Alloy nanoparticles are a potential candidate for research among different types of nanoparticles, due to their simple synthesizing method, promising features and diverse applications. Here, we have studied truncated octahedron FeCo alloy nanoparticles using atomistic simulations. The work is carried in the framework of classical spin Hamiltonian. Monte-carlo method and Landau Lifshitz Gilbert (LLG) equation are employed for sampling of chosen atomistic system. In this study, it has been observed that magnetic ordering temperature in the FeCo alloy is dependent on the composition of alloy in addition to the size of the system. The magnetic ordering temperature is calculated for each composition and size. Moreover, switching of the magnetic field in the FeCo alloy is studied using the combination of classical spin Hamiltonian and LLG equation as a function of composition of alloy and temperature of system. [ABSTRACT FROM AUTHOR]