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SQUID Magnetometry

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Superconducting Quantum Interference Device

SQUID magnetometry is one of the most effective and sensitive ways of measuring magnetic properties. In particular, it is the only method which allows to directly determine the overall magnetic moment of a sample in absolute units. The term SQUID is an abbreviation and stands for Superconducting QUantum Interference Device.

Following the equations established by Brian David Josephson in 1962, the electrical current density through a weak electric contact between two superconductors depends on the phase difference Δφ of the two superconducting wave functions. Moreover, the time derivative of Δφ is correlated with the voltage across this weak contact. In a superconducting ring with one (so-called rf SQUID) or two (dc SQUID, fig. 1, blue) weak contacts, Δφ is additionally influenced by the magnetic flux Φ through this ring. Therefore, such a structure can be used to convert magnetic flux into an electrical voltage. This is the basic working principle of a SQUID magnetometer.

Flux-to-Voltage Converter (jpeg, 17k)
Fig. 1: SQUID = flux-to-voltage converter

Quantum Design MPMS XL-7

We operate a commercial SQUID magnetometer system from Quantum Design, San Diego (magnetic properties measurement system MPMS XL-7, fig. 2). The sample is located in the center of a superconducting solenoid producing magnetic fields up to 7 Tesla. The sample space is filled with helium at low pressure at temperatures ranging from 2 to 400 Kelvin. The sensitivity of the system is 10-8 emu or 10-11 Joule per Tesla in RSO mode. This value is equivalent to the saturation magnetization of an extremely small amount of six billionth (6/1,000,000,000) mm3 of iron or 1012 Bohr magnetons. The whole system is fully computer-controlled and operated 24 hours a day. Measuring sequences can be programmed in advance and will be executed automatically without users' intervention - except for changing samples and refilling liquid nitrogen and helium.

Quantum Design MPMS XL-7 (jpeg, 21k)
Fig. 2: Quantum Design MPMS XL-7

Theory of Operation

The magnetic signal of the sample is obtained via a superconducting pick-up coil with 4 windings (fig. 3). This coil is, together with a SQUID antenna (red in fig. 1), part of a whole superconducting circuit transferring the magnetic flux from the sample to an rf SQUID device which is located away from the sample in the liquid helium bath. This device acts as a magnetic flux-to-voltage converter (blue in fig. 1). This voltage is then amplified and read out by the magnetometer's electronics (green in fig. 1).

When the sample is moved up and down it produces an alternating magnetic flux in the pick-up coil which leads to an alternating output voltage of the SQUID device. By locking the frequency of the readout to the frequency of the movement (RSO, reciprocating sample oscillation), the magnetometer system can achieve the extremely high sensitivity for ultra small magnetic signals as described above.

pick-up coil (jpeg, 11k)
Fig. 3: Pick-up coil