Showing papers by "Birabar Nanda published in 2010"

Magnetic and orbital order in LaMnO 3 under uniaxial strain: A model study

Abstract: The effect of uniaxial strain on electronic structure and magnetism in ${\text{LaMnO}}_{3}$ is studied from a model Hamiltonian that illustrates the competition between the Jahn-Teller, superexchange, and double-exchange interactions. We retain in our model the three main octahedral distortions (${Q}_{1}$, ${Q}_{2}$, and ${Q}_{3}$), which couple to the Mn $({e}_{g})$ electrons. Our results show the ground state to be a type A antiferromagnetic (AFM) insulating state for the unstrained case, consistent with experiments. With tensile strain (stretching along the $c$ axis), the ground state changes into a ferromagnetic and eventually into a type ${\text{G}}^{\ensuremath{'}}$ AFM structure, while with compressive strain, we find the type A switching into a type G structure. The orbital ordering, which displays the well-known checkerboard ${x}^{2}\ensuremath{-}1/{y}^{2}\ensuremath{-}1$ structure for the unstrained case, retains more or less the same character for compressive strain, while changing into the ${z}^{2}\ensuremath{-}1$ character for tensile strains. While ${Q}_{1}$ and ${Q}_{3}$ are fixed by the strain components ${\ensuremath{\epsilon}}_{xx}$ and ${\ensuremath{\epsilon}}_{zz}$ in our model, the magnitude of the in-plane distortion mode $({Q}_{2})$, which varies to minimize the total energy, slowly diminishes with tensile strain, completely disappearing as the FM state is entered. Within our model, the FM state is metallic, while the three AFM states are insulating.

Polar catastrophe, electron leakage, and magnetic ordering at the LaMnO 3 / SrMnO 3 interface

Abstract: Electronic reconstruction at the polar interface ${\text{LaMnO}}_{3}/{\text{SrMnO}}_{3}$ (LMO/SMO) (100) resulting from the polar catastrophe is studied from a model Hamiltonian that includes the double and superexchange interactions, the Madelung potential, and the Jahn-Teller coupling terms relevant for the manganites. We show that the polar catastrophe, originating from the alternately charged LMO layers and neutral SMO layers, is quenched by the accumulation of an extra half electron per cell in the interface region as in the case of the ${\text{LaAlO}}_{3}/{\text{SrTiO}}_{3}$ interface. In addition, the $\text{Mn}\text{ }{e}_{g}$ electrons leak out from the LMO side to the SMO side, the extent of the leakage being controlled by the interfacial potential barrier and the substrate induced epitaxial strain. The leaked electrons mediate a Zener double exchange, making the layers adjacent to the interface ferromagnetic, while the two bulk materials away from the interface retain their original type A or G antiferromagnetic structures. A half-metallic conduction band results at the interface, sandwiched by the two insulating bulks. We have also studied how the electron leakage and consequently the magnetic ordering are affected by the substrate-induced epitaxial strain. Comparisons are made with the results of the density-functional calculations for the ${(\text{LMO})}_{6}/{(\text{SMO})}_{4}$ superlattice.

Polar catastrophe and the spin-polarized electron gas at the LaMnO3/SrMnO3 interface

Abstract: From calculations using ab initio density-functional methods, the authors show that analogous to the much-studied polar LaAlO3/SrTiO3 interface, there also forms a two-dimensional electron gas at the LaMnO3/SrMnO3 (100) interface due to the polar catastrophe. The electron gas originates from the accumulation of half an electron per cell to quench the polar catastrophe and from the leakage of Mn(eg) electrons from the LaMnO3 side to the SrMnO3 side. These electrons mediate a ferromagnetic double-exchange interaction between the Mn moments at the interface, causing the interface region to be ferromagnetic and in turn the electrons becoming spin-polarized owing to the Zeeman field of the Mn moments.