19 Squid

Introduction

Magnetic fields and their properties can be measured by so many ways among which induction coils, magneto resistive magnetometers, Hall effect magnetometers, magneto-optical magnetometers, and optically pumped magnetometers are frequently used sensing methods. The sensitivities range of these sensing methods lies from pico-Tesla to micro-Tesla levels. Out of all these devices superconducting quantum interference device (SQUID) is considered as a most sensitive magnetic flux detector. The field resolution of SQUIDs devices is 10−17 T, the device can be operated at low temperatures such as cryogenic temperatures with quantum-limited sensitivity. In the development and commercialization of ultrasensitive electric and magnetic measurement systems, superconducting quantum interference devices SQUIDs have been considered as a major factor. In many of the cases no methodology works top measure magnetic properties other than SQUID. In the present module we examine the main features of developing, constructing, and operating SQUID measurement systems. Materials used to fabricate SQUID sensors with the various types of SQUID sensors description as well as the operating principles and the properties are listed. Most of the applications of the SQUID sensors are based on lower frequencies, however they can be operated well above 1 MHz frequency. Different detection coil configuration for the manufacturing of SQUID sensors and various applications in the area of environment, medicines etc. are described.

SQUID device is the most fundamentally used application of flux quantization and Josephson phenomena. Therefore before going into the depth analysis of SQUIDs devices, let us first understand some basic terms.

1.  Superconductivity

H. Kamerlingh Onnes in 1908, measures the electrical resistance of metals at very low temperatures by successfully liquefy helium. In 1911, scientists discovered that as the temperature of Hg was reduced at around 4.2 K, the resistance of Hg did not fall continuously as expected, but in its place dropped suddenly to zero over a range of a few hundredths of a degree. This phenomenon of non-zero resistance of a metal was termed as superconductivity. Later on the same phenomena was also discovered for other metal elements such as Pb, Sn and Al at critical temperatures lying between 4 to10K. However, the microscopic mechanism of such an unexpected behaviour of the metals found at some critical temperatures known as superconductivity was not discovered until 1957. In 1957, three of the scientists Bardeen, Cooper and Schrieffer try to investigate this microscopic behaviour. In 1933, Meissner & Ochsenfeld was also discovered one another important property of superconductors called Meissner effect. Superconductors exhibits the property of perfect conductivity, such that the magnetic flux should be excluded from entering a superconductor, whereas it is also found that when the superconducting material was cooled below its temperature of transition, the magnetic flux was expelled from the interior of the material. This phenomenon is known as ‘Meissner effect’.

2.  Meissner effect

In the presence of the magnetic field, when a superconductor is cooled down through its transition temperature a distinct property is observed. When a non-superconducting material is placed in the magnetic field, the magnetic lines of flux penetrate within the material as shown in Fig. 1(a). In a perfectly conducting material, to prevent any change in the magnetic field in the interior of the conductor an induced current is produced within the conductor. On the other hand, for superconductor the magnetic flux lines exist only for a shallow surface layer, called the penetration depth and are not present within the superconducting region. The magnetic flux is excluded from the conductor when it becomes superconducting whereas the flux traps in the interior of a perfect conductor as shown in fig. 1(b). This essential quality of flux expulsion of the superconductors is called Meissner effect. In case of a continuous solid the flux is expelled out of a superconductor whereas it is trapped in the interior of the material if it forms a ring (Fig. 1(b)). A current is induced around the ring to keep the magnetic flux inside the ring constant when the magnetic field is then turned off (Fig. 1(c)).

3. Flux quantization

The magnetic flux is trapped within a superconducting ring until it is in the superconducting state. Some extraordinary properties are shown by this trapped flux. Firstly, the intensity of the magnetic flux cannot be change in the superconducting ring and only discrete levels of the magnetic flux can be trap as shown in Fig. 2. Thus we can say that the magnetic flux exists in the form of the flux quantum and said to be quantized.

