The remarkable success of the local density and generalized gradient approximations within Kohn-Sham approach have led to widespread interest in Density Functional Theory (DFT( as the most promising approach for theoretical study of materials.Wien2k package employs the highly accurate all-electron full-potential linearized augmented plane wave plus local orbital method to solve the single particle Kohn-Sham equations. We have used Wien2k to study two interesting magnetic materials; half metal Co 2 MnSi Heusler alloy and antiferromagnetic dilute Cr alloys. Theoretical prediction of half-metallicity in ferromagnetic full Heusler Co 2 MnSi alloy along with its high measured curie temperature has made this alloy a promising material for spintronic applications. In a ferromagnetic half metal the majority spins have metallic behavior while the Fermi energy is located in a gap of the minority band structure. We have studied the structural, electronic and magnetic properties of the ideal and modified Co 2 MnSi(001) surfaces. For the ideal MnSi and CoCo terminations we found that some surface states arising mainly from surface or subsurface cobalt break half metallicity In the MnSi termination surface exchange enhancement is the main origin of the surface states while in CoCo termination, the potential rise up at the surface push some minority states into the half metallic gap. In order to study disorder effects, we have investigated the stability, the electronic, and the magnetic properties of Co 2 MnSi(001) thin films for 13 different modified terminations in a 1x1 supercell. The phase diagram obtained by ab initio atomistic thermodynamics shows that in practice the ideal MnSi, modified pure Mn, or pure Si terminated surfaces can be stabilized under suitable conditions. Analyzing the surface band structure, we found that the pure Mn termination, due to its strong surface-subsurface coupling, preserves the half metallicity of the system, while surface states appear for the other terminations The second subject is magnetic properties of dilute impurities in chromium host. Cr alloys reveal great variety of antiferromagnetic properties originating from the peculiar magnetic behavior of simple bcc chromium, an incommensurate spin density wave ground state. In this research we have investigated Fe, Ta, and 4d impurities (Nb--I) in a commensurate antiferromagnetic Cr host. The effects of these impurities on the structural, electronic, and magnetic properties of chromium are discussed with special emphasis on the hyperfine interaction. It is shown that the filled d shell 4d atoms are nonmagnetic with weak bonding while other impurities have considerable magnetic moments and stronger coupling with the nearest neighbors. Two different magnetic states were identified for Fe impurity in the Cr host, one with high and the other with low magnetic moments, the