Offset Field Research Articles

Magnetic field dependence of threshold electric field for magnetoelectric switching of exchange-bias polarity

We report the magnetic field dependence of the threshold electric field Eth for the magnetoelectric switching of the perpendicular exchange bias in Pt/Co/Au/Cr2O3/Pt stacked films using a reversible isothermal electric tuning approach. The Eth values for the positive-to-negative and negative-to-positive switching are different because of the unidirectional nature of the interfacial exchange coupling. The Eth values are inversely proportional to the magnetic-field strength, and the quantitative analysis of this relationship suggests that the switching is driven by the nucleation and growth of the antiferromagnetic domain. We also find that the magnetic-field dependence of Eth exhibits an offset electric field that might be related to the uncompensated antiferromagnetic moments located mainly at the interface.

Robust optimal control of two-level quantum systems

We investigate the time and the energy minimum optimal solutions for the robust control of two-level quantum systems against offset or control field uncertainties. Using the Pontryagin Maximum Principle, we derive the global optimal pulses for the first robustness orders. We show that the dimension of the control landscape is lower or equal to 2N for a field robust to the N th order, which leads to an estimate of its complexity.

In situ absolute magnetometry in an UHV scanning probe microscope using conducting polymer-thin film

The in situ measurement and control of the direction and magnitude of the magnetic field is demonstrated within the sample plane of a low-temperature ultra-high vacuum scanning probe microscope. These measurements utilized electrically detected magnetic resonance magnetometry based on the spin-dependent recombination current in a conducting polymer-thin film. The presented magnetometry approach allows the absolute measurement of systematic magnetic offset fields with a resolution on the order of ≈5μT/Hz with an angular resolution below ≈1°. As the polymer film covers a macroscopic area within the sample plane, magnetometry becomes possible at various locations within the sample plane and thus the determination of magnetic field gradients.

The Impact of Small Field’s Off-Set on Output Factor in Intensity Modulated Radiation Treatment

In intensity modulated radiation treatment (IMRT) planning, the use of asymmetrically collimated fields that are placed on central axis or its off-set is mostly required. Output is the main topic discussed today for extremely small and/or severe irregularly shaped fields. The air scatter data are involved directly or indirectly in obtaining the output. Despite the fact that extensive data have been published in many studies to provide a guide on the magnitude of output factor for clinical accelerators, there are very few data reviewed about output factor in-air or phantom for off-set fields. This study was aimed to investigate the impact of these conditions for small fields. This study was conducted in Elekta Synergy linear accelerator which produces 6 MV X-ray energy. The in-air output factor (Sc) has been measured by CC04 ion chamber with brass-alloy “build-up” cap and Dose-1 electrometer, and the total output (Scp) measurements were carried on at dose maximum depth in phantom by the same chamber and Thermoluminescence dosimeter (TLD) for 1 - 10 cm2 fields. The all measurements at center of isocenter and off-set fields at three directions (X2, Y1, Diagonal) were done. By decreasing field size from 10 to 2 cm2 at isocenter, the Sc value using CC04 was decreased to 5.4% and Scp using CC04 and TLD to 14.5% and 11% respectively. By increasing off-set value, the Sc and Scp values were increased in all directions comparing to central fields. The maximum increase was obtained in Y1 direction for Sc and Scp. TLD results for Scp is slightly higher than CC04. The dosimetric properties of small fields and their off-set should be evaluated and modelled appropriately in the treatment planning system to ensure accurate dose calculation in Intensity Modulated Radiation Treatment.

Spin valves with Conetic based synthetic ferrimagnet free layer

The detection of low magnetic field requires hysteresis-free magnetoresistive sensor with high sensitivity. Giant magnetoresistance spin valve (SV) sensors with CoFeB based synthetic antiferromagnetic (SAF) pinned layer and Conetic based synthetic ferrimagnet (SF) free layer are developed in this paper. Low free layer coercivity (HC) of 0.27 Oe is achieved through adapting synthetic antiferromagnetic pinned layer in SV with Conetic single free layer. In SV with SAF pinned layer and SF free layer, the MR ratio, HC, switching field (HSW) and offset field (HO) increase with the thickness of the Conetic alloy layer right above the Cu spacer. The HC, HO, and HSW show oscillatory behavior with the Ru spacer thickness in the SF free layer. Through introducing CoFeB/SF free layer, an increased sensitivity of 0.27%/Oe and a small coercivity of 0.3 Oe are attained, which holds promise for low field sensing applications.

