## 3 Magnetic Characterization Instruments
### Superconducting Quantum Interference Device (SQUID)  
A superconducting quantum interference device (SQUID) is a magnetic flux to voltage or current transducer and consists of a superconducting loop interrupted by two Josephson junctions [100; 101]. The existence of the magnetic flux transducer increases the sensitivity of this type of magnetometer, in comparison to the conventional magnetometers that contain conventional amplifiers. This well-known magnetometer was developed around 1962 with the contribution of [PERSON] [62]. The sensitivity of this device is high [42; 47], and according to [62], it is the most sensitive field sensor and can measure magnetic flux per unit bandwidth less than \(10^{-6}\)\(\Phi_{0}\) (where \(\Phi_{0}\) = 2.07 \(\times\)\(10^{-15}\) Wb), corresponding to a magnetic field less than 1 fT over an effective area of about 2 mm\({}^{2}\)[100]. Consequently, the range of the SQUID magnetometer is between 1 fT and 9 T, a fact that leads to the use of this device for medical applications because the neuromagnetic field of the human brain is a few tenths of a fT [62]. Therefore this instrument has a great variety of applications in many fields like medicine [102] (liver iron stores detection, sentinel lymph node biopsy, and breast cancer diagnostics), biomagnetism, magnetic microscopy, quantum computing, the non-destructive evaluation test, geophysics, the detection of axion dark-matter, the Hawking radiation, the dynamical Casimir effect, and the Majorana fermions [100]. According to [58; 103; 104; 105], SQUIDs can be categorized in two types: the low temperature superconductor (LTS) SQUIDs and the high temperature superconductor (HTS) SQUIDs. The magnetometers of the first category are very sensitive and therefore can detect weak magnetic properties. The disadvantage of this type of sensor is that the liquid He systems need maintenance, resulting in the increment of the operating cost and making this device inappropriate for simple measurements. The materials by which it is constructed are conventional metal low transition temperature superconductors, operating at 4.2 K, which is the temperature that liquid helium boils [103]. However, when these devices are used to measure samples at room temperature, the difference in the temperatures of the sample and the operating temperature of the sensor is high. Except for the strong cooling need, there is also a need for either insulation, which is several millimeters thick, or operation in a vacuum, both of which result in worse resolution [103]. The magnetometers of the second category are characterized by a simple cooling system in high temperatures; they are compact and present similar performance and reliability compared to the magnetometers of the first category. There are simpler, economical, higher power, and lower weight devices. In contrast to the low-temperature SQUIDs, there is no need for high insulation, which reduces the distance between the sensor and sample enabling higher accuracy measurements. One of the most well-known sensors of this category is the high-temperature SQUID made by YBa\({}_{2}\) Cu\({}_{3}\) O\({}_{7-\delta}\) (YBCO) superconductor [32; 103]. This material is a ceramic transition metal oxide, with a 92K transition temperature. Although, the YBCO material presents high anisotropy, and difficulties in growing, patterning, and etching [103]. Another classification of SQUIDs is given by [106], in which it states that there are several types of SQUIDs, and among them, the most widespread are the bulk and the thin film SQUIDs. The bulk SQUIDs present high inductance and thus have the drawback of high flux noise, in comparison to thin film SQUIDs. The bulk SQUIDs, constructed by niobium, can be easily built, are economical devices, and have important reliability, but they do not have as high a sensitivity when compared to other SQUID types.