Low Field Region Research Articles

Magnetotransport properties of noncentrosymmetric CaAgBi single crystal

We report on the single crystal growth and transport properties of a topological semimetal CaAgBi which crystallizes in the hexagonal ABC-type structure with the non-centrosymmetric space group P63mc (No. 186). The transverse magnetoresistance measurements with current in the basal plane of the hexagonal crystal structure reveal a value of about 30% for I∥[10̄0] direction and about 50% for I∥[1̅10] direction at 10 K in an applied magnetic field of 14 T. The magnetoresistance shows a cusp-like behavior in the low magnetic field region, suggesting the presence of weak antilocalization effect for temperatures less than 100 K. The Hall measurements reveal that predominant charge carriers are p-type, exhibiting a linear behavior at high fields. The magnetoconductance of CaAgBi is analyzed based on the modified Hikami–Larkin–Nagaoka model. Our first-principle calculations within a density-functional theory framework reveal that the Fermi surface of CaAgBi consists of both the electron and hole pockets and the size of the hole pocket is much larger than electron pockets suggesting the dominant p-type carriers in accordance with our experimental results.

Thickness effect on low-power driving of MoS2 transistors in balanced double-gate fields

In field-effect transistors (FETs), when the thickness of the semiconducting transition metal dichalcogenides (TMDs) channel exceeds the maximum depletion depth, the entire region cannot be completely controlled by a single-gate electric field. The layer-to-layer carrier transitions between the van der Waals interacted TMD layers result in the extraordinary anisotropic carrier transport in the in-plane and out-of-plane directions. The performance of the TMD FETs can be largely enhanced by optimizing the thickness of the TMD channel as well as increasing the effective channel area through which the gate field is delivered. In this study, we investigated the carrier behavior and device performance in double-gate FETs fabricated using a 57 nm thick MoS2, which is thicker than the maximum depletion depth of about 50 nm, and a much thinner 4 nm thick MoS2. The results showed that in the thick MoS2, the gate voltages at both ends formed two independent channels which had no synergistic effect on the device performance owing to the inefficient delivery of the vertical electric field. On the other hand, in the thin MoS2 channel, the double-gate voltages effectively controlled one channel, resulting in twice the carrier mobility and operation in a low electric field region, i.e. below 0.2 MV cm−1.

Studies on magnetocaloric effect of Tb2Ni0.90Si2.94 compound

A comparative study has been carried out on the magnetocaloric properties of as-cast and annealed Tb2Ni0.90Si2.94 intermetallic compound. While the as-cast material exhibits ferromagnetic cluster-glass behaviour below 9.9 K coexisting with antiferromagnetic (AFM) interaction, the annealed system shows AFM ordering below 13.5 K and spin freezing occurs below 4 K. The compound exhibits moderate magnetocaloric performance with maximum isothermal entropy changes (−ΔS M) 8.8 and 10.9 J kg−1 K−1, relative cooling power (RCP) 306 and 365 J kg−1, along with adiabatic temperature change (ΔT ad) 5.5 and 8.15 K for 70 kOe magnetic field change in as-cast and annealed forms, respectively. The estimated magnetic entropy change is found to be larger for annealed sample in comparison to that of as-cast analogue. However, the full width at half maxima (FWHM) of −ΔS M(T) behaviour is larger in as-cast compound due to the presence of inherent structural disorder which reduces with thermal annealing. A positive isothermal entropy change (−ΔS M) and adiabatic temperature change (ΔT ad) is observed for the as-cast compound in the measured field and temperature region. In contrast, the annealed system exhibits inverse magnetocaloric effect in the low field and temperature region where AFM interactions dominate. Magnetocaloric effect (MCE) is used as a tool to establish a subtle correlation between the observed magnetocaloric effect and the reported magnetic properties of the system.

