Ferrites are the series of magnetic materials that have many application in the industry, and so much research is being done on them. In different studies, the aim is to improve the magnetic properties of these samples, which is done through various methods. In this research, the fabrication and study of chromium substituted strontium ferrite (SrCr x Fe 12-x O 19 ) was first carried out and then core-shell structures by strontium ferrite (SrFe 12 O 19 ) and cobalt-zinc ferrite (Co 0.6 Zn 0.4 Fe 2 O 4 ) were fabricated and investigated. In this research, electrospinning method were used to fabricate fibers. Electrospinning is a method for producing polymeric fibers with diameters ranging from micrometer to nanometer. After preparing the raw fibers, they are subjected to heat treatment in the furnace in order to form the desired composition. In the first part of this study, the effect of chromium substitution (in the iron site) on the magnetic properties of strontium ferrite was investigated and it was found that by increase of chromium in the iron site, due to the decrease in the energy of the spin-orbit interaction and because of the lower atomic momentum of the chromium ion than the iron ion, coercive field and saturation magnetization were reduced. This study showed that chromium adding in the site of iron, does not enhance the magnetic properties of strontium ferrites. In the second part of this study, strontium ferrite and cobalt-zinc ferrite were fabricated as core-shell nanofibers. Here, strontium ferrite was used as hard magnetic material and cobalt-zinc ferrite was used as soft magnetic material. In core-shell nanofibers that contain hard and soft magnetic phases, there is an interaction called exchange-spring interaction between two hard and soft magnetic phases that will improve the magnetic properties of the sample. In this research, the measurement and analysis of structural and magnetic properties of samples, especially the study of exchange-spring interaction, have been investigated. It was found that various factors such as temperature would affect the mode of exchange-spring interaction. Based on the theories of exchange-spring interaction developed by Kneller and Skomsky, it was found that as dimensions of samples approach to the critical dimensions of exchange-spring interaction, this interaction between the hard and the soft magnetic phases will be established desirably. According to the measurements, it was found that the temperature drop increased the critical dimensions of the exchange-spring interaction, for this reason, the (BH) max enhancement was observed in the remanent curve measurements of samples measured at 10 K. It was also observed that, as the fibers diameter of the hard magnetic phase diminished, the dimensions of the sample were close to the single domain critical dimensions which is caused to coercive field enhancement.