Magnetic structure and transport properties of noncollinearLaMn2X2(X=Ge,Si)systems

Abstract

Electronic, magnetic, and transport properties of the noncollinear naturally multilayered compounds ${\mathrm{LaMn}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{LaMn}}_{2}{\mathrm{Si}}_{2}$ are addressed by first-principles calculations based on the density-functional theory. At low temperatures, these systems show a magnetic state with the $\mathrm{Mn}$ moments ordered in a conical arrangement (spin spiral) with a ferromagnetic coupling along the $c$ axis and an in-plane antiferromagnetic coupling. The magnetic structures are studied by means of the full-potential linearized augmented-plane-wave method within both the generalized-gradient approximation and the local-density approximation. In both compounds, a conical magnetic state is obtained with energies lower than canted and collinear structures. The trends in the experimentally observed magnetic configuration when replacing $\mathrm{Ge}$ by $\mathrm{Si}$ are discussed. The origin of the experimentally observed inverse giant magnetoresistance in ${\mathrm{LaMn}}_{2}{\mathrm{Ge}}_{2}$ is traced back to the presence of many noncollinear low-energy magnetic configurations.