SQUIDs, or super conducting Quantum interference devices are used to measure minute changes in magnetic flux by means of a pair of Josephson junctions, often used to detect extremely small changes in magnetic fields, electric current and voltages. They are currently the most sensitive devices (e.g. magnetometer) known, with noise levels as low as.

CONSTRUCTION: Most SQUIDs are fabricated from lead or pure niobium. The lead is usually in the form of an alloy with 10% gold or indium, as pure lead is unstable when its temperature is repeatedly changed. The base electrode of the SQUIDs is made of a very thin niobium layer, formed by deposition, and the tunnel barrier is oxidized, onto this niobium surface. The top electrode is a layer of lead alloy deposited on the top of the other two, forming a sandwich arrangement. To achieve the necessary superconducting charactistics the entire device is then cooled to within a few degrees of absolute zero with liquid helium. More recently developed “high temperature” SQUIDs are made of a substance called Yttrium Barium copper Oxide or YBCO and are cooled by liquid nitrogen which is cheaper and more easily handled then liquid helium. They are less sensitive than conventional “low temperature” SQUIDs but the superconductivity property of YBCO IS very sensitive to its oxygen present.

TYPES

There are two main types of SQUIDs:-

DC SQUIDs

RF (AC) SQUIDs

DC SQUIDs: The dc SQUIDs consists of two Josephson tunnel junctions connected in parallel on a superconducting loop. A small applied current flows through a junction as a super current, without developing a voltage, by means of copper Pairs of electrons tunnelling through the barriers. However, when the applied current surpasses a certain critical value, a voltage is produced. When a magnetic field is applied in order that a magnetic flux threads the loop, the critical value oscillates as the magnetic flux varies, with a period of one flux quantum, meter. The oscillation arises from the interface between two coherent electromagnetic waves.      

RF (AC) SQUID: The RF SQUIDs consists of a single junction interrupting a superconducting loop. In operation, it is coupled to the inductor of an AC tank circuit exited at its resonant frequency by a radio frequency current. The RF voltage across the tank circuit oscillates as a function of the magnetic flux in the loop again for a period of one flux quantum.

Applications: The extreme sensitivity of SQUIDs makes them ideal for studies in biology. Magneto encephalography (MEG), FOR EXAMPLE uses measurements from an array Of SQUIDs to make interfaces about neural activity inside brains. Because SQUIDs can operate acquisition rates much higher than the highest temporal frequency of interest in the signals emitted by the brain (KHz), MEG achieves good temporal resolution (accuracy of a particular measurement with respect to time). The most common use of SQUIDs is in magnetic property measurement system. These are turn-key systems, made by several manufactures that measure the magnetic properties of a material sample. SQUIDs are also used in the scanning SQUIDs microscope, which uses a SQUIDs immersed in liquid helium as a probe. The use of SQUIDs in oil prospecting, earthquake prediction and geothermal energy surveying is becoming more widespread as super conductor technology developed. They are also used as precision moment sensors in a verity of scenic applications, such as the detection of gravity waves. Both DC and RF SQUIDs are utilized as magnetometers to identify small changes in magnetic field. They are also used for logic and switching elements in experimental digital circuits and high-speed analogue to digital converters.