Low Values Of Magnetic Field Research Articles

Thermodynamic behaviour of Rashba quantum dot in the presence of magnetic field**Project support by the University Grants Commission, India, the Department of Science and Technology, and the University Grants Commission–Basic Science Research (UGC-BSR).

The thermodynamic properties of an InSb quantum dot have been investigated in the presence of Rashba spin–orbit interaction and a static magnetic field. The energy spectrum and wave-functions for the system are obtained by solving the Schrodinger wave-equation analytically. These energy levels are employed to calculate the specific heat, entropy, magnetization and susceptibility of the quantum dot system using canonical formalism. It is observed that the system is susceptible to maximum heat absorption at a particular value of magnetic field which depends on the Rashba coupling parameter as well as the temperature. The variation of specific heat shows a Schottky-like anomaly in the low temperature limit and rapidly converges to the value of 2kB with the further increase in temperature. The entropy of the quantum dot is found to be inversely proportional to the magnetic field but has a direct variation with temperature. The substantial effect of Rashba spin–orbit interaction on the magnetic properties of quantum dot is observed at low values of magnetic field and temperature.

Reversible adiabatic temperature changes at the magnetocaloric and barocaloric effects in Fe49Rh51

We report on the adiabatic temperature changes (ΔT) associated with the magnetocaloric and barocaloric effects in a Fe49Rh51 alloy. For the magnetocaloric effect, data derived from entropy curves are compared to direct thermometry measurements. The agreement between the two sets of data provides support to the estimation of ΔT for the barocaloric effect, which are indirectly determined from entropy curves. Large ΔT values are obtained at relatively low values of magnetic field (2 T) and hydrostatic pressure (2.5 kbar). It is also shown that both magnetocaloric and barocaloric effects exhibit good reproducibility upon magnetic field and hydrostatic pressure cycling, over a considerable temperature range.

Weak magnetic field effects on chiral critical temperature in a nonlocal Nambu–Jona-Lasinio model

In this paper we study the nonlocal Nambu–Jona-Lasinio model with a Gaussian regulator in the chiral limit. Finite temperature effects and the presence of a homogeneous magnetic field are considered. The magnetic evolution of the critical temperature for chiral symmetry restoration is then obtained. Here, we restrict ourselves to the case of low magnetic field values, being this a complementary discussion to the existing analysis in nonlocal models in the strong magnetic field regime.

Fallback accretion onto magnetized neutron stars and the hidden magnetic field model

The observation of several neutron stars with relatively low values of the surface magnetic field found in supernova remnants has led in recent years to controversial interpretations. A possible explanation is the slow rotation of the proto-neutron star at birth which is unable to amplify its magnetic field to typical pulsar levels. An alternative possibility, the hidden magnetic field scenario, seems to be favoured over the previous one due to the observation of three low magnetic field magnetars. This scenario considers the accretion of the fallback of the supernova debris onto the neutron star as the responsible for the observed low magnetic field. In this work, we have studied under which conditions the magnetic field of a neutron star can be buried into the crust due to an accreting fluid. We have considered a simplified toy model in general relativity to estimate the balance between the incoming accretion flow an the magnetosphere. We conclude that the burial is possible for values of the surface magnetic field below 1013 G. The preliminary results reported in this paper for simplified polytropic models should be confirmed using a more realistic thermodynamical setup.

An ultraluminous nascent millisecond pulsar

Abstract If the ultraluminous source (ULX) M82 X-2 sustains its measured spin-up value of $\dot{\nu }= 10^{-10}\,{\rm s^{-2}}$, it will become a millisecond pulsar in less than 105 yr. The observed (isotropic) luminosity of 1040 erg s−1 also supports the notion that the neutron star will spin up to a millisecond period upon accreting about 0.1 Mȯ – the reported hard X-ray luminosity of this ULX, together with the spin-up value, implies torques consistent with the accretion disc extending down to the vicinity of the stellar surface, as expected for low values of the stellar dipole magnetic field (B ≲ 109 G). This suggests a new channel of millisecond pulsar formation – in high-mass X-ray binaries – and may have implications for studies of gravitational waves, and possibly for the formation of low-mass black holes through accretion-induced collapse.

