## 1 Introduction  
Magnetic nanoparticles play a role in a great variety of applications like medicine because they can provide magnetic hyperthermia and magnetic drug targeting [1; 2; 3]. In addition, beyond medical applications, their presence is valuable in the industry fields of microelectronics, sensor technology, heterogeneous catalysis, and other important fields [2; 4]. For this reason, the existence of rapid, economical, and simple methods for the characterization of magnetic nanoparticles is vital to have the possibility to know the moment and their properties and to detect possible faults as they appear. The methods for catalyst characterization are many. The main categories of nanoparticle characterization methods are adsorption methods, thermal analysis, electron microscopy methods, X-ray and neutron methods, ion scattering techniques, vibrational spectroscopies, electron spectroscopies, magnetic measurements, etc. [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Adsorption methods are capable of providing information related to the catalyst's total surface area, the surface area of the phase carrying the active sites, and the type and number of active sites. There are both methods related to adsorption: the flow and the dynamic methods. Consequently, the adsorption can be static or dynamic. The microscopy methods can characterize the nanoparticle materials, providing details about the phase state of the crystal and the crystal structure. However, sometimes the image interpretation can be accompanied by complexities that make the characterization process difficult. In addition, some other difficulties derive from the procedure of preparation of specimens and the microscope environment itself. The main microscopy methods are conventional transmission electron microscopy (CTEM), dark field methods, high-resolution electron microscopy (HREM), reflection electron microscopy (REM) and reflection high energy electron diffraction (RHEED), scanning electron microscopy (SEM), the atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) [5; 9; 10; 17; 18]. CTEM can provide material characterization for nanoparticles with sizes smaller than 2-3 nm, as well as provide information about the particle location [5]. However, this method does not allow the investigation of the crystal's periodic structure. This possibility is partially given using another technique, the HREM. According to [5], the HREM in some cases can be used for nanoparticle characterization smaller than 1 nm. STEM is similar to CTEM, but it is able to perform microanalysis at very high resolution. Nevertheless, all the methods that are based on microscopes are not economical, give exclusively local information and the need for time to prepare the sample [19; 20]. Other structure investigation methods are the X-ray and neutron methods (neutron diffraction). X-ray diffraction [6; 9; 18; 21] (XRD), also called WAXS, is one of the most important techniques for catalyst characterization. This method can give accurate results about the material structure when utilized for both zeolites and catalysts with good crystallinity [5; 17]. One of the method's drawbacks is that the XRD is limited to powder-pattern identification of crystalline phases. In materials, obtaining crystals with sizes smaller than 1-1.5 nm or in amorphous materials, the method cannot provide accurate results and because of this, the radial electron distribution (RED) is utilized. Another method is the extended X-ray absorption fine structure (EXAFS), which has application mainly in the case of multimetallic catalyst characterization, where the other methods do not provide satisfying results. Referring to the ion scattering techniques are ion scattering spectroscopy (ISS), low-energy ion scattering (LEIS), Rutherford backscattering spectroscopy (RBS) [17], nuclear backscattering spectroscopy (BNS), and high-energy ion scattering (HEIS).