An automatic frequency-sweeping SQUID susceptometer

Abstract. A new. sQc1D-based, measuring system has been developed for the investigation of the dynamic properties of magnetic materials. Its main advantages. compared to conventional mutual inductance systems, are its high sensitivity and its nearly frequency independent response. down to extremely low frequencies. With the SQUID system it is now possible to measure directly and rapidly the frequency dependence of the dynamic susceptibility of weakly magnetic materials in the range from 0.002 Hz to 5 kHz at a fixed value of a constant background field of up to 5 T. The frequency-swept operation allows experiments which are beyond the capabilities of conventional systems. for example the investigation of thermally isolated samples or the study of the dynamics at magnetic phase transitions in cases where the magnetic properties change very rapidly in a small field interval. 1

Elimination of flux-transformer crosstalk in multichannel SQUID magnetometers

Multichannel SQUID magnetometers are being developed for signal-field mapping in biomagnetic experiments. A problem that becomes more serious as the number of channels is increased is the crosstalk caused by the mutual inductances between the individual sensing coils. A simple and effective method for eliminating this crosstalk is presented in this Paper. The method is based on a rearrangement of the feedback loops which causes the flux-transformer circuits to become currentless. The feasibility of the method is verified experimentally

Partial lattice participation in the spin-lattice relaxation of potassium chromium alum

We developed a SQUID-based frequency sweeping system for a.c. susceptibility measurements. Using this instrument we found that in Potassium Chromium Alum only a part of the lattice system is involved in the spin-lattice relaxation process. This partial lattice participation amounts 60–75% of the total lattice specific heat

Closed-cycle gas flow system for cooling of high Tc d.c. SQUID magnetometers

A high Tc.d.c SQUID based magnetometer for magnetocardiography is currently under development at the University of Twente. Since such a magnetometer should be simple to use, the cooling of the system can be realized most practically by means of a cryocooler. A closed-cycle gas flow cooling system incorporating such a cooler has been designed, constructed and tested. The aimed resolution of the magnetometer is 0.1 pT Hz−1/2. The required operating temperature for the SQUIDs is 30 to about 77 K with a stability of 2 × 10−4 K Hz−1/2. After a cool-down time of 1–2 h, a stationary cooling power of at least 0.2 W is required. In the design, helium gas is cooled by a Leybold Heraeus RG 210 cryocooler, transported through a gas line, and subsequently passed through a heat exchanger on which SQUIDs can be installed. The lowest obtainable SQUID heat exchanger temperature is 31 ± 2 K. This can be reached in roughly 2–3 h with an optimal mass flow with respect to the cooling power of 6 × 10−6 kg s−1. At this mass flow the cooling power at the SQUID heat exchanger is 0.2 W at 42 K and roughly 1.2 W at 77 K. A temperature stability of 0.05 K was measured at a SQUID heat exchanger temperature of 54 K and a mass flow of 3 × 10kg s−5. The experience gained with this large cooling system will be used in the design of a smaller configuration cooling system, incorporating miniature Stirling cryocoolers. In this paper the design and the construction of the present closed-cycle system are described and test results are presented.\ud \u

Cooling a low noise amplifier with a micromachined cryogenic cooler

The sensitivity of antenna systems increases with increasing active area, but decreases at higher noise figure of the low-noise amplifier (LNA). Cooling the LNA locally results in significant improvement in the gain and in lowering the noise figure of the LNA. Micromachined Joule-Thomson (JT) coolers can provide a cryogenic environment to the LNA. They are attractive because they have no cold moving parts and can be scaled down to match the size and the power consumption of LNAs. The performance of a LNA mounted on a JT microcooler with dimensions of 60.0 × 9.5 × 0.72 mm3 is reported in this paper. The microcooler is operated with nitrogen gas and the cold-end temperature is controlled at 115 K. The measured net cooling power of the microcooler is about 43 mW when the LNA is not operating. The power dissipation of the LNA is 26 mW, with a supply voltage of 2 V. At room temperature the noise figure of the LNA is 0.83 dB and the gain lies between 17.9 and 13.1 dB, in the frequency range of 0.65 and 1.05 GHz. Upon cooling to 115 K, the noise figure drops to 0.50 dB and the increase in gain varies in the range of 0.6–1.5 d

A 19-channel d.c. SQUID magnetometer system for brain research

A 19-channel d.c. SQUID magnetometer system for neuromagnetic investigations is under constuction. The first-order gradiometers for sensing the signal are placed in a hexagonal configuration. D.c. SQUIDs based on niobium/aluminium technology have been developed, leading to a field sensitivity of about 5 fT/ Hz. SQUID read-out is realized with a resonant transformer circuit at 100 kHz. The multichannel control and detection electronics are compactly built

On the fetal magnetocardiogram

Fetal magnetocardiography is a non-invasive technique for studying the electrical activity of the fetal heart. Fetal magnetocardiograms (fMCG) can be used to diagnose and classify fetal cardiac arrhythmias reliably. An averaged fMCG shows a QRS-complex, a P-wave, and a T-wave. However, it is still unknown if the currents in the tissues surrounding the fetal heart disturb these features. Furthermore, the measuring technique is not yet optimised for fMCG registrations. Simulation studies may provide guidelines for the design of an appropriate magnetometer system. Therefore, finite-element and boundary-element models were constructed in order to study the possible influence of the volume conductor. Especially, the influence of the layer of vernix caseosa, a fatty layer that covers the fetus, was investigated. The computations showed that the layer of vernix caseosa will affect the waveform of the fMCG. The signal processing procedure used is also discussed. It turned out to be difficult to deduce the onset and offset of the T-wave from the resulting averaged signals. Possibly, the QRS-complex does not provide a correct trigger to obtain a distinguishable T-wave in the averaged signal, because the RT-interval may be variable

Progress in Micro Joule-Thomson Cooling at Twente University

At the University of Twente, research on the development of a sorption-based micro cooler is in progress. Because of the absence of moving parts, such a cooler is virtually vibration free and highly durable, which potentially results in a long lifetime. A miniature cryogenic cooler with these properties would be appealing in a wide variety of applications including the cooling of vibration-sensitive detectors in space missions, low-noise amplifiers and semi- and superconducting circuitry. The objective of the present project is to scale down a Joule-Thomson (JT) cold stage to a total volume of a few hundredths of a cm3. This size reduction introduces many problems. The proposed cold stage volume results in a restriction cross-sectional area of about a thousandth of a mm2 which may cause clogging problems. Flow channels with a cross-sectional area of a few hundredths of a mm2 will produce high pressure drops influencing the JT cycle. Furthermore, the micro channels must be capable of withstanding high pressures and maintaining a large temperature gradient over a relatively short length. The project aim is to develop a reliable micro JT cold stage that is fabricated out of one material with a relatively simple and reproducible fabrication method. The length of the cold stage is calculated at about 20 mm with a width of 1.7 mm and height of about 0.3 mm. The mass flow is in the order of one mg per second to create a net cooling power of 10 mW at 96 K. The final objective of the project is to integrate the cold stage, vacuum chamber and device into one compact design. This paper discusses possible solutions to the problems mentioned and presents a concept design of such a miniature JT cold stage