Magnetic properties of ferromagnetic Heusler alloy Co2NbGa

The compound Co2NbGa crystallizes in the Heusler-type structure. Magnetization measurements of Co2NbGa have been made under ambient pressure using a SQUID magnetometer. The spontaneous magnetic moment at 5 K and the Curie temperature are 1.81 μB/f.u. and 351 K, respectively. Many magnetic properties of the itinerant electron ferromagnets are explained by the Takahashi’s spin fluctuation theory. In the Takahashi’s spin fluctuation theory, spectral parameters of spin fluctuations, T0 and TA, are involved in various magnetic properties derived theoretically. In this work, we estimated T0 and TA of Co2NbGa by using the results of magnetization measurements at 5 K: T0 = 2.1 × 103 K and TA = 4.7 × 103 K. This experiment proved that the squared spontaneous magnetization Ms(T)2 is proportional to T2 at low temperature. The TA value estimated from the Ms(T)2 vs. T2 curve is 6.6 × 103 K. Furthermore, the field-induced magnetization M4 around TC is found to be proportional to H/M in the low magnetic field region.

Effect of the proton irradiation on the thermally activated flux flow in superconducting SmBCO coated conductors

We investigate changes in the vortex pinning mechanism caused by proton irradiation through the measurement of the in-plane electrical resistivity for H//c in a pristine and two proton-irradiated (total doses of 1 × 1015 and 1 × 1016 cm−2) SmBa2Cu3O7-δ (SmBCO) superconducting tapes. Even though proton irradiation has no effect on the critical temperature (Tc), the resulting artificial point defect causes an increase in normal state electrical resistivity. The electrical resistivity data around Tc shows no evidence of a phase transition to the vortex glass state but only broadens with increasing magnetic field due to the vortex depinning in the vortex liquid state. The vortex depinning is well interpreted by a thermally activated flux flow model in which the activation energy shows a nonlinear temperature change {boldsymbol{U}}{boldsymbol{(}}{boldsymbol{T}},{boldsymbol{H}}{boldsymbol{)}}{boldsymbol{=}}{{boldsymbol{U}}}_{{boldsymbol{0}}}{boldsymbol{(}}{boldsymbol{H}}{boldsymbol{)}}{{boldsymbol{(}}{bf{1}}-{boldsymbol{T}}{boldsymbol{/}}{{boldsymbol{T}}}_{{boldsymbol{c}}}{boldsymbol{)}}}^{{boldsymbol{q}}} (q = 2). The field dependence of activation energy shows a {{boldsymbol{U}}}_{{bf{0}}}{boldsymbol{ sim }}{{boldsymbol{H}}}^{-{boldsymbol{alpha }}} with larger exponents above 4 T. This field dependence is mainly due to correlated disorders in pristine sample and artificially created point defects in irradiated samples. Compared with the vortex pinning due to correlated disorders, the vortex pinning due to the appropriate amount of point defects reduces the magnitude of Uo(H) in the low magnetic field region and slowly reduces Uo(H) in high magnetic fields.

Numerical estimation of the oxygen impurity transport in the Aditya tokamak

The present study illustrates an approach to solving the one-dimensional impurity transport equation in tokamak plasma, without the loss of generality, using a semi-implicit numerical method. The radial profiles of number density distribution of different ionization states of oxygen (Z = 1 to Z = 8) have been estimated using the ionization and recombination rate coefficient data from the Atomic Data and Analysis Structure database. The rate coefficients are determined based on the electron temperature and number density distribution in the Aditya tokamak. The radial number density profile of the O4+ ion, obtained using the semi-implicit impurity transport equation, has been applied further in determining the radial emissivity profiles of the characteristic 650.024 nm transition of the Be-like O4+ ion. The simulated O4+ emissivity is then matched with respect to the experimental emissivity values along inboard (high toroidal magnetic field) and outboard (low toroidal magnetic field) regions of the Aditya tokamak. The comparison estimates the nature of the radial diffusivity profile of oxygen ions, which in turn demonstrates the nature of oxygen transport in Aditya plasma. Large values of oxygen ion diffusivity (> neo-classical values) are observed toward the plasma edge in both regions of the Aditya tokamak. These diffusivity values point toward a fluctuation induced transport in the edge region of Aditya plasma. The simulated results are benchmarked with the STRAHL code. The semi-implicit form of the radial impurity transport equation can be extended to deduce the distributions of other light impurities in tokamak plasma.

