Thick film resistors in low-temperature resistance thermometry in high magnetic fields

Thick film resistors exhibit a strong dependence on temperature in the cryogenic range below 20 K. In this range the temperature coefficient of resistance is estimated to be comparable to that of commonly used resistance thermometers.The resistance change by the action of the magnetic field is observed to be only small and independent of magnetic field direction. By using these resistors in cryogenic resistance thermometry in intense magnetic field the temperature error due to the magnetoresistance is shown to be only small in comparison with frequently used sensors.According to these results thick film resistors are reported to offer an interesting alternative in cryogenic resistance thermometry within intense magnetic fields.     

Vapour pressure thermometry in high magnetic fields using a Pirani gauge

Vapour pressure thermometry has the advantage of not being affected by magnetic fields. Therefore, a study was carried out of thermometry below 1 K using a Pirani pressure gauge in the vapour of He³. Its sensitivity and error in temperature measurements above 0.55 K are dV/dT ≥ K^?1 and 0.4% respectively.Additionally, a semiconductor pressure sensor in the vapour of He⁴ was examined for the vapour pressure thermometry.     

Instrumentation and methods for low temperature measurements in high magnetic fields

There are several difficulties associated with the accurate measurement of low temperature in the presence of an intense magnetic field B. Most of the problems stem from the direct effect of the field on the thermometric properties of almost all of the comcommonly used sensors. Because the magnitude of the field effect, eg magnetoresistance, varies widely as a function of B, T, and the thermometer itself, a careful selection process is necessary to minimize the error. As an aid to such a process, a detailed comparison is presented of the field-dependent errors, Δ/T, as a function of T, of carbon, carbon-glass, germanium, and platinum resistance thermometers, thermistors, Si and GaAs diodes, thermocouples, capacitance thermometers, and several other less popular devices. Specific recommendations are made on the basis of the comparison. The related problem of magnetic field measurement is also examined, with emphasis on the recent characterizations of commercially available InAs, InSb, and GaAs Hall effect probes. From the results of measurements over the 1.5–300 K range and to fields as high as 23 T, several encouraging conclusions may be drawn concerning the performance of the sensors as magnetometers in the 1% accuracy range.     

Resistance thermometry in magnetic fields 1. Thermistors and platinum thermometers at 77 K

The effects of static magnetic field up to 16 kG on seven midget disc thermistors and three miniature platinum resistance thermometers were measured at 77 K. Corrections for the magnetoresistance of the electrical leads and solder junctions were made from data taken on a copper wire coil. A virtual temperature rise of the order of millikelvins occurs for both temperature sensors; the lead wire correction is less than a few microkelvins. The thermistors have larger magnetoresistance effects than similar types recommended for use at lower temperatures. A figure of merit comparison suggests that they are slightly less desirable than platinum thermometers in magnetic fields at liquid nitrogen temperatures.     

Low temperature magnetic measurements of NENP

The temperature dependent magnetic susceptibility, (T), of two samples of the S=1 linear-chain Heisenberg antiferromagnet Ni(C₂H₈N₂)₂NO₂(ClO₄), known as NENP, has been measured from 400 K to 264 µK. Our measurements are in agreement with existing results of other researchers who worked above 1.2 K. Below 1.2 K, (T) increases with decreasing temperature. The results suggest that this increase is intrinsic to the NENP and is not a consequence of a single source of paramagnetic impurities in the samples. Scientific Instruments Graduate Student Fellow     

强磁场下铑铁温度计的磁致电阻效应研究

以铑铁温度计为研究对象,研究了其在0-16 T磁场下、2-300 K温区内的磁致电阻效应.结果表明:铑铁温度计在0-16 T场强下,2-77 K温区内磁效应随场强的增强和温度的降低而明显升高,77-300 K温区内温度计受磁场的影响较小,其中在16 T、2 K处,以及16 T、300 K处的磁效应分别为70.94%和-...     

Iron chalcogenide superconductors at high magnetic fields

Iron chalcogenide superconductors have become one of the most investigated superconducting materials in recent years due to high upper critical fields, competing interactions and complex electronic and magnetic phase diagrams. The structural complexity, defects and atomic site occupancies significantly affect the normal and superconducting states in these compounds. In this work we review the vortex behavior, critical current density and high magnetic field pair-breaking mechanism in iron chalcogenide superconductors. We also point to relevant structural features and normal-state properties.     

