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

The interplay between the structure and A-type spin ordering in La_0.46Sr_0.54 Mn_0.98Cr_0.02O₃ (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≪b/√2 to monoclinic P2₁/n with b/√2≪a≈c (β≈90.4°). Moreover, the first-order A-type spin ordering is characterized by a two-dimensional-like (2D-like) anisotropic structural distortion [Δ(a,c)≈ + 1.05 %, Δb/√2≈ -2.2%] and the unusual volume discontinuity (ΔV≈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_x2-y2 orbital ordering.