Reduction of Low-Frequency Noise in Tunneling-Magnetoresistance Sensors With a Modulated Magnetic Shielding

The high-noise level at low frequency in magnetic tunnel junction (MTJ) sensors is the key problem for their applications in measuring the low values of static magnetic field. In this paper, we investigated a shielding-type flux chopper to resolve the 1/f noise issue. By the using a cylindrical chopper with its axis perpendicular to the sensing direction of MTJ sensor, the noise at 1 Hz was reduced from 4 to 0.3 nT/√Hz by chopping the magnetic flux at 5 kHz. The reduction in noise can be qualitatively explained by the 1/f behavior and the chopping efficiency. The proposed method is promising in enhancing the detectivity of MTJ sensor for sensing the static geomagnetic field.

Very young neutron stars and millisecond pulsars: the role of the accretion

AbstractWe consider the evolution of the very young neutron stars (NS) with moderate and low magnetic field values around 1E8 G to know how large is the share of the these objects among the those attributed as the millisecond pulsars (MSP). To exclude the contamination of accreted NS and young NS with moderate magnetic fields we study the observational evidences of the accretion on NS in the binary systems and different methods of age determinations. It was concluded that only central compact objects are appropriate candidates for NSs with small initial magnetic fields.

Supersolid Phases in a Realistic Three-Dimensional Spin Model

Supersolid phases, in which a superfluid component coexists with conventional crystalline long range order, have recently attracted a great deal of attention in the context of both solid helium and quantum spin systems. Motivated by recent experiments on 2H-AgNiO2, we study the magnetic phase diagram of a realistic three-dimensional spin model with single-ion anisotropy and competing interactions on a layered triangular lattice, using classical Monte Carlo simulation techniques, complemented by spin-wave calculations. For parameters relevant to experiment, we find a cascade of different phases as a function of magnetic field, including three phases which are supersolids in the sense of Liu and Fisher. One of these phases is continuously connected with the collinear ground state of AgNiO2, and is accessible at relatively low values of magnetic field. The nature of this transition, and its possible observation, are discussed.

The origin of the diffuse non-thermal X-ray and radio emission in the Ophiuchus cluster of galaxies

We present high-resolution 240 and 607 MHz Giant Metrewave Radio Telescope radio observations, complemented with 74 MHz archival Very Large Array radio observations of the Ophiuchus cluster of galaxies, whose radio mini-halo has been recently detected at 1400 MHz. We also present archival Chandra and XMM–Newton data of the Ophiuchus cluster. Our observations do not show significant radio emission from the mini-halo, hence we present upper limits to the integrated, diffuse non-thermal radio emission of the core of the Ophiuchus cluster. The XMM–Newton observations can be well explained by a two-temperature thermal model with temperatures of ≃1.8 and ≃9.0 keV, respectively, which confirms previous results that suggest that the innermost central region of the Ophiuchus cluster is a cooling core. This result is consistent with the occurrence of a mini-halo, as expected to be found in hot clusters with cool cores. We also used the XMM–Newton data to set up an upper limit to the (non-thermal) X-ray emission from the cluster. We also emphasize that the non-thermal X-ray emission obtained with XMM–Newton and International Gamma-Ray Astrophysics Laboratory (INTEGRAL) cannot be produced by the putative active galactic nucleus of the galaxy at the cluster centre. The combination of available radio and X-ray data has strong implications for the currently proposed models of the spectral energy distribution (SED) from the Ophiuchus cluster. In particular, a synchrotron + inverse Compton model is in agreement with the currently available data, if the average magnetic field is in the range of 0.02–0.3 μG. A pure weakly interacting massive particle annihilation scenario can in principle reproduce both radio and X-ray emission, but at the expense of postulating very large boost factors from dark matter substructures, jointly with extremely low values of the average magnetic field. Finally, a scenario where synchrotron and inverse Compton emission arise from PeV electron–positron pairs (via interactions with the cosmic microwave background) can be ruled out, as it predicts a non-thermal soft X-ray emission that largely exceeds the thermal bremsstrahlung measured by INTEGRAL.

