## 3 Magnetic Characterization Instruments
### Superconducting Quantum Interference Device (SQUID)
Compared to the fluxgate magnetometers, SQUIDs present low magnetic field noise [20; 32; 47; 101] as well, and they are characterized by high bandwidths (of the order of MHz), while the corresponding fluxgate magnetometers have bandwidth in the order of kHz [32]. More specifically, according to [47], the conventional Nb SQUIDs present magnetic field noise values down to and below 1 \(\mathrm{fT}/\mathrm{Hz}^{1/2}\), while the corresponding values for YBCO SQUIDs are down to a few \(\mathrm{fT}/\mathrm{Hz}^{1/2}\). In addition, as the SQUIDs are cooled, one can achieve measurements that are not dependent on temperature [46; 107]. Moreover, this device can measure at a close distance to the sample [44; 108; 109] and has a magnetorelaxometry dead time of approximately 200 \(\mathrm{\SIUnitSymbolMicro s}\)[1]. Some important disadvantages of SQUIDs are the cooling demand [1; 26; 47; 110], under the superconducting critical temperature, in either helium or nitrogen temperature; their disability to measure absolute magnetic fields, as they measure variations on the field; and their susceptibility to magnetic disturbances and the need of heavy magnetic shielding for preventing interference by environmental magnetic noises [42; 103; 110]. Moreover, when SQUIDs are used in magnetorelaxometry measurements, an important issue is their stability under high magnetic fields due to the rapid variations of the fields. In these cases, the noise performance of the device is downgraded, especially in the 1/f region, while the dynamic range of the device is limited [32]. Another important fact to take into consideration is the behavior of the SQUID under a large excitation magnetic field when a large offset interference signal is produced. In this case, the dynamic range of the device is reduced and a small magnetic response cannot be successfully investigated. Consequently, the ability of the SQUID to detect nanoparticles presents restrictions to some extent. In [104] a solution to this problem is attempted, using two detection coil configurations, one for AC and another for DC measurements, which increased the complexity of the system, demanding the use of two SQUIDs for concurrent measurements. Next, the most important studies in the recent international literature using SQUID magnetometers are presented. More specifically, in [20], two systems that characterize magnetic nanoparticles through magnetorelaxation method using a differential fluxgate magnetometer and a low-temperature SQUID are compared, as depicted in Figure 2.  
In [29], an improvement of an already existing AC/DC high-temperature SQUID magnetometer [104] is proposed. The aim of the optimization is the increment of the dynamic range of the device when an AC magnetic field is applied to it, using a tuned compensation coil, as well as the increment of the resolution of the DC magnetic field  
Figure 2: (**a**) The differential fluxgate measurement system, (**b**) The SQUID measurement system [20]. with the use of a secondary excitation coil. The compensation coil will contribute to the system's capability to use only one SQUID for both AC and DC measurements, and it will increase the sensitivity level for the AC measurements. In [41], a thin film LTS SQUID magnetometer and the magnetorelaxation method are used to determine the magnetic nanoparticle distribution in large objects such as animals in a distributed environment. In [32], a high-temperature YBCO SQUID, connected with a slotted pick-up loop, has been used for magnetic nanoparticle characterization using the magnetorelaxometry method. The investigation showed that the system can achieve satisfactory noise levels, while it is appropriate for static magnetic fields. In [46], a thin film planar low-temperature SQUID magnetometer of the second order is used for magnetorelaxometry measurements in an unshielded environment. In [100], two nanoSQUIDs with 0.5 \(\upmu\)m\({}^{2}\) sensitive area are presented.