Relation between crystal and magnetic structures of the layered manganites La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50)

TL;DR

This study investigates how the crystal and magnetic structures of layered manganites La2-2xSr1+2xMn2O7 change with composition, revealing a transition from ferromagnetic to antiferromagnetic states linked to Jahn-Teller distortion and orbital occupancy.

Contribution

It provides detailed neutron diffraction analysis showing the correlation between Jahn-Teller distortion, orbital state, and magnetic structure across different compositions.

Findings

Planar ferromagnetic structure appears at x ≤ 0.32.

Canted antiferromagnetic structure emerges at x ≤ 0.39.

The magnetic canting angle increases with x, reaching 180° at x=0.48.

Abstract

Comprehensive neutron-powder diffraction and Rietveld analyses were carried out to clarify the relation between the crystal and magnetic structures of La2-2xSr1+2xMn2O7 (0.30 =< x =< 0.50). The Jahn-Teller (JT) distortion of Mn-O6 octahedra, i.e., the ratio of the averaged apical Mn-O bond length to the equatorial Mn-O bond length, is Delta_JT=1.042(5) at x=0.30, where the magnetic easy-axis at low temperature is parallel to the c axis. As the JT distortion becomes suppressed with increasing x, a planar ferromagnetic structure appears at x =< 0.32, which is followed by a canted antiferromagnetic (AFM) structure at x =< 0.39. The canting angle between neighboring planes continuously increases from 0 deg (planar ferromagnet: 0.32 =< x < 0.39) to 180 deg (A-type AFM: x=0.48 where Delta_JT=1.013(5)). Dominance of the A-type AF structure with decrease of JT distortion can be ascribed to the change in the eg orbital state from d3z^2-r^2 to dx^2-y^2.