SQUID MAGNETOMETER

The S700X SQUID Magnetometer is the most sensitive instrument for the measurement of magnetic properties as a function of magnetic field and temperature. Numerous different experiments may be performed with this unique instrument.

This instrument is the product of more than 20 years development and is fully engineered to be robust and reliable. It is suited for both routine measurement by non-specialists and, in the right configuration, for the most advanced research on the magnetic properties of materials.

Cooling for the system is now available in both cryogen free or liquid helium based environment. Cryogenic has been the leading supplier of high field measurement systems without the using liquid cryogens, and has now extended its cryogen free range to include the 7 Tesla SQUID Magnetometer. This gives great advantages to user in terms of reduced running costs and ease of operation for the user.

Great care has been taken to make the S700X as user friendly as possible. LabVIEW software operates in an open environment that allows the user direct control of all parts of the system with real-time graphical displays of all the relevant functions. The transparent nature of the operating system greatly improves the user’s understanding of the experimental set-up, as well as providing unparalleled control for the most demanding measurements.

The SQUID is the most sensitive detector of magnetic signals available, with an input noise power sensitivity of about 10-30 Joules per root Hz. This value of energy sensitivity is 10⁸ better than any semiconductor device, such as an FET, and accounts for the instrument’s greater sensitivity and its ability to resolve small magnetic signals quickly.

The S700X has several modes of operation. The most widely used is the measurement of total magnetic moment made by moving the sample through the pick-up coils. This method is known as the extraction method.

A second order pick-up coil in the S700X which is not sensitive to changes in the background field is used to detect the background field.

For materials that show magnetic hysteresis it is important that the sample remains in a highly uniform field during the scan. The normal scan length is set to 4 mm, over which the field is uniform to 10-4 but measurements can be made with movements from 2mm up to 130mm.

S700X SQUID MAGNETOMETER

The S700X SQUID magnetometer utilizes a variable temperature sample space insert which is mounted inside the superconducting magnet with the SQUID and magnetic detection coils.

At the top of the VTI there is the sample movement system, an airlock to facilitate changing the sample and all the electrical feed-throughs for the magnetometer. The sample is mounted on a long rod with low magnetic moment which passes through a helium tight sliding seal into the sample space. Vertical translation and rotation of the sample are performed by stepper motors.

The sample space is sealed at the top with a gate valve and airlock so that samples can be changed while the system is cold without contamination of the cold space. The air-lock is made of clear transparent plastic, so that the condition and position of the sample can be checked during the loading procedure just prior to lowering the sample to the measurement position.

The major components are machined from stainless steel, which gives the S700X SQUID magnetometer its excellent immunity to vibration and RF interference. The cryostat has a liquid nitrogen cooled radiation shield to provide a very low liquid helium consumption.

S700X SYSTEM WITH LOW LOSS LIQUID HELIUM CRYOSTAT

The S700X liquid helium based system includes all of the standard capabilities of the S700X series
SQUID magnetometer and includes a low loss liquid helium cryostat for operation.

The S700X liquid helium based system is a fully shielded system which is supplied with a Nb shield around the magnet with full shielding from EMF noise to a factor of 10⁸.

R-S700X SYSTEM WITH RECONDENSING CRYOSTAT

Cryogenic offers a zero boil-off cryostat for both the S700X series of SQUID Magnetometers. The cryostat is supplied with a dry pump and all the necessary pumping kit for the variable temperature sample space that collects the gas from the VTI and returns it to the cryostat where it is cooled and re-liquified.

The cryostat is similar to the conventional liquid helium cryostat fitted to the standard model in that it has a reservoir of 45 litres of liquid helium. It also has the full magnetic shield built into the cryostat to protect the measurement system from external influence and provide a background field of typically 100 micro-Tesla at the sample measurement point.

The cryostat is cooled by a newly developed ultra quiet pulse tube cooler which provides 1 Watt cooling power at 4K and has a special built in facility for liquifying helium gas from room temperature. The pulse tube cooler provides refrigerator at 4K for the liquid helium bath and at 50K for a radiation shield to intercept external radiation. There is optimized radiation shielding to reduce external heat loads to the minimum.

Tests have shown that there is no effect on the measurement performance of the SQUID Magnetometer from the operation of the pulse tube cooler.

In normal operation the cryostat is filled initially once only with liquid helium. A 100 litre storage vessel should be sufficient for this operation. Thereafter the liquid helium reservoir is filled and maintained by the Pulse Tube cryocooler, which also re-liquifies the gas used for the VTI. If the cryocooler is turned off or there is a power-cut then the liquid helium level in the reservoir will fall. Liquid will be lost at the rate of 7 litres per day. Once the cryocooler is turned on it can be used to re-liquify helium gas from a cylinder at the rate of about 2 litres per day. Short term fluctuation in heat load from ramping the magnet etc, will not result in liquid helium losses since there is a buffer tank full of reserve of helium gas charge supplied with the system.

The zero boil-off Magnetometer system reduces operating costs very substantially and makes the operation and use of a SQUID Magnetometer practical in all locations, especially those areas where helium is expensive and regular supplies are difficulty to acquire. The recommended maintenance for the pulse tube cryocooler is every 20,000 hours of use which is only every 3 to 4 years of continuous operation. This helps to keep all operating costs to minimal.

CF-S700X CRYOGEN FREE SQUID MAGNETOMETER

The CF-700X SQUID Magnetometer system consists of the cryogen free, 7 Tesla superconducting magnet, an integrated variable temperature sample space (CFVTI), cryocooler and compressor, multi-channel temperature monitor, sample temperature controller and power supply. The cooling for the magnet and sample is provided by the cryocooler with a base temperature of less than 4K, providing up to 1 Watt of cooling power on the second stage. The current to the magnet is supplied by a Cryogenic’s specially designed power supply using current leads of HTS conductor. A Lakeshore model 340-temperature controller (or equivalent) controls the temperature of the sample.

The cryogen free will be delivered as a turnkey working system tested and ready to operate on delivery. No additional special equipment will be needed nor will previous cryogenic experience be necessary to operate the system.

The magnet is produced using our proven technology for high field superconducting magnets. These magnets are manufactured using copper stabilized filamentary superconductor which is then vacuum impregnated with epoxy resin to form a composite structure of excellent mechanical strength and electrical insulation. The filament size and conductor type is chosen to eliminate flux jumps.

The magnet operates in vacuum and is cooled by conduction. It is passively protected against damage caused by accidental quench whether this is caused by mains power failure or by exceeding the operating temperature of the magnet or by exceeding critical current of the superconductor.