Anisotropy of static and dynamic order-disorder transition in YBa2Cu3O7-δ single crystal

Transport measurements were performed on YBa2Cu3O7-δ single crystal at the temperature 86.7 K. Magnetic field was oriented at different angles to the ab-plane and the vortex velocities were v = 10-4-2 m/s. Obtained voltage-current characteristics show that the pinning force Fp depends non-monotonously on magnetic field both at thermal assisted creep and in the flux flow regime. These dependences can be described in context of static and dynamic order-disorder (OD) transitions. The static transition corresponds a to minimum of Fp (H) dependence and dynamic transition corresponds to the peak in the ρd-dependences. Angular variation shifts static OD transition to lower values of magnetic field with increasing angle while the value of magnetic field corresponding to the dynamic OD transition increased. These behaviors can be possibly explained by the formation of oxygen vacancies clusters.

Length-scale dependent superconductor-insulator quantum phase transitions in one dimension

We study the dissipation physics of a one dimensional mesoscopic superconducting quantum interference device array using the field-theoretical renormalization group method. We observe length scale dependent, superconductor-insulator quantum phase transition at very low temperature and also observe the dual behavior of the system for higher and lower values of magnetic field. At a critical magnetic field, we also observe a critical behavior where the resistance is independent of length.

Magneto-optical studies on the molecular cluster Fe4 in different polymeric environments

The magnetization of a molecular cluster of the Fe4 family embedded in two different polymeric matrices was investigated at low temperatures employing a magnetic circular dichroism (MCD) setup. The light wavelength used for the experiments was found to have a strong influence on the magnetic field dependence of the MCD intensities. The MCD response of this cluster embedded in a silicone elastomer and in poly(methyl methacrylate) (PMMA) exhibited at 476.5 nm a field dependence similar to the one found in the corresponding magnetization data while at other wavelengths (904 and 632.8 nm) the MCD could not be related to the cluster magnetization. Furthermore, the different polymers used as matrix induced slightly different magnetic and MCD behaviors with the samples of Fe4 in poly(methyl methacrylate) saturating at lower magnetic field values than the ones in the silicone elastomer. This result is believed to arise from the selection rules of the electronic transitions in the MCD measurement process. Furthermore, anisotropies in the spatial distribution of magnetic easy-axes, as well as inhomogeneities in cluster concentration induced by the PMMA casting process not present in the silicone elastomer could also contribute to the different magnetic behaviors observed.

Magnetically tunable microstrip linear resonator on polycrystalline ferrite substrate

AbstractA low cost conventional half‐wave microstrip linear resonator has been designed at 2 GHz central frequency and fabricated photolithographically by depositing metallic films on prepared LiMnTi polycrystalline ferrite substrate. Resonator characteristics have been examined by keeping it in the low biasing magnetic field values (from 0 to 298 Guass) applied in transverse direction of wave propagation and normal to ground plane. Tunability of 339 MHz (17%) was achieved which is the best reported so far for such a structure for low magnetic field values with a minimum insertion loss of 10.68 dB. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2868–2870, 2007; Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.22858

Characteristics of high altitude oxygen ion energization and outflow as observed by Cluster: a statistical study