4.   The Josephson effect

The Cooper pair wave function of a superconducting wire interrupted by a normal region would quickly decay across the resistive barrier and the resultant superconducting current would die down as shown in fig. 3. Electrons can tunnel through a junction of two superconducting regions separated by a resistive barrier, this possibility of tunnelling of electrons is given by Josephson in 1962.

Josephson junction describes the contact between two superconductors, in which there is found to be a thin (< 2 nm) dielectric tunnel barrier between the two superconductors. The effect explains the tunneling of Cooper pairs through a barrier. The dc Josephson effects, relates the tunnel current to critical current through a Josephson junction as,

The maximum Josephson tunnel current flowing through the barrier is called the critical current. The value of the tunnel current is measured by the density of Cooper pairs, tunnel barrier thickness, area of the tunnel junction and by the phase difference ∅.

The ac Josephson effect relates the voltage across a Junction to the temporal change of the phase difference thus a voltage across a Josephson junction leads to a current.

Superconducting quantum interference devices: working principle

SQUID is a superconducting ring of intermittent by one or more Josephson junctions and used to measure very small variations in magnetic flux by utilising Josephson effect phenomena (Fig. 4).

The operational point is marked on the I-V curve midway between superconducting and resistive behaviours and the corresponding bias current is applied (Fig. 5). The hysteretic behaviour of the I-V curve is prevented by putting the Shunt resistors. The inductively coupled magnetic flux creates screening currents in the SQUID loop, which depending on the direction of the induced flux either increase or decrease the output current. The bias current is fixed at a little higher value than the output current. Also, the voltage drop across the Josephson junction will change when an external magnetic flux is coupled with it. This change in the voltage caused due to the increases or decreases in the external flux is periodic in nature with the flux quantum as a period (Fig. 5). The magnetic flux coupled with the SQUID loop can be determined by recording the change in the voltage. The SQUID can be locked at a unique point on the voltage-flux curve by using the external feedback electronics and thus the feedback current is also measured with the help of the externally applied flux. The steepest part of the voltage-flux curve having maximum value of V/ϕ is the appropriate portions of the SQUIDs operation.

SQUID SENSORS

1.       MATERIALS

a)       Low temperature superconducting materials (LTS):

LTS materials are metallic in nature, they shows isotropic behaviour, chemically stable in air and found to have large coherence lengths which is found to be tens to hundreds of interatomic distances. Although, at liquid helium temperatures some nonmetallic and organic compounds have been found to be superconducting but none of them is used to fabricate SQUID devices. On the basis of the unique properties shown by these materials, three-dimensional structured devices can be fabricated by using LTS materials. Thus crossovers, multilayer structures and multiturn devices can be made over single turn devices which offer higher sensitivity.

b)       High temperature superconducting materials (HTS):

HTS materials are ceramics in nature, having brittleness, shows an-isotropic behaviour with planar geometry. The coherence lengths of the HTS materials lies in the c direction that is considerably small in dimension. Larger dimensions are required by the HTS crossovers (needed for multiturn coils) and thus a significant 1/ f noise is introduced due to the formation of a Josephson or insulating junction. HTS structures are degraded very fast in the presence of the moisture, which adds another disadvantage to it. Thus, to protect them passivation layers or overcoatings are necessary that adds manufacture complexity.

HTS devices have the ability to be operated at liquid nitrogen rather than liquid helium temperatures which gives a significant operational advantage of HTS materials over LTS materials.

2. Flux transformers

The area of the SQUID detection loop is very small and also inductance (10-10 H) is associated with it, this is the major problem faced in using the SQUID detection loop. On the other hand, the sensitivity would be increased by increasing the area of the loop or connecting a larger loop in series. However, increased inductance of the larger loop produces the impedance mismatch which cancels out the sensitivity gain majorly. Multilayer flux transformer is used by most of the LTS SQUID sensors to couple an externally detected flux into the SQUID loop (Fig. 6).

Fig. 6.  External flux coupler flux transformer (two turn “detection coils”).

3.  Fractional turn SQUIDs

By connecting a number of single detection coils in parallel not in series as in case traditional multiturn coils is found to be another way to increase the sensitivity of a bare SQUID loop (Fig. 7). The fundamental concept behind this thought is to have a large area for coupling to an external coil by keeping the inductance of the SQUID loop itself very small.

