Magnetometer Superconducting Quantum Interference Device

Magnetic properties of a magnetic material could be measured by various types of facilities, among which a vibrating sample magnetometer (VSM) or a superconducting quantum interference device (SQUID) magnetometer is one of the most commonly used tools for purposes of the magnetic measurements. A VSM is a magnetometer based on the mechanism of the electromagnetic induction. A sample is vibrated in the vicinity of a set of pick-up coils. The flux change caused by the moving magnetic sample causes an induction voltage across the terminals of the pick-up coils that is proportional to the magnetization of the sample. The VSMs have a comparatively high sensitivity for measuring the magnetic moment, which can be up to 5x10-5-2x10-9 Am2.

SQUID is a highly sensitive detector of the flux designed based on the concept of the flux quantization and the superconducting Josephson effect. The so-called Josephson junction is a junction at which a thin insulator is connected with two superconductors. When the magnetic field is applied to a superconducting ring with a Josephson junction, a stable current may pass the thin insulator between the two superconductors due to the tunneling. The sum of the flux due to the Josephson current density js and that of the external field, namely, the total flux of the superconducting ring, is the integer times the flux quanta = K/2\e\. The Josephson current density js is the periodicity function of the external magnetic flux $ as described as js = -jc sin(2w$/$0), where jc is the critical current density. SQUID can measure not only the extremely weak magnetic fields with a resolution of 10-19 Wb, but also susceptibilities, magnetic moments, magnetization curves, hystersis loops, and their temperature dependences. SQUID can be used to measure very weak voltages and currents, to establish the voltage benchmark, and to measure directly the elementally physical constant \e\/h, where e is electronic charge and h is the Planck constant. SQUID is a powerful tool for validating in macroscopic range the basic principles of quantum mechanics. Figure 19 shows zero-field-cooled (ZFC) and field-cooled (FC) (in a field of 100 mT) magnetization curves of the Fe(B) nanocapsules [122].

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