Abstract. The results of a statistical study of oxygen ion outflow using Cluster data obtained at high altitude above the polar cap is reported. Moment data for both hydrogen ions (H+) and oxygen ions (O+) from 3 years (2001-2003) of spring orbits (January to May) have been used. The altitudes covered were mainly in the range 5–12 RE geocentric distance. It was found that O+ is significantly transversely energized at high altitudes, indicated both by high perpendicular temperatures for low magnetic field values as well as by a tendency towards higher perpendicular than parallel temperature distributions for the highest observed temperatures. The O+ parallel bulk velocity increases with altitude in particular for the lowest observed altitude intervals. O+ parallel bulk velocities in excess of 60 km s-1 were found mainly at higher altitudes corresponding to magnetic field strengths of less than 100 nT. For the highest observed parallel bulk velocities of O+ the thermal velocity exceeds the bulk velocity, indicating that the beam-like character of the distribution is lost. The parallel bulk velocity of the H+ and O+ was found to typically be close to the same throughout the observation interval when the H+ bulk velocity was calculated for all pitch-angles. When the H+ bulk velocity was calculated for upward moving particles only the H+ parallel bulk velocity was typically higher than that of O+. The parallel bulk velocity is close to the same for a wide range of relative abundance of the two ion species, including when the O+ ions dominates. The thermal velocity of O+ was always well below that of H+. Thus perpendicular energization that is more effective for O+ takes place, but this is not enough to explain the close to similar parallel velocities. Further parallel acceleration must occur. The results presented constrain the models of perpendicular heating and parallel acceleration. In particular centrifugal acceleration of the outflowing ions, which may provide the same parallel velocity increase to the two ion species and a two-stream interaction are discussed in the context of the measurements.

Effect of preparation parameters on the properties of La 0.9Ce 0.1CoO 3 catalysts: An EMR investigation

A few La 1− x Ce x CoO 3 ( x = 0; 0.1) catalysts, prepared by the traditional sol–gel (SG) or by flame pyrolysis (FP) procedures have been analysed by EPR spectroscopy at 120 ≤ T ≤ 300 K. No EPR line was noticed with all of the x = 0 samples. The x = 0.1 SG catalysts gave a single EPR feature at g ≅ 2, both when fresh and after catalytic reaction. This line was Lorentzian shaped at 120 K, broadening with increasing temperature, and also becoming a bit asymmetric after catalytic use. The x = 0.1 FP samples gave an EPR spectrum only after catalytic reaction. This pattern was rather similar to that obtained with the fresh SG sample with the same composition, but shifted towards lower magnetic field values. Furthermore, all the FP samples showed an intense ferromagnetic signal (FMR) after degassing treatment, while they remained EPR silent after oxygenation. An interpretation of most of these observations is here proposed on the base of polaron propagation and of formation of ferromagnetic spin bags. A correlation between catalytic performance, XRD patterns and electron magnetic resonance (EMR) (i.e. EPR or FMR) spectra, has been also highlighted.

Non-uniform columnar defect implantation in YBCO coplanar resonators for the control ofvortex-induced microwave dissipation and nonlinearity

This paper reports on 4.2 GeV Au ion irradiation of YBCO coplanar resonators aimed atcontrolling vortex-induced microwave dissipation and nonlinearity by means of asuitable distribution of columnar defects. Two procedures were used: (i) uniformirradiation at a given dose and (ii) non-uniform irradiation, obtained by means of amoving-target technique, producing a tailored profile of columnar defect density. The lastconfiguration is needed in order to attain an optimal compromise between defect-inducedenhancement of vortex pinning and increase of dissipative scattering. The resonatorswere characterized before and after irradiation in order to evaluate penetrationdepth and surface impedance, by means of a data analysis also suitable to accountfor the substrate properties. It turns out that in the case of uniform irradiationthe induced damage, attributed to the addition of dissipative regions, alwaysprevails over the improvement due to vortex pinning. In contrast, in the case of thetailored dose profile, significant damage is absent, while the onset of non-linearity isshifted towards higher circulating power. The need to control vortex-inducedeffects is confirmed by magneto-optical analysis that puts in evidence that vortexpenetration occurs even at low values of transverse dc magnetic field and temperature.