Iron chalcogenide superconductors at high magnetic fields

Abstract

Iron chalcogenide superconductors have become one of the most investigated superconducting materials in recent years due to high upper critical fields, competing interactions and complex electronic and magnetic phase diagrams. The structural complexity, defects and atomic site occupancies significantly affect the normal and superconducting states in these compounds. In this work we review the vortex behavior, critical current density and high magnetic field pair-breaking mechanism in iron chalcogenide superconductors. We also point to relevant structural features and normal-state properties.     

X-ray diffraction measurements in high magnetic fields and at high temperatures

A system was developed measuring x-ray powder diffraction in high magnetic fields up to 5 T and at temperatures from 283 to 473 K. The stability of the temperature is within 1 K over 6 h. In order to examine the ability of the system, the high-field x-ray diffraction measurements were carried out for Si and a Ni-based ferromagnetic shape-memory alloy. The results show that the x-ray powder diffraction measurements in high magnetic fields and at high temperatures are useful for materials research.     

X-ray diffraction measurements in high magnetic fields and at high temperatures

Abstract

A system was developed measuring x-ray powder diffraction in high magnetic fields up to 5 T and at temperatures from 283 to 473 K. The stability of the temperature is within 1 K over 6 h. In order to examine the ability of the system, the high-field x-ray diffraction measurements were carried out for Si and a Ni-based ferromagnetic shape-memory alloy. The results show that the x-ray powder diffraction measurements in high magnetic fields and at high temperatures are useful for materials research.     

FRP Dewar for measurements in high pulsed magnetic fields

We describe a liquid helium glass-fibre reinforced plastic (G-FRP) Dewar which we designed, fabricated and tested for excitation spectrum measurements in high pulsed magnetic fields of up to 50 T. The sensitivity of high-resolution magnetic measurements carried out at low temperatures in such high fields is limited inevitably by magnetic and electric properties of the structural Dewar materials involved. Magnetic properties of various G-FRP Dewar-purpose materials are explored with a χ-meter furnished with a RF SQUID magnetometer. The Dewar materials and multilayer insulation effects contributing to the magnetic response signal are analyzed. It has also been discovered that field noise caused by the magnetization of the Dewar materials can be suppressed substantially by using special glass-epoxy technologies. The liquid helium evaporation rate is 3.8 l/day while the hold time is 27 h, the influencing factors are discussed.     

A temperature-variable sample rotating cryostat in high magnetic fields

A temperature-variable sample rotating cryostat has been developed in order to measure the angular and temperature dependence of the upper critical field in several single crystal Chevrel phase superconductors. In the cryostat, the temperature of the sample can be varied from 2 to 15 K within an accuracy of ±10 mK. The sample can be rotated in an adiabatic vacuum can around a horizontal axis at the centre of our superconducting magnets up to 16.5 T within an accuracy of ±1°.     

A nanoscale SQUID operating at high magnetic fields

A washer-free Nb nanoSQUID has been developed for measuring magnetization changes from nanoscale objects. The SQUID loop is etched into a 250 nm wide Au/Nb bilayer track and the diameter of the SQUID hole is ～70 nm. In the presence of a magnetic field perpendicular to the plane of the SQUID, vortex penetration into the 250 nm wide track can be observed via the critical current-applied field characteristic and the value at which vortex first penetrates is consistent with the theoretical prediction. Upon removing the applied field, the penetrated vortices escape the track and the critical current at zero field is restored.     

Saturation magnetostriction measurements of magnetic thin films under high magnetic fields

Saturation magnetostriction measurements of magnetic thin films have been studied using a system equipped with a motor driven electromagnet and a laser displacement meter. A new method to reduce the errors caused by applied rotating magnetic field is proposed for high-sensitive and accurate measurements of saturation magnetostriction. It is shown that accurate measurements are possible for magnetic fields up to around 1 kOe, by extracting the 2nd harmonic output through Fourier analysis as a function of applied rotating magnetic field and then by taking the extrapolation of 2nd harmonic output to zero magnetic field. This method is applied to measure a saturation magnetostriction of single-crystal fcc-Co(111) film and the value of − 30 × 10^− 6 is obtained.     

