Correlation induced half-metallicity in a ferromagnetic single-layered compound: Sr$_2$CoO$_4$

The electronic and magnetic properties of Sr$_2$CoO$_4$ compound have been studied using $\emph{ab initio}$ electronic structure calculations. As opposed to GGA calculation, which gives ferromagnetic metallic solution, GGA+$U$ calculations provide two kind of ferromagnetic solutions: (i) half-metallic and (ii) metallic. The half-metallic solution is a ground state of the system and the metallic one is a metastable state. The strong hybridization between Co 3$d$ and O 2$p$ orbitals decides the electronic and magnetic properties of the compound. The total magnetic moment per formula unit is found to be $\sim$ 3 $\mu_B$ ($S$ = 3/2). Our calculations give the magnetocrystalline anisotropy energy of $\sim$ 2.7 meV, which provides a good description of experimentally observed large magnetocrystalline anisotropy. The Heisenberg exchange parameters up to fourth nearest neighbours are also calculated. The mean-field theory gives the $T_C$ = 887 K. The possible physical implications of the ferromagnetic half-metallic ground state are also discussed.