explained in terms of photoinduced hole-doping and demagnetization effects of La7/8Sr1/8MnO3.

The Characteristics of the Superconducting and Magnetic Phases in the Polycrystalline Samples of Ruthenocuprates of Nominal Compositions RuSr2GdCu2O8, Ru0.98Sr2GdCu2O8 and Ru0.5Sr2GdCu2.5O8−δ

The temperature dependencies of the resistivity for the superconducting ruthenocuprates of nominal compositions RuSr2GdCu2O8, Ru0.98Sr2GdCu2O8 and Ru0.5Sr2GdCu2.5O8-d were examined for the magnetic field dependent characteristics of the superconducting transitions. The effect of the insignificant diminishing of the Ru/Cu ratio in parent RuSr2GdCu2O8 was confirmed as relevant for the stabilisation of the superconducting phase. Noted differences in the compared characteristics are interpreted for possible inhomogeneous nucleation of the superconducting phase in the parent ruthenocuprate. The phase anisotropy in RuSr2GdCu2O8 and Ru0.98Sr2GdCu2O8 , in presence of the compounds Ru magnetism, appears to be a cause of a significant softening of the Hc2(T) phase line. An anomalous lowering of the magneto-resistivity was observed in the approx. 10 K range above the onset of the superconducting transition, which may suggest the presence of enhanced superconducting fluctuations in the samples. The positive magnetic field shift of the temperatures, which limit the magneto-resistivity and the specific heat signatures of the magnetic ordered state of the Ru sub-lattice, suggests probing the influence of the ferromagnetic Ru interactions in an effective metallic-like conduction channel present in the samples. Superconducting characteristics of the Ru0.5Sr2GdCu2.5O8-d reveal a significant contribution of the Gd paramagnetic signal at low temperatures, interpreted for the presence of a significant anisotropy of the superconducting phase. It is concluded that the Ru-Cu substituted phases of ruthenocuprates may present an opportunity to investigate the effectively anisotropic superconducting phase despite its comparatively high Tc in the compounds related to the 123-type cuprate superconductor.

Magneto-optical behavior of microstructure spun fibers

The simple design of a microstructured spun fiber with air channels has been developed. For the first time a magneto-optical response of the microstructured spun fiber with high built-in linear birefringence has been experimentally studied. Most valued magneto-optical properties of this fiber to be discovered are high sensitivity resistance at bent deformations and sensitivity value is nearly to a theoretical value. It is showed that such microstructured fiber accumulates effectively Faraday phase shift in magnetic field even when the fiber is coiled into a loop of a very small diameter. For example, a fiber coil of diameter 5 mm with 100 turns has a magneto-optical sensitivity of about 70% of “ideal” value and well agrees with model calculations. Results are of a great significance for a fiber-optic current sensor.

In vivo quantification of contrast agent concentration using the induced magnetic field for time‐resolved arterial input function measurement with MRI

For pharmacokinetic modeling of tissue physiology, there is great interest in measuring the arterial input function (AIF) from dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) using paramagnetic contrast agents. Due to relaxation effects, the measured signal is a nonlinear function of the injected contrast agent concentration and depends on sequence parameters, system calibration, and time-of-flight effects, making it difficult to accurately measure the AIF during the first pass. Paramagnetic contrast agents also affect susceptibility and modify the magnetic field in proportion to their concentration. This information is contained in the MR signal phase which is discarded in a typical image reconstruction. However, quantifying AIF through contrast agent susceptibility induced phase changes is made difficult by the fact that the induced magnetic field is nonlocal and depends upon the contrast agent spatial distribution and thus on organ and vessel shapes. In this article, the contrast agent susceptibility was quantified through inversion of magnetic field shifts using a piece-wise constant model. Its feasibility is demonstrated by a determination of the AIF from the susceptibility-induced field changes of an intravenous bolus. After in vitro validation, a time-resolved two-dimensional (2D) gradient echo scan, triggered to diastole, was performed in vivo on the aortic arch during a bolus injection of 0.1 mmol/kg Gd-DTPA. An approximate geometrical model of the aortic arch constructed from the magnitude images was used to calculate the spatial variation of the field associated with the bolus. In 14 subjects, Gd concentration curves were measured dynamically (one measurement per heart beat) and indirectly validated by independent 2D cine phase contrast flow rate measurements. Flow rate measurements using indicator conservation with this novel quantitative susceptibility imaging technique were found to be in good agreement with those obtained from the cine phase contrast measurements in all subjects. Contrary to techniques that rely on intensity, the accuracy of this signal phase based method is insensitive to factors influencing signal intensity such as flip angle, coil sensitivity, relaxation changes, and time-of-flight effects extending the range of pulse sequences and contrast doses for which quantitative DCE-MRI can be applied.