SQUID SENSORS: OPERATION AND PERFORMANCE

On the basis of the operation, SQUID sensors are divided into two broad categories:

1.   rf SQUID

Single Josephson junction is used in the fabrication of rf SQUID where input coil is inductively coupled with the SQUID loop and flux is thus induced. The coupled input coil is thus joined the SQUID loop to the electronic circuitry and to the rf coil which becomes the part of a high-Q resonant (tank) circuit, measures the corresponding SQUID loop current changes. A constant current radio-frequency oscillator is used to driven the this tuned circuit which also couples SQUID loop weakly to it. The output of the rf amplifier increases with the increase in the amplitude of the oscillator until a critical level is reached.

2. dc SQUID

In case of a dc SQUID, SQUID loop is inductively coupled with the input circuit, feedback electronics and modulation coils and they are not wound around the as in case of rf SQUID. Figure 9 shows the schematic of a dc SQUID. The SQUID loop is biased with a dc current which is about twice to that of the critical current and thus a dc voltage is developed across the junctions. A wave function phase change is induced due to the change in the magnetic flux linked with the SQUID loop which increases the current through one Josephson junction and decreases the current in the other one.

With the increase or decrease in the external flux, voltage will change in a periodic manner having flux quantum as period.

rf SQUID Vs. dc SQUID

• dc SQUID offers much lower noise as compared to the rf SQUID.
• dc SQUID is more sensitive than rf SQUID, however this increase the electronics complexity requires to operate dc SQUID.
• Two nearly identical Josephson junctions in a single device is the major challenge in dc SQUID fabrication whereas rf SQUID require only one junction.
• The early LTS development was performed with the rf SQUIDs, however the first type of SQUID magnetometer was made using LTS dc SQUID.

Applications of SQUID

SQUID devices are majorly used to identify very small amount of magnetic field, current, voltage, inductance and magnetic susceptibility etc. These devices show excellent sensitivity in the magnetic flux detection. The devices have been used to detect small. These devices are used in the form of magnetometers and gradiometers. Various areas of Squid devices are as follows:

1.       Magnetoencephalography (MEG)

Weak magnetic fields are produced in the brain by the presence of the electrical currents occurring naturally in the brain. The SQUID magnetometers are used to measure this magnetic field and thus perform brain activity mapping, this technique is known as MEG.

2.   Magnetogastrography

In this technique SQUIDs devices are used to record the magnetic fields produced due to electrical activity from the stomach.

3.  Magnetic Marker Monitoring (MMM)

SQUID sensors are used to detect the dosage form of an orally applied drug through the intestinal tract, containing a little amount of magnetite (Fe3O4), and is magnetized by a high-energy magnetic field.

4.  Magnetic property measurement systems (MPMS)

Measurement of magnetic properties of the systems is the fundamental commercial use of SQUID sensors.

5.  Superparamagnetic Relaxometry (SPMR), NMR and EPR

In these techniques, a sample is placed in the centre of the SQUID detection coils and by applying the external field or rf excitation to the sample corresponding signals are measured.

6. Scanning SQUID Microscope

By moving a SQUID sensor across examine area, weak magnetic fields are measured.

7. Magnetic Anomaly Detector (MAD)

In this technique SQUID magnetometers are used to detect infinitesimal deviations in the Earth’s magnetic field. The technique is most commonly used by military forces to detect submarines.

Limitations on SQUID Technology

1. Since the voltage output of a SQUID loop is a periodic function of the flux. Thus the SQUID sensors are susceptible only to the relative changes of magnetic field and current.
2. The operating point of the SQUID sensors can shift by one or more flux quanta. If the corresponding electronics of the feedback does not follow appropriately to the signal changes (i.e., the slew rate is exceeded). This happens when the total signal change exceeds ½ Φo.
3. The maximum bandwidth of the SQUIDs is approximately half the bias frequency due to the use of ac biasing which generates 1/ƒ noise in the sensors.

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