Magnetic structure and transport properties of noncollinear LaMn2X2 (X=Ge,Si) systems

Electronic, magnetic, and transport properties of the noncollinear naturally multilayered compounds LaMn2Ge2 and LaMn2Si2 are addressed by first-principles calculations based on the density-functional theory. At low temperatures, these systems show a magnetic state with the 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 Ge by Si are discussed. The origin of the experimentally observed inverse giant magnetoresistance in LaMn2Ge2 is traced back to the presence of many noncollinear low-energy magnetic configurations