Compact two-dimensional CMOS Hall sensor based on switchable configurations of four three-contact elements

This paper reports a novel CMOS sensor of compelling design simplicity for the measurement of both in-plane magnetic field components Bx and By at the same location. The device is operated through only four contacts and using a single control signal. It consists of four separate n-wells arranged as two orthogonal pairs. Each n-well segment forms a three-contact vertical Hall element (3CVHE) whose two outer contacts are used for the electrical interconnection of the four 3CVHEs into a conducting loop. The central contacts of each 3CVHE serve as drive and sense contacts enabling offset reduction by the spinning-current method. The operating principle of the novel device is derived from the magnetic field dependent resistance matrix of the individual 3CVHEs. The two-axis sensing capability of the sensor is achieved by selectively switching the interconnections among the four 3CVHEs, whereby the device is made sensitive alternatingly to Bx and By. Integrated transmission gates (TG) serve as switches. Appropriate design ensures the electrical symmetry of the sensor, which results in a low equivalent offset field of 0.2 to 0.4mT. For a nominal B field of 3.7mT the RMS error of the measured magnitude and angle of B are extracted to be only 42 μT and 0.7°, respectively.

Mitigating Ionospheric Artifacts in Coseismic Interferogram Based on Offset Field Derived From ALOS-PALSAR Data

Ionospheric total electron content (TEC) disturbances can seriously influence the signal of low-frequency spaceborne synthetic aperture radar (SAR) systems, e.g., Advanced Land Observation Satellite (ALOS)-phased array-type L-band synthetic aperture radar (PALSAR). With regard to coseismic studies using interferometric synthetic aperture radar (InSAR), it is vital to mitigate the ionospheric artifacts in the contaminated coseismic interferogram. In this paper, we propose a new method for the integral constant calculation, and we then aim to improve the estimation of the ionospheric phase screen (IPS). The proposed method is based on both azimuth and range displacement field maps. At present, the azimuth displacement field can be generated by an offset-tracking procedure or multiple-aperture InSAR (MAI), but the range displacement field can only be estimated by an offset-tracking procedure. We applied ALOS-PALSAR data that were acquired before and after the 2008 Wenchuan earthquake and the 2010 Darfield earthquake to test the proposed method. This case study further showed that ionospheric azimuth streaks were clearly visible in the azimuth displacement field maps of these two cases, one of which was generated using the MAI algorithm and the other using an offset-tracking algorithm. The results confirmed that the long-wavelength ionospheric artifacts in the non-coseismic regions could be corrected by the use of the proposed method. The line-of-sight (LOS) displacement corrections of these two cases, Wenchuan and Darfield, were in the range of $-35.9$ to 21.0 cm and $-6.6$ to 10.0 cm along the LOS direction, respectively.

An improved pixel-tracking method for monitoring mining subsidence

ABSTRACTUnderground mining always induces large vertical displacements in the ground surface and, because of the large subsidence gradient, the phase-unwrapping methods of interferometric synthetic aperture radar (InSAR) are unable to give accurate results. Pixel tracking based on normalized cross-correlation maximization overcomes the limitations imposed by the subsidence gradient and can be used to monitor large displacements. This paper introduces and analyses each component of the offset field and then describes the improved method used to efficiently remove orbital error and topography-related offset caused by rugged terrain with the help of a digital elevation model (DEM). Eleven TerraSAR-X images in spotlight mode and Shuttle Radar Tomography Mission (SRTM) DEM data are used to monitor mining subsidence by the improved pixel-tracking method in Shenmu County of Yulin City. The root mean square error (RMSE) between the improved method and Global Positioning System (GPS) data in the strike and dip directions are 0.143 and 0.108 m, respectively. The approach presented here is shown to be appropriate for monitoring large vertical displacements in mining subsidence.

Nanoslit design for ion conductivity gradient enhanced dielectrophoresis for ultrafast biomarker enrichment in physiological media.