Thermohydraulic analysis of Tore Supra/WEST TF coil quench: Associated smooth quench occurrence in tokamak

The Toroidal Field (TF) system of the Tore Supra/WEST tokamak comprises 18 NbTi superconducting coils, cooled by a static superfluid helium bath at 1.8 K and carrying a nominal current of 1255 A. The 19th December 2017, at the end of plasma run #52205, a quench of TFC-09 was detected first on a secondary thermohydraulical signal (helium liquid level) and triggered the current Fast Safety Discharge (FSD). A numerical model of this quench has been developed with SuperMagnet (CryoSoft). The whole TFC-09 circular coil is modelled by THEA as a single large Cable-In-Conduit Conductor (CICC) with 2028 large rectangular strands (monolithic conductors of length equal to coil average perimeter). The external quench helium relief circuit (cold and warm safety valves, rupture disk and magnetic valve with corresponding pressure set) is modelled by FLOWER.The helium pressure in the coil, upstream of the cold safety valve (maximum value of 9 bar), and the expulsed helium temperature have been used as comparison between measurements and calculations which depend on the Minimum Quench Energy (MQE) used in THEA with small initial heat deposition length (few tens of centimeters) at low field region (external leg). This energy, in the order of few kJ, is compared to the real shape and energy of neutron and gamma flux caused by highly energetic runaway electrons colliding the outboard plasma facing components and which induced the quench. The expulsed helium mass flow rate of nearly 6 kg/s during 5 s has also been calculated. This event confirms on one hand the criticality and the possible occurrence of a so called “smooth quench” caused by small initial heat deposition length and at low field region and on the other hand the important interest of secondary quench detection which can be useful for other tokamak magnets safe operation and protection.

Barrier Layer Induced Switching Stability in Ga:ZnO Nanorods Based Electrochemical Metallization Memory

The effect of the TiW barrier layer on the switching properties of Ga-doped ZnO (GZO) nanorods based on Electrochemical Metallization Memory is investigated. Vertically well-aligned and uniform GZO nanorods having a diameter of approximately 35 nm are hydrothermally grown on a seeding layer of ZnO deposited on indium tin oxide (ITO) coated glass substrate, to fabricate Cu/TiW/nanorods/ITO/Glass devices. The remarkable enhancement in the memory window (on/off ratio) is achieved in the 5 nm TiW barrier layer embedded device. This device exhibits endurance of more than 103 cycles and a large memory window of ∼103. The conduction mechanism at different current regions is studied, and it is found that Schottky emission is dominated in the low field region. The TiW barrier layer helps to retain the Cu ions and control the Cu ions diffusion, hence control the filament growth into the resistive layer, confirmed from the X-ray photoelectron spectroscopy (XPS) analysis. This device is suitable for the future low power non-volatile memory devices.

Study on the Film of Ti-Si-C-O Oxides and Its Application in Memristor

The thin films of Ti-Si-C-O oxides are fabricated via direct-current magnetron co-sputtering using a Ti3SiC2 target with oxygen (O2). The X-ray photoelectron spectroscopy is used to verified the composition of the Ti-Si-C-O thin films. A memristor is designed as the structure of Pt/TSCO/ITO/glass, and the I-V curves as well as the switching behaviors are studied for the first time. The results indicated that there appear typical pinched hysteresis I-V loops and continuous resistance changes under positive and negative sweeping bias, and that the device conductance can be modulated by consecutive potentiating and depressing programming. The carrier transport mechanism of the device is also investigated and it is found that the Ohmic connection is the leading factor in the linear low electric field region while Schottky emission in the non-liner high electric field region.

