A SQUID (Superconducting Quantum Interference Device) is the most sensitive available device for measuring magnetic fields. Based on this sensitive device the so called ‘SQUID magnetometers’ have been developed. SQUID magnetometers are used to characterize materials when the highest detection sensitivity over a broad temperature range and using applied magnetic fields up to several Tesla is needed. Nowadays, this instrument is widely used worldwide in research laboratories. The system is designed to measure the magnetic moment of a sample, from which the magnetization and magnetic susceptibility can be obtained. Therefore, SQUID magnetometers are versatile instruments that perform both, DC and AC magnetic moment measurement.

SQUID magnetometers are classified within the flux methods of measuring magnetization of a sample. Fig. 1 illustrates schematically its principle: the measurement of the flux change through a pick-up coil system with a SQUID. This signal is proportional magnetic moment of a sample which is magnetized by the magnetic field produced by a superconducting magnet. The main components of a SQUID magnetometer are: (a) superconducting magnet (that must be acquired together its programmable bipolar power supply); (b) superconducting detection coil which is coupled inductively to the sample; (c) a SQUID connected to the detection coil. In the following a description of each one is given; (d) superconducting magnetic shield.

Fig. 1. SQUID magnetometer principle.

The main components of a SQUID magnetometer are: (a) superconducting magnet (that must be acquired together its programmable bipolar power supply); (b) superconducting detection coil which is coupled inductively to the sample; (c) a SQUID connected to the detection coil. In the following a description of each one is given; (d) superconducting magnetic shield.

Superconducting magnet. A superconducting magnets is a solenoid made of superconducting wire. Fig. 2 shows a cut view of a typical superconducting magnet used in a SQUID magnetometer. This solenoid must be kept at liquid helium temperature in a liquid-helium dewar The uniform magnetic field is produced along the axial cylindrical bore of the coil. Currently, superconducting solenoids that produce magnetic fields in the range 5-18 Tesla are commercially available. To operate a superconducting magnet requires an appropriate programmable bipolar power supply.

Superconducting detection coil. This is a single piece of superconducting wire configured as a second-order gradiometer (see Fig. 3). In this geometry
This pick-up coil system is placed in the uniform magnetic field region of the solenoidal superconducting magnet.

Fig. 2. Cut view of a typical superconducting magnet.		Fig. 3. Second-order gradiometer superconducting pick-up coils.

SQUID. High sensitivity is possible because this device responds to a fraction of the flux quantum. The SQUID device is usually a thin film that functions as a extremely sensitive current-to-voltage-converter. A meadurement is done in this equipment by moving the sample through the second-order gradiometer. Hence, the magnetic moment of the sample induces an electric current in the pick-up coil system. A change in the of magnetic flux in these coils changes the persistent current in the detection circuit. So, the change in the current in the detection coils produce variation in the SQUID output voltage proportional the magnetic moment of sample.

Superconducting magnetic shield. Is used to shield the SQUID sensor from the fluctuations of the ambient magnetic field of the place where the magnetometer is located and from the large magnetic field produced by the superconducting magnet.Fig. 4 shows a commercial SQUID magnetometer. Applications. Using this kind of equipment one can measure: (a) The real and imaginary components of the AC magnetic susceptibility as a function of frequency, temperature, AC magnetic field amplitude and DC magnetic field value. (b) The DC magnetic moment as a function of temperature, DC magnetic field, and time. Using a specially designed sample holder the magnetic moment as a function of angle can be also measured. High sensitivity is needed when samples with low intrinsic magnetic moment or low mass are measured. In thin films, for instance, the mass may be smaller than 1 microgram. These materials are therefore difficult to characterize using a extraction or vibrating sample magnetometer but not with a SQUID magnetometer. Also for measurements of magnetic viscosity in permanent magnets, where small changes of magnetization as a function of time must be recorded, a SQUID magnetometers is the best choice.
	Fig. 4.	Commercial SQUID magnetometer: the most sensitive available instrument for measuring in a research laboratory the magnetic moment of a sample.