Selective and rapid enrichment of biomolecules is of great interest for biomarker discovery, protein crystallization, and in biosensing for speeding assay kinetics and reducing signal interferences. The current state of the art is based on DC electrokinetics, wherein localized ion depletion at the microchannel to nanochannel interface is used to enhance electric fields, and the resulting biomarker electromigration is balanced against electro-osmosis in the microchannel to cause high degrees of biomarker enrichment. However, biomarker enrichment is not selective, and the levels fall off within physiological media of high conductivity, due to a reduction in ion concentration polarization and electro-osmosis effects. Herein, we present a methodology for coupling AC electrokinetics with ion concentration polarization effects in nanoslits under DC fields, for enabling ultrafast biomarker enrichment in physiological media. Using AC fields at the critical frequency necessary for negative dielectrophoresis of the biomarker of interest, along with a critical offset DC field to create proximal ion accumulation and depletion regions along the perm-selective region inside a nanoslit, we enhance the localized field and field gradient to enable biomarker enrichment over a wide spatial extent along the nanoslit length. While enrichment under DC electrokinetics relies solely on ion depletion to enhance fields, this AC electrokinetic mechanism utilizes ion depletion as well as ion accumulation regions to enhance the field and its gradient. Hence, biomarker enrichment continues to be substantial in spite of the steady drop in nanostructure perm-selectivity within physiological media.

Spin-exchange relaxation-free magnetometer with nearly parallel pump and probe beams

We constructed a spin-exchange relaxation-free (SERF) magnetometer with a small angle between the pump and probe beams facilitating a multi-channel design with a flat pancake cell. This configuration provides almost complete overlap of the beams in the cell, and prevents the pump beam from entering the probe detection channel. By coupling the lasers in multi-mode fibers, without an optical isolator or field modulation, we demonstrate a sensitivity of 10 f T for frequencies between 10 Hz and 100 Hz. In addition to the experimental study of sensitivity, we present a theoretical analysis of SERF magnetometer response to magnetic fields for small-angle and parallel-beam configurations, and show that at optimal DC offset fields the magnetometer response is comparable to that in the orthogonal-beam configuration. Based on the analysis, we also derive fundamental and probe-limited sensitivities for the arbitrary non-orthogonal geometry. The expected practical and fundamental sensitivities are of the same order as those in the orthogonal geometry. We anticipate that our design will be useful for magnetoencephalography (MEG) and magnetocardiography (MCG) applications.

From Three-Contact Vertical Hall Elements to Symmetrized Vertical Hall Sensors with Low Offset

This paper gives a detailed account of vertical Hall sensors (VHS) with an electrical symmetry equivalent to that of planar Hall plates (PHP). Conventional five-contact VHS (5CVHS) have shown strongly nonlinear offset characteristics due to the junction field effect combined with an inherent electrical asymmetry which have prevented efficient offset compensation by current spinning. We show that this deficit can be cured by appropriate device design. The novel sensor achieves a higher symmetry by four identical three-contact vertical Hall elements (3CVHE) interconnected into what we call the fully symmetric VHS (FSVHS). In this device offset compensation by current spinning is found to be at least ten times more efficient than in comparable 5CVHS. Quantitatively, at a drive current of 0.5mA the residual magnetic offset field of the FSVHS extracted from 50 samples is 0.14±0.41mT. In 5CVHS devices the corresponding value was found to be 2.04±1.32mT. Moreover, by orthogonal coupling of four FSVHS the device is symmetrized even further, which results in an equivalent magnetic offset field of 0.04±0.05mT. This represents the best results to date achieved by a stand-alone VHS in standard CMOS technology. For comparison the magnetic offset of conventional PHP fabricated in CMOS technology lies in the range of 0.05 to 0.1 mT, between the results of the two novel concepts.

Easily magnetic anomalies earthquake prediction

Low power consumption long time offset magnetic field detector (earthquake prediction) .The design of the hardware circuit of the magnetic field detector seismic geomagnetic acquisition and pre processing module mainly includes. Electronic compass, compass. monitoring device while the magnetic azimuth for monitoring and analyzing the object, GSM, but it can also be applied to other seismic precursor information analysis, such as earthquake precursory infrasound abnormality, only need infrasound abnormality intelligent sensor replace geomagnetic anomaly intelligent sensor, and modify the relevant parameters can be.

Experimental evidence of electrification processes during the 2009 L'Aquila earthquake main shock

AbstractTwo types of coseismic magnetic field events are simultaneously observed: transient offset events and magnetic field signal that occurred at the destructive, Mw6.1 L'Aquila earthquake main shock. The offset event, conventionally interpreted as a signature of piezomagnetic effects, however, could not be explained as such. In the second type of coseismic event, the transient magnetic signal starts simultaneously with the offset event and reaches amplitude of 0.8 nT in the total magnetic field. The signal is a local one; its amplitude shape resembles diffusion‐like form with time scale characteristics that are indicative for a source deep in the crust. The polarity of the transient signal is in the horizontal plane and nearly parallel to the L'Aquila fault strike.