The effect of the addition of CaF2 and PbF2 on boro-tellurite glasses doped with chromium ions

This article presents the impact of CaF2 and PbF2 on boro-tellurite glasses, together with preparation of glass, primary experimental techniques, modern technology and progress, particularly the development of transition metal ion doped boro-tellurite glasses for optical glass related applications. Physical and structural investigations on chromium doped boro-tellurite glasses have been studied as a function of metal fluorides. Several physical and optical parameters have been obtained and their results are explained clearly. High density and refractive index values were observed for lead fluoride doped boro-tellurite glasses which are the essential properties of the materials for applications in fiber optical communication. Electron Paramagnetic Resonance (EPR) spectrum have shown two resonance signals at low and high field regions. FTIR spectra revealed that the B–O–B vibrations are due to various BO3 and BO4 units and Te–O vibrations are from different TeO3 and TeO4 units. Deconvoluted Raman spectra being used to locate the band positions in the spectra. Raman spectra confirmed that the presence of stretching vibrations of Te–O–B links, B–O links in BO4 units. The conversion of Te–O linkages to Te–F linkages by the addition of metal fluorides is observed from of Raman spectra.

Prolonged signals from silicon strip sensors showing enhanced charge multiplication

P-type silicon strip sensors have been irradiated and annealed to predict their performance as particle detectors. After long annealing times and at high voltages, the charge collection increases due to charge multiplication and an unexpected slow signal collection was observed. In addition to beta source measurements, an edge-transient current technique was used to investigate the origin of these slow pulses. It was discovered that sensors showing slow pulses exhibit a low but substantial electric field even beyond the high field region beneath the implants. Here, the created free electrons spread while slowly traveling to the depletion region. They subsequently get multiplied and create a broad secondary cloud of free holes, which moves toward the sensor backplane. Trapping is suspected to play a crucial role in reducing and slowing down the carrier drift. The features of the signal pulses also indicate that the large amount of free charge carriers created leads to a self-screening effect and behaves like a plasma. The results are correlated well with the observation of a ballistic deficit and an enlarged cluster size observed in charge collection measurements. The conclusions drawn by this investigation can be extended to every semiconductor detector exploiting enhanced charge multiplication but having low field regions.

Collinear magnetic structure and multiferroicity in the polar magnet Co2Mo3O8

Among numerous multiferroic phenomena observed in spin frustrated lattice, giant magnetoelectricity in honeycomb lattice ${(\mathrm{Fe},\phantom{\rule{0.28em}{0ex}}\mathrm{Mn})}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ has stimulated great interest and substantial effort in searching for novel members in this 238 family. In this work, we synthesize successfully compound ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, a structural analogue of ${\mathrm{Fe}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, and present a series of characterizations on its structural, magnetic, and electric properties. An antiferromagnetic transition takes place at the Neel temperature ${T}_{N}=39\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with appearance of electric polarization and dielectric anomaly, which provides clear evidence of simultaneous magnetic and ferroelectric transitions. The neutron powder diffraction (NPD) and magnetic susceptibility data confirm the $c$-axis collinear antiferromagnetic orders and emergent ferroelectric polarization. In particular, such antiferromagnetic order is relatively robust against magnetic field up to 9 T, different from ${\mathrm{Fe}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ with ferrimagnetic transition or ${\mathrm{Mn}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ with spin flop in the low-field region. Our data on single crystals demonstrate the second-order magnetoelectric effect in terms of magnetic field dependence of ferroelectric polarization response, while no linear magnetoelectric response is allowed. It is suggested that ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ provides a unique platform on which rich multiferroic physics in the presence of collinear magnetic order can be explored.