New scientific opportunities at very high magnetic fields

The establishment of the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, FL and at the Los Alamos National Laboratory NM will provide the U.S. with a unique opportunity to explore new scientific phenomena in magnetic fields that are significantly higher than presently available. The facilities to be developed within the new NHMFL will include continuous fields to 45 tesla and pulsed fields to 200 tesla. We will review some of the promising new areas of the physics of quantum fluids and solids, superconductivity, materials science and engineering, and advanced technologies that will be able to be studied and developed at the new facility. An overview of the scope of the laboratories facilities, planned scheduling and the proposed timetable for the installation of magnet systems for this new national user facility is presented.The NHMFL is supported by the State of Florida and the National Science Foundation under cooperative agreement #DMR9016241.     

Phase transitions in two-dimensional electron-hole systems in high magnetic fields

We consider phase transitions with an excitonic condensate formation in quasi-two-dimensional semiconductors in high magnetic fields. The phase diagram is constructed and thermodynamic quantities are calculated for all densities. In the Hartree-Fock approximation all thermodynamic potentials prove to be the same as those of the electron-hole (e-h) drop state. Therefore the transition from a tenuous e-h gas to a dense e-h liquid can be treated either as a first-order transition or as two successive second-order transitions. With allowance for the correlation corrections the thermodynamic properties of the excitonic phase become slightly different from those of the drop state.     

Model of the rotor temperature of turbo-molecular pumps in magnetic fields

The KATRIN neutrino experiment operates about 20 turbo-molecular pumps (TMP) in the vicinity of super-conducting magnets, pumping out tritium gas from the electron beam-line of the experiment. In a dedicated test setup with Helmholtz coils systematic studies have been conducted, investigating the rotor temperature and stability of operation of TMPs at full speed as a function of gas load, magnetic field strength and direction of the field. The temperature of the magnetically levitated spinning rotor was measured in vacuum with an infra-red pyrometer. A simple model has been developed, which describes quantitatively the temporal progression of the rotor temperature as a function of gas flow and field strength of an external static magnetic field. The model requires 5 pump-specific parameters, characterising the heating effects of eddy currents and gas friction and cooling by radiation loss and convection. When designing a vacuum system with TMPs in a critical environment (e.g. magnetic beam-line, fusion reactor), the model can be used to predict the maximum temperature of the rotor, to ensure a safe operation of the pump.     

Conducting polymers at low temperatures and high magnetic fields

Advances in the synthesis of organic conducting polymer systems has increased the electrical conductivity of these systems by several orders of magnitude in the last decade. Several practical applications are envisioned for such systems, but a thorough understanding of the conduction mechanisms and identification of the charge carriers is lacking, making design and implementation for bulk synthesis difficult. In order to clarify our understanding of the electrical properties of these systems, the resistivity and magnetoresistivity of various polymers doped near the metal - insulator transition, such as polyaniline protonated by camphor sulfonic acid (PANi-CSA) and polypyrrole doped with PF₆ (PPy-PF₆), have been studied down to 25 mK in magnetic fields up to 16 T.     

Low temperature growth of carbon nanotubes in a magnetic field

We report on the growth of carbon nanotubes on a glass substrate at a low temperature of 450 °C by plasma-enhanced chemical vapor deposition in the presence of a magnetic field. The growth of carbon nanotubes can be realized at 450 °C only when a magnetic field is applied to the substrate. Carbon nanotubes cannot be grown in the absence of a magnetic field at the same temperature. An NH₃ plasma pretreatment significantly improved the uniformity of the grain size of the Ni catalyst under the magnetic field. The enhancement in the growth of CNTs at low temperature can be attributed to the magnetic moment pre-alignment of the ferromagnetic catalyst film under high magnetic field. A high emission current density of 20 mA/cm² was obtained at 6 V/μm and a stable emission current was observed. This method permits the growth of carbon nanotubes directly on glass substrate at much more reliable low temperatures for the fabrication of high-density field emitter arrays.     

54Mn in Fe for ultralow temperature thermometry

Experimental investigations into the accuracy of nuclear orientation thermometry are reported.⁵⁴Mn in iron, a primary thermometer useful from 2–40 mK, gives consistent thermometry using both scintillation (3×3 in.-NaI) and solid-state (40-cc-Ge) detectors.⁶⁰Co-in-iron temperatures are consistent with the⁵⁴Mn temperatures above 10 mK, but are about 10% higher at 4 mK.¹²⁵Sb in iron registers a temperature that is perhaps 2 or 3% lower over the 4–15 mK region. Some of the techniques required for the utilization of this type of thermometry are discussed.Work performed under the auspices of the U.S. Atomic Energy Commission.