Accurate Determination of Glacier Surface Velocity Fields with a DEM-Assisted Pixel-Tracking Technique from SAR Imagery

We obtained accurate, detailed motion distribution of glaciers in Central Asia by applying digital elevation model (DEM) assisted pixel-tracking method to L-band synthetic aperture radar imagery. The paper firstly introduces and analyzes each component of the offset field briefly, and then describes the method used to efficiently and precisely compensate the topography-related offset caused by the large spatial baseline and rugged terrain with the help of DEM. The results indicate that the rugged topography not only forms the complex shapes of glaciers, but also affects the glacier velocity estimation, especially with large spatial baseline. The maximum velocity, 0.85 m∙d−1, was observed in the middle part on the Fedchenko Glacier, which is the world’s longest mountain glacier. The motion fluctuation on its main trunk is apparently influenced by mass flowing in from tributaries, as well as angles between tributaries and the main stream. The approach presented in this paper was proved to be highly appropriate for monitoring glacier motion and will provide valuable sensitive indicators of current and future climate change for environmental analysis.

Stability of a trapped-atom clock on a chip

We present a compact atomic clock interrogating ultracold 87Rb magnetically trapped on an atom chip. Very long coherence times sustained by spin self-rephasing allow us to interrogate the atomic transition with 85% contrast at 5 s Ramsey time. The clock exhibits a fractional frequency stability of $5.8\times 10^{-13}$ at 1 s and is likely to integrate into the $1\times10^{-15}$ range in less than a day. A detailed analysis of 7 noise sources explains the measured frequency stability. Fluctuations in the atom temperature (0.4 nK shot-to-shot) and in the offset magnetic field ($5\times10^{-6}$ relative fluctuations shot-to-shot) are the main noise sources together with the local oscillator, which is degraded by the 30% duty cycle. The analysis suggests technical improvements to be implemented in a future second generation set-up. The results demonstrate the remarkable degree of technical control that can be reached in an atom chip experiment.

Performance, simulation and field application modeling of rollercone bits

Abstract Drilling simulation technology has been used extensively to optimize drilling operations so as to minimize the associated costs and risks. Optimum bit types and designs with corresponding drilling parameters can be recommended utilizing a simulator with rate of penetration (ROP) models. There have been several attempts to develop ROP models that can deliver the most reliable outputs, required for the pre-planning and post analysis applications, using various sets of drilling parameters. However, due to the existing modeling complexities, these attempts have not been successful. In this study, a new ROP model is developed for the rollercone bits, which properly integrates the effect of main drilling parameters as well as cutting structure of the bit. The model is mathematically derived based on the mechanism of single cutter–rock interaction, and calibrated utilizing sets of full scale laboratory data. Also, the bit wear effect for simulating accurate rock strength and ROP values is included in the analysis using a previously published model. One of the most important features of the newly introduced ROP model is that it can be easily inverted to generate accurate rock strength values using offset and/or real-time field data. This unique characteristic of the ROP model makes it a valuable candidate for drilling simulation studies to optimize drilling operations in the most cost effective manner. The verification of the introduced ROP model is performed through series of simulation analysis and comparing the generated rock strength logs to the outputs of a commercially available drilling simulator. The comparison of the results obtained from the simulator and the ROP model as well as field data has been quite encouraging which signifies the application of the developed model in determining the best case scenario for planning and/or drilling of future wells with lowest possible expenditures.

Switching methods in magnetic random access memory for low power applications

Effect of saturation magnetization (Ms) of the free layer (FL) on the switching current is analyzed for spin transfer torque (STT) magnetic random access memory (MRAM). For in-plane FL, critical switching current (Ic0) decreases as Ms decreases. However, reduction in Ms also results in a low thermal stability factor (Δ), which must be compensated through increasing shape anisotropy, thus limiting scalability. For perpendicular FL, Ic0 reduction by using low-Ms materials is actually at the expense of data retention. To save energy consumed by STT current, two electric field (EF) controlled switching methods are proposed. Our simulation results show that elliptical FL can be switched by an EF pulse with a suitable width. However, it is difficult to implement this type of switching in real MRAM devices due to the distribution of the required switching pulse widths. A reliable switching method is to use an Oersted field guided switching. Our simulation and experimental results show that the bi-directional magnetization switching could be realized by an EF with an external field as low as ±5 Oe if the offset field could be removed.

