In this project the structural and magnetic properties of manganese ferrite nanoparticles, MnFe 2 O 4 have been investigated. Large scale synthesis method was one of the aims of this study. In the first part of this work, the nanoparticles were prepared successfully by using thermal decomposition of metal nitrates in presence of citric acid. Although the XRD analysis confirmed crystalline structure of samples without noticeable trace of impurity phases, the saturation magnetization ( M s ) was smaller than that for bulk Mn-Ferrite. Such a decrease in M s is mainly arised from the presence of dead layer in the shell of nanoparticle. Because of agglomeration and direct contact of nanoparticles, these samples are highly interacting. At low temperature, these interactions lead to frustrated super-spin galss ( SSG ) state. AC magnetic susceptibility at different frequencies and memory experiments confirmed SSG behavior of these samples. In the following, by grinding the aggregates, compaction of nanoparticles was decreased and SSG behavior almost diminished. In the next section of this project, the effect of Ag and Co doping on the structural and magnetic properties of MnFe 2 O 4 nanoparticles have been investigated. Because of non-magnetic nature of Ag 1 + cations, doping of silver in spinel structure leads to reduction in magnetic response of sample to external applied fields. Decrease in crystallite’s size with increase in Ag doping level and consequently increase in surface/volume ratio is the main reason of decrease in M s . Also in this section we showed that there is a solubility limit in substitution of Ag 1 + in iron and manganese sites because of larger ionic radius of Ag 1 + cations than Fe 3 + and Mn 2 + . Metallic silver phase was formed at higher doping level. On the other hand by doping the highly anisotropic Co 2 + in manganese sites we found that M s of samples is not influenced while the coersivity showed a remarkable increase by increasing the Co content in the samples. In the last section, we focused on improving the quality of samples by change in sample preparation method. We used Tri-ethylene glycol ( TEG ) as a solvent of metal nitrates in order to increase the saturation magnetization and decrease the contribution of disordered surface spins. It was found that molecular coating of nanoparticles decreases the surface spins canting by improving the metal-oxygen-metal bond in the interface of nanoparticles. As same as thermal decomposition, this method can be applied for large scale syntheses of nanoparticles. By using magnetic characterization it has been shown that increase in solvent to nitrates molar ratio leads to decrease in interactions strength between super-spins so that there is no trace of SSG behavior in these samples. In the following of this part, by applying hydrostatic pressure the magneto-static energy was increased. By using imaginary part of ac magnetic susceptibility and calculation of heat power density ( P ) it was realized that increase in dipole-dipole interactions create a decrease in P values, which is an important result in the field of using the magnetic nanoparticles in the hyperthermia based cancer therapy method.