Terfenol,Samfenol and other rare earth iron compounds are currently used materials for magnetostrictive stress or torque sensor applications.However,poor mechanical properties and high cost of these compounds restrict their use for sensors. Recently substituted cobalt ferrites are found suitable to some extent for these applications due to their high negative magnetostriction, moderate positive crystalline anisotropy and good mechanical strength. This book describes a systamatic study of cobalt manganese and cobalt silicon ferrites for obtaining a promising composition with the desired properties.The necessary theoretical background for understanding the magnetic properties of sensors with an emphasis on magnetostrictive properties has been given. The materials were prepared by ceramic method. The beneficial nature of substituents in reducing the Curie temperature and anisotropic contribution of cobalt due to the cobalt migration has been discussed.