Control of offset field and pinning stability in perpendicular magnetic tunnelling junctions with synthetic antiferromagnetic coupling multilayer

In a magnetic tunnelling junction (MTJ) with perpendicular magnetic anisotropy (PMA), offset field (Ho) of the free layer is usually controlled by using a synthetic antiferromagnetic (SAF) coupling structure, which is composed of an antiferromagnetic coupling (AFC) layer sandwiched by two ferromagnetic (FM) layers. However, Ho increases significantly as the size of MTJ devices shrinks to accommodate high density. In addition, magnetostatic field in PMA SAF structure tends to destabilize the antiferromagnetic (AFM) alignment of the SAF layers, in contrast to the in-plane anisotropy SAF, where the closed flux forms stable AFM magnetic configuration. Here, we present a double SAF structure to control Ho, while maintaining high magnetic stability of the reference layer (RL). The double SAF consists of FM1/AFC/FM2/AFC/FM3 multilayer. An AFM layer like PtMn is added to further stabilize the magnetic configuration of the double SAF. As the magnetization of other FM layers (FM1 and FM2) is aligned oppositely, the magnetostatic field acting on the RL (FM3) layer is significantly reduced due to cancellation effect from its adjacent layers. Both simulation and experimental results demonstrate that the double SAF layers provide high stability for the RL in addition to the reduction of Ho. Our results on MTJ devices show that the AFM pinned double SAF has the highest RL stability. The RL switch rate decreases as the thickness of the CoFe inserted layer between AFM and the pinned layer (Co/Pt multilayer) increases due to improved exchange coupling.

Scalable and thermally robust perpendicular magnetic tunnel junctions for STT-MRAM

Thermal budget, stack thickness, and dipolar offset field control are crucial for seamless integration of perpendicular magnetic junctions (pMTJ) into semiconductor integrated circuits to build scalable spin-transfer-torque magnetoresistive random access memory. This paper is concerned with materials and process tuning to deliver thermally robust (400 °C, 30 min) and thin (i.e., fewer layers and integration-friendly) pMTJ utilizing Co/Pt-based bottom pinned layers. Interlayer roughness control is identified as a key enabler to achieve high thermal budgets. The dipolar offset fields of the developed film stacks at scaled dimensions are evaluated by micromagnetic simulations. This paper shows a path towards achieving sub-15 nm-thick pMTJ with tunneling magnetoresistance ratio higher than 150% after 30 min of thermal excursion at 400 °C.

Phase cycling schemes for finite-pulse-RFDR MAS solid state NMR experiments

The finite-pulse radio frequency driven dipolar recoupling (fp-RFDR) pulse sequence is used in 2D homonuclear chemical shift correlation experiments under magic angle spinning (MAS). A recent study demonstrated the advantages of using a short phase cycle, XY4, and its super-cycle, XY414, for the fp-RFDR pulse sequence employed in 2D 1H/1H single-quantum/single-quantum correlation experiments under ultrafast MAS conditions. In this study, we report a comprehensive analysis on the dipolar recoupling efficiencies of XY4, XY412, XY413, XY414, and XY814 phase cycles under different spinning speeds ranging from 10 to 100kHz. The theoretical calculations reveal the presence of second-order terms (T10T2,±2, T1,±1T2,±1, etc.) in the recoupled homonuclear dipolar coupling Hamiltonian only when the basic XY4 phase cycle is utilized, making it advantageous for proton–proton magnetization transfer under ultrafast MAS conditions. It is also found that the recoupling efficiency of fp-RFDR is quite dependent on the duty factor (τ180/τR) as well as on the strength of homonuclear dipolar couplings. The rate of longitudinal magnetization transfer increases linearly with the duty factor of fp-RFDR for all the XY-based phase cycles investigated in this study. Examination of the performances of different phase cycles against chemical shift offset and RF field inhomogeneity effects revealed that XY414 is the most tolerant phase cycle, while the shortest phase cycle XY4 suppressed the RF field inhomogeneity effects most efficiently under slow spinning speeds. Our results suggest that the difference in the fp-RFDR recoupling efficiencies decreases with the increasing MAS speed, while ultrafast (>60kHz) spinning speed is advantageous as it recouples a large amount of homonuclear dipolar couplings and therefore enable fast magnetization exchange. The effects of higher-order terms and cross terms between various interactions in the effective Hamiltonian of fp-RFDR are also analyzed using numerical simulations for various phase cycles. Results obtained via numerical simulations are in excellent agreement with ultrafast MAS experimental results from the powder samples of glycine and l-alanine.