Characterization of a high-density electron-cyclotron resonance plasma source operating in nitrogen

Parametric characterization of a low-pressure and high-density electron-cyclotron resonance plasma source operating in nitrogen over a range of low-pressure 10−3–10−4 mbar, dissipated power 500 W, and magnetic field intensity 200–400 G, is presented. Internal parameters like electron energy distribution function, plasma density, electron temperature, plasma potential, and floating potential, were measured using a compensated Langmuir probe. The radial variation of internal parameters evidences two discharge modes, that is, a constricted mode for low-magnetic field values and a diffuse mode for high-magnetic field values. The electron energy distribution function is Maxwellian within the energy range of 0–15 eV and presents a structure with two prominent dips around 3.5 and 9.0 eV. The form of the structure and the position of the dips do not change significantly with the radial position or by changing the external parameters.

Nonvolatile magnetoresistive memory in phase separatedLa0.325Pr0.300Ca0.375MnO3

We have measured magnetic and transport response on the polycrystalline La$_{5/8-y}$Pr$_y$Ca$_{3/8}$MnO$_3$ ($y=0.30$, average grain size 2 microns) compound. In the temperature range where ferromagnetic metallic and insulating regions coexist we observed a persistent memory of low magnetic fields ($<$ 1 T) which is determined by the actual amount of the ferromagnetic phase. The possibility to manipulate this fraction with relatively small external perturbations is related to the phase separated nature of these manganese oxide based compounds. The colossal magnetoresistance figures obtained (about 80%) are determined by the fraction enlargement mechanism. Self-shielding of the memory to external fields is found under certain described circumstances. We show that this non-volatile memory has multilevel capability associated with different applied low magnetic field values.

The non-thermal radiation–cluster merger connection

There is compelling evidence that at least some clusters of galaxies are powerful sources of non-thermal radiation. In all cases where this radiation has been detected, a general trend is that high energy densities of cosmic rays and correspondingly low values of the average magnetic field are required in order to have a self-consistent picture of the multiwavelength observations. Mergers of clusters of galaxies might provide large enough cosmic ray injection rates and at the same time provide the mechanism for the heating of the intracluster medium to the observed temperatures. In this paper we analyze critically all the components that play a role in the non-thermal emission of clusters during a merger, with special attention to mergers occurred in the past of the cluster. We outline the consequences of this model for high energy gamma ray observations and for Faraday rotation measurements of the intracluster magnetic field.

Model of peak separation in the gamma light curve of the Vela pulsar

The separation Δpeak between two peaks in the gamma-ray pulse profile is calculated as a function of energy for several polar cap models with curvature-radiation induced cascades. The Monte Carlo results are interpreted with the help of analytical approximations and discussed in view of the recent data analysis for the Vela pulsar. We find that the behaviour of Δpeak as a function of photon energy ε depends primarily on local values of the magnetic field, Blocal, in the region where electromagnetic cascades develop. For low values of Blocal (<1012G), Δpeak(ε) is kept constant. However, for stronger magnetic fields (≳1012G) in the hollow-column model, Δpeak decreases with increasing photon energy at a rate dependent on the maximum energy of the beam particles, as well as on viewing geometry. There exists a critical photon energy εturn above which the relation Δpeak(ε) changes drastically: for , in hollow-column models the separation Δpeak increases (whereas in the filled-column model it decreases) rapidly with increasing ε, at a rate of ∼0.28 of the total phase per decade of photon energy. The existence of critical energy εturn is a direct consequence of one-photon magnetic absorption effects. In general, εturn is located close to the high-energy cut-off of the spectrum, thus photon statistics at εturn should be very low. This will make it difficult to verify the existence of εturn in real gamma-ray pulsars. Spectral properties of the Vela pulsar would favour those models that use low values of magnetic field in the emission region which in turn implies a constant value of the predicted Δpeak within the range of EGRET.