SQUID magnetometry is well-known as one of the most sensitive methods of magnetometry. This technique uses a combination of superconducting materials and Josephson junctions to measure magnetic fields with resolutions up to 10^-14 T or better.

The DC SQUID   A DC SQUID is actually a rather simple device. It consists of two Josephson junctions connected in parallel on a closed superconducting loop as shown in Fig. 1(a) [1]. A fundamental property of superconducting rings is that they can enclose magnetic flux only in multiples of a universal constant called the flux quantum, h/2e=2.07×10^-15 Wb. Because the flux quantum is very small, this physical effect can be exploited to produce an extraordinarily sensitive magnetic detector known as the SQUID. SQUIDs actually function as magnetic flux-to-voltage transducers where the sensitivity is set by the magnetic flux quantum. Applying current to the SQUID (biasing it) sends Cooper pairs of electrons tunnelling through the junctions. A magnetic field applied to the ring, however, alters the flow. Specifically, it changes the quantum-mechanical phase difference across each of the two junctions. These phase changes, in turn, affect the critical current of the SQUID. A progressive increase or decrease in the magnetic field causes the critical current to oscillate between a maximum value and a minimum one. The maximum occurs when the flux administered to the SQUID equals an integral number of flux quanta through the ring; the minimum value corresponds to a half-integral number of quanta. The flux applied to the SQUID can assume any value, unlike the flux contained within a closed superconducting ring, which must be an integral number. In practice, we do not measure the current but rather the voltage across the SQUID, which also swings back and forth under a steadily changing magnetic field as shown in Fig. 1(b). This quantum interference effect provides us with a digital magnetometer. Each "digit" represents one flux quantum. In fact, conventional electronics can detect voltages corresponding to changes in magnetic flux of much less than one flux quantum. Thus the SQUID in essence is a flux-to-voltage transducer, converting a tiny change of magnetic flux into a voltage.

Figure 1: The DC SQUID construction and principle: (a) Shows the two Josephson junctions forming a superconducting ring, which forms the DC SQUID.(b) Shows the output voltage as a function of applied flux. A tiny flux signal produces a corresponding voltage swing across the SQUID, which conventional electronics can measure. Figure inspired by Ref. [1].

Practical SQUID Magnetometer   Although in some applications it is convenient to expose the SQUID directly to the magnetic field of interest, more often the magnetic signal is conveyed to the SQUID by a flux transformer. A flux transformer is a closed superconducting circuit consisting of two coils in series. One coil, the input coil, is magnetically coupled to the SQUID and is usually fabricated along with it; the second, or pick-up coil, is exposed to the field to be measured. This second coil acts as a magnetic antenna that couples external signals into the SQUID. It is a basic principle of superconductivity that the flux inside a closed superconducting circuit cannot change. Consequently, a change in field that causes the flux in the pick-up coil to change also causes a change in the flux in the input coil. The SQUID senses this latter flux change. The area of the pick-up coil is usually much greater than the area of the SQUID. The prime function of the transformer is to convert the high magnetic flux sensitivity of the SQUID itself into a high magnetic field sensitivity. Another advantage of using a flux transformer is that the input coil, which can be made as a wire or a thin film structure, can be configured to suit the measurement at hand. In particular, it can be wound so as to be sensitive not to the magnetic field itself, but to the gradient of the field in a chosen direction, or to a higher derivative of the field. In these cases, the flux transformer is referred to as a gradiometer. Since the gradient of the magnetic field falls off more rapidly with distance from the magnetic source than the field itself, a gradiometer tends to reject magnetic interference from distant sources, while remaining sensitive to closer objects. Again, a gradiometer is essentially sensitive to changes in the field gradient rather than its absolute value, and the technique of controlled resetting can be applied to yield a large dynamic range. The system used in this work implements a second-derivative gradiometer that minimizes background drifts in the SQUID detection system caused by relaxation in the magnetic field of the superconducting magnet. The second-derivative gradiometer is also more noise immune than a first-order gradiometer.

The MPMS SQUID system [2] used in this work is composed of several units: the dewar, the probe and SQUID assembly, and the electronic control system. The probe contains a high-precision temperature control system that allows measurements between 1.9 K to 400 K with an accuracy of 0.01 K (valid at low temperature). A superconducting electromagnet can deliver a field of up to 5×10⁴ G with a field accuracy of 0.1 G (for small magnetic fields). The dewar consists of an inner liquid helium reservoir, and an outer liquid nitrogen jacket to reduce excessive liquid helium boil-off. Liquid helium is used both to maintain the electromagnet in a superconducting state and to cool the sample space. The samples are mounted within a plastic straw and connected to one end of a sample rod which is inserted into the dewar. The other end is attached to a stepper motor which is used to position the sample within the center of the SQUID pickup coil. The generated magnetic field is well-shielded from the surroundings.

Bibliography

[1]

J. Clarke, Scientific American 271, 46 (1994).

[2]

MPMS from Quantum Design, see http://www.qdusa.com/products/mpms.html.

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On 20 Oct 2005, 16:32.

Last Updated Wednesday October 26, 2005