Mapping out the Degree of Freedom of Hosted Enzymes in Confined Spatial Environments

Summary The integration of enzymes with solid materials is crucial for promoting their industrialization. Understanding the enzyme behavior upon association within a confined space, though of fundamental importance for biocomposite development, remains a persistent, unresolved challenge. Here, we present a comprehensive elucidation of the spatial environment’s impact on hosted enzymes’ degree of freedom and, consequently, their accompanying reactivity. Site-directed spin labeling in combination with electron paramagnetic resonance spectroscopy allows the direct detection of host-guest interactions at atomic resolution, while the tailorable synthesis of covalent organic frameworks (COFs) enables an evaluation of factors affecting such interactions. Specifically, lysozyme is found to be more constrained and less active along with increasing hydrophilicity of the COFs. These results support the establishment of a connection between the hydrophilicity of the spatial environment and the resulting biocomposites’ reactivity, enabling the prediction of the performance of unknown biocomposites. This study provides a unique insight into the mechanistic pathways underpinning biocatalysis.

The Anisotropic van Hemmen model with a random field in a random network

In this paper we investigate the three-state spins van Hemmen model with a crystalline field in the random network. The van Hemmen model is known for being treated without the use of replicas. The same method used by van Hemmen is utilized here to average over disordered exchange interactions. To deal with averages over the realizations of the lattice we utilized the replica symmetry formalism of order parameter functions. The order parameters calculated to detect which phase the system encounters are calculated numerically by means of a population dynamics algorithm. Firstly we obtained phase diagrams in low fixed temperature in the diagram crystalline field versus random field, we observe higher connectivities to give rise to segregation between high activity and low activity spin glass (SG) phases, also we verify the appearance of a tricritical point in the low crystalline field region. Finally to account the effects of high thermal fluctuations we have drawn phase diagrams in the temperature versus random field plane for fixed values of crystalline field. Here we observe an important modification of the phase plane topology by the increment of network connectivity. It is also notable the presence of a reentrant behavior in first and second order phase transitions when the crystalline field is sufficiently large.

Design and optimization of a magnet for magnetocaloric refrigeration

The magnetic field source is the most expensive element of a magnetic refrigeration device. The design of an efficient magnet with high and low field regions, using reduced volumes of permanent magnets, is of paramount importance for the practical application of this new technology. A two-pole rotary magnet formed by sectors of oriented hard magnets and soft iron pieces is optimized in angular width and magnetization direction of each sector. A quadratic approximation is used for the optimization of the defining angles. This procedure provides the parameters that result in extreme values of a given functional F, defined as an efficiency variable of the working device. The procedure also gives the correlation between parameters and the relative importance of the deviations with respect to their mathematical optimal values. An optimized magnet has also been built, and the resulting calculated fields are compared with experimental measurements in the real system.

Experimental characterization of magnetically focused electron contamination at the surface of a high-field inline MRI-linac.

The fringe field of the Australian MRI-linac causes contaminant electrons to be focused along the central axis resulting in a high surface dose. This work aims to characterize this effect using Gafchromic film and high-resolution detectors, MOSkinTM and microDiamond. The secondary aim is to investigate the influence of the inline magnetic field on the relative dose response of these detectors. The Australian MRI-linac has the unique feature that the linac is mounted on rails allowing for measurements to be performed at different magnetic field strengths while maintaining a constant source-to-surface distance (SSD). Percentage depth doses (PDD) were collected at SSD 1.82m in a solid water phantom positioned in a low magnetic field region and then at isocenter of the MRI where the magnetic field is 1T. Measurements for a range of field sizes were taken with the MOSkinTM , microDiamond, and Gafchromic® EBT3 film. The detectors' relative responses at 1T were compared to the near 0T PDD beyond the region of electron contamination, that is, 20mm depth. The near surface measurements inside the MRI bore were compared among the different detectors. Skin dose in the MRI, as measured with the MOSkinTM , was 104.5% for 2.1×1.9cm2 , 185.6% for 6.1×5.8cm2 , 369.1% for 11.8 × 11.5cm2 , and 711.1% for 23.5×23cm2 . The detector measurements beyond the electron contamination region showed agreement between the relative response at 1T and near 0T. Film was in agreement with both detectors in this region further demonstrating their relative response is unaffected by the magnetic field. Experimental characterization of the high electron contamination at the surface was performed for a range of field sizes. The relative response of MOSkinTM and microDiamond detectors, beyond the electron contamination region, were confirmed to be unaffected by the 1-T inline magnetic field.

