The effect of Mn content on magnetism and half-metallicity of off-stoichiometric Co2MnAl

Using the first-principles calculations within density functional theory (DFT), we investigate the influence of Mn content on magnetism and half-metallicity of off-stoichiometric Co2MnAl. From our calculation, the Mn-poor structure most likely results from antisite disorders where Mn atoms are partially substituted by Co (CoMn antisite) or Al (AlMn antisite) due to their lower formation energy than the structure missing Mn atom. Besides, the half-metallicity is immune to AlMn antisite, while the impurity Co atom in CoMn antisite is responsible for the dramatic decrease in spin polarization. Besides, in the Mn-rich structure where excess impurity Mn occupy the Co sites, impurity Mn atom exhibits antiparallel coupling with other magnetic atoms, resulting in ferrimagnetism. With increasing of Mn content, the spin polarization of Mn-rich structure increases from 75% to 100%. When Mn content rises up to α = 1.875, the corresponding compound Co1.125Mn1.875Al owns the perfect spin polarization and stable half-metallicity due to the reason that its Fermi level is situated nearly in the middle of the spin down gap. Hence, a large tunneling magnetoresistance (TMR) of magnetic tunnel junctions (MTJs) could be obtained by using Mn-rich Co2MnAl electrode. Furthermore, when Mn content reaches up to α = 2, the compound converts to inverse Heusler compound Mn2CoAl with an unique band structure that the conduction and valence band edges of the spin up electrons touch at the Fermi level, it is therefore classified to be spin gapless semiconductors.