Magnetic fields SQUIDS

A SQUID [2] provides two basic advantages for measuring small variations in the magnetic field caused by cracks [3-7]. First, its unsurpassed field sensitivity is independent of frequency and thus dc and ac fields can be measured with an resolution of better than IpT/VHz. Secondly, the operation of the SQUID in a flux locked loop can provide a more than sufficient dynamic range of up to 160 dB/VHz in a shielded environment, and about 140 dB/>/Hz in unshielded environment [8]. 

The static magnetic susceptibilities of the aligned CNTs with keeping tube s cylindrical direction parallel x and perpendicular x to the magnetic field were measured by SQUID as shown in Fig. 4 [31]. The CNTs are diamagnetic with... 

The modern approach to measuring magnetic properties is to use a superconducting quantum interference device (a SQUID), which is highly sensitive to small magnetic fields and can make very precise measurements on small samples. 

Magnetic field detectors (superconducting quantum interference devices or SQUIDS). 

In the DC SQUID, two Josephson junctions are made in the loop, and a dc current is passed through the parallel circuit so formed. The voltage required to produce the current is then a function of the magnetic field trapped inside the loop. 

The SQUID obviously has several potential advantages for on-line applications. It can operate at very low inhomogeneous magnetic fields, avoiding the need for large expensive shimmed magnets. It also does not require tuning and has very low power requirements. However there are also severe technical limitations to be overcome before it can be used in on-line commercial operations. The whole SQUID needs to be immersed in liquid helium and a conventional RF coil is still needed to excite the NMR resonance. It remains to be seen whether such devices will find future on-line application in horticulture. 

Figure 6.39(a) shows the vs. T curve, normalized to the RT value, for a 100 nm thick a-/ -NPNN/glass film obtained from electron paramagnetic resonance (EPR) measurements with the static magnetic field applied perpendicular to the substrate plane. As previously shown in Fig. 3.19, the molecular a -planes are parallel to the substrate s surface. The data points closely follow the Curie-Weiss law = (T — w)/C, where C stands for the Curie constant. In this case w — —0.3 K, indicating that the net intermolecular interactions are weakly anfiferromagnetic. No hint of a transition at low temperature is observed. These results coincide with those derived from SQUID measurements on a single a-p-NPNN crystal (Tamura etal, 2003), where 0.5 < w < 0, which are displayed in Fig. 6.40. 

For the size determination of spin polarons from the field-dependent magnetization data (SQUID results in Fig. 6) we first employed a fitting based on the Brillouin function /3(./), which is justified since the respective temperatures around T]T are sufficiently above the ferromagnetic transition. The paramagnetic magnetization as a function of field is given by... 

There are several factors that have made SQUID magnetometers less attractive than other methods for NQR explosives detection. The SQUID magnetometers used for NQR detection were cooled to 4.2K for operation, as were the flux transformers. SQUIDs are generally broadband devices and must be shielded from stray magnetic fields. Optimal performance is obtained at low frequency, with sensitivity decreasing with increasing frequency. 

X-ray diffraction investigation (Co-Ka radiation) was made on the powder samples for the phase identification both the parent compound and its hydride and to determine the unit cell parameters. The magnetisation measurements were carried out with a SQUID (Quantum Design MPMS 5-S) magnetometer from 5 to 340 K in magnetic fields up to 50 kOe. 

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