Timing optimization for 3D silicon sensors

Looking forward to future High Luminosity LHC experiments, efforts to develop new tracking detectors are increasing. A common approach to improve track reconstruction efficiency in high pile-up conditions is to add time measurement per pixel with resolution smaller than 50 ps. Different sensor technologies are under development in order to achieve those performances, like low gain avalanche diodes and 3D sensors. 3D sensors are characterized by very fast charge collection times, but present some critical issues in timing due to their electrode configurations. The presence of zero electric field volumes inside the electrodes themselves and low electric field regions between same sign electrodes causes that the 3D sensor technology presents potentially a large time walk contribution which negatively affects time resolution. In order to reduce this error drastically, a detailed study based mostly on simulation has been done with main focus on the exploration for a timing optimized 3D sensor electrode configuration. To have a more detailed view of the timing performances, sensor operation was also simulated, using TCAD and other simulation tools developed specific for this application, and the results analysed. In this presentation a detailed overview of the modelling and simulation activity as well as their results, including also future steps will be presented. The output of this studies defines the optimal sensor layout for timing applications.

Temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on first-order reversal curve analysis

A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 °C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well confirmed by using the soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well discussed by the temperature dependent local demagnetization field and the saturation field of multidomain state. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.

Properties of the Umbral Filament Observed in Active Region NOAA 12529

Abstract Recent observations of the solar photosphere revealed the presence of elongated filamentary bright structures inside sunspot umbrae, called umbral filaments (UFs). These features differ in morphology, magnetic configuration, and evolution from light bridges (LBs) that are usually observed to intrude in sunspots. To characterize a UF observed in the umbra of the giant leading sunspot of active region NOAA 12529, we analyze high-resolution observations taken in the photosphere with the spectropolarimeter on board the Hinode satellite and in the upper chromosphere and transition region with the IRIS telescope. The results of this analysis definitely rule out the hypothesis that the UF might be a kind of LB. In fact, we find no field-free or low-field strength region cospatial to the UF. Conversely, we recognize the presence of a strong horizontal field larger than 2500 G, a significant portion of the UF with opposite polarity with respect to the surroundings, and filaments in the upper atmospheric layers corresponding to the UF in the photosphere. These findings suggest that this structure is the photospheric manifestation of a flux rope hanging above the sunspot and forming penumbral-like filaments within the umbra via magneto-convection. This reinforces a previously proposed scenario.

On the weighting field of irradiated silicon detectors

The understanding of the weighting field of irradiated silicon sensors is essential for calculating the response of silicon detectors in the radiation environment at accelerators like at the CERN LHC. Using 1-D calculations of non-irradiated pad sensors and 1-D TCAD simulations of pad sensors before and after irradiation, it is shown that the time-dependence of the weighting field is related to the resistivity of low field regions with ohmic behaviour in the sensor. A simple formula is derived, which relates the time constant of the time-dependent weighting field, $\tau $, with the resistivity and the extension of the low-field region for pad detectors. As the resistivity of irradiated silicon increases with fluence and finally reaches the intrinsic resistivity, $\tau $ becomes much larger than the charge-collection time and the weighting field becomes essentially independent of time. The TCAD simulations show that the transition from a time-dependent to a time-independent weighting field occurs at a neutron-equivalent fluence of $ \approx 5 \times 10^{12}$ cm$^{-2}$ for a 200 $\mu$m thick pad diode operated at 40 V and $- 20^\circ$C. It is therefore concluded that the use of a time-independent weighting field calculated with the same method as for a fully-depleted non-irradiated sensor is also appropriate for the simulation of highly irradiated silicon sensors.