Density functional theory with local spin density approximation has been used to propose possible room temperature ferromagnetism in N-doped NaCl-type BaO. Pristine BaO is a wide bandgap semiconductor, however, N induces a large density of states at
the Fermi level in the nonmagnetic state, which suggests magnetic instability within the Stoner mean field model. The spin-polarized calculations show that N-doped BaO is a true half- metal, where N has a large magnetic moment, which is mainly localized around the N atoms and a small polarization at the O sites is also observed. The origin of magnetism is linked to the electronic structure. The ferromagnetic(FM) and antiferromagnetic (AFM) coupling between the N atoms in BaO reveal that doping N atoms have a FM ground state, and the calculated transition temperature ($T_{C}$), within the Heisenberg mean field theory, theorizes possible room temperature FM in N-doped BaO. Nitrogen also induces ferromagnetism when doping occurs at surface O site and has a smaller defect formation energy than the bulk N-doped BaO. The magnetism of N-doped BaO is also compared with Co-doped BaO, and we believe that N has a greater potential for tuning magnetism in BaO than Co.
First-principles based on density functional theory is used to study the phase stability, elastic, magnetic, and electronic properties of cubic (c)-Fe$_4$C. Our results show that c-Fe$_{4}$C has a ferromagnetic (FM) ground state structure compared wi
th antiferromagnetic (AFM) and nonmagnetic (NM)states. To study the phase stability of c-Fe$_4$C, BCC Fe$_4$C, FCC Fe$_4$C, and BCC Fe$_{16}$C, where C is considered at tetrahedral and octahedral interstitial sites, are also considered. Although, the formation energy of c-Fe$_4$C is smaller than BCC Fe$_4$C, but the shear moduli of c-Fe$_4$C is negative in the FM and AFM states indicating that c-Fe$_4$C is dynamically not stable in the magnetic (FM/AFM) states. However, NM state has positive shear moduli which illustrates that instability in c-Fe$_4$C is due to magnetism and can lead to soft phonon modes. The calculated formation energy also shows that c-Fe$_4$C has higher formation energy compared with the FCC Fe$_4$C indicating no possibility of c-Fe$_4$C in low carbon steels at low temperature. The magnetic moment of Fe in c-Fe$_4$C is also sensitive to lattice deformation. The electronic structure reveals the itinerant nature of electrons responsible for metallic behavior of c-Fe$_4$C.
Density functional theory with local density approximation for exchange and correlation functional is used to tune the electronic band structure of silicene monolayer. The cohesive energy of free standing monolayer is increasing (decreasing) with ext
ernal electric field (distortion). Electrons in silicene behave like Dirac fermions, when the bond angle between the Si atoms is larger than $sim 102^{0}$. Large distortions destroy the electronic structure of silicene and silicene is no longer a semi-metallic material, and the distorted silicene acts like an $n$-doped system. Electric field opens a band gap around $K$ point in the Brillouin zone, which increases with electric field. The bond angle between the Si atoms is a key player to determine the presence or absence of Dirac cones in silicene.
