Electricity and magnetism play a central role in operation of various nanotechnology devices such radio, receivers, televisions, electric motors, computers, high energy accelerators and most of electronic devices used in medicine. Scientists and researchers have worked hard to develop electromagnetic science and to find out theories to explain properties of the materials. Materials were classified into their interaction with applied magnetic field into diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic. Study of polycrystalline ferrites is important in view of their successively extensively used over a wide range of frequency. This is due to their high initial permeability, large resistivity, high dielectric constant and low dielectric loss. This book divides into two parts. Part I provide: description of ferrimagnetic materials, structure, magnetic and electric properties; and applications of ferrimagnetic. Part II describes structure, magnetic and electric properties of Cu-Zn spinel ferrite. This book should be useful to professionals and students in science, engineering and technology, or anyone else who may be considering utilizing materials science.