Structural origin of the pseudo-two-dimensional metallic state in theA-type antiferromagnetic manganites

Abstract

The interplay between the structure and A-type spin ordering in ${\mathrm{La}}_{0.46}{\mathrm{Sr}}_{0.54}{\mathrm{Mn}}_{0.98}{\mathrm{Cr}}_{0.02}{\mathrm{O}}_{3}$ (LSMO) has been studied using neutron powder diffraction. The antiferromagnetic transition is accompanied by a structural transition from tetragonal $I4/mcm$ with lattice constant $a=c\ensuremath{\ll}b/\sqrt{2}$ to monoclinic ${P2}_{1}/n$ with $b/\sqrt{2}\ensuremath{\ll}a\ensuremath{\approx}c$ $(\ensuremath{\beta}\ensuremath{\approx}90.4\ifmmode^\circ\else\textdegree\fi{}).$ Moreover, the first-order A-type spin ordering is characterized by a two-dimensional-like (2D-like) anisotropic structural distortion $[\ensuremath{\Delta}(a,c)\ensuremath{\approx}+1.05%,$ $\ensuremath{\Delta}b/\sqrt{2}\ensuremath{\approx}\ensuremath{-}2.2%]$ and the unusual volume discontinuity $(\ensuremath{\Delta}V\ensuremath{\approx}0.13%).$ These results demonstrate that the pseudo-2D metallic state in LSMO can be realized by a 2D-like anisotropic lattice distortion correlated with the A-type spin ordering and the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital ordering.