Spin and orbital ordering in TlMnO3: Neutron diffraction study

TL;DR

This neutron diffraction study reveals the crystal and magnetic structures of TlMnO3, highlighting unique orbital and spin ordering patterns, and differences from similar LnMnO3 manganites, with implications for magnetic interactions.

Contribution

The paper provides the first detailed neutron diffraction analysis of TlMnO3's structure and magnetic ordering, showing distinct orbital patterns and magnetic coupling compared to related manganites.

Findings

Orbital ordering involves a+b-b-octahedral tilting and reduces symmetry to triclinic P -1.

Magnetic structure exhibits A-type antiferromagnetism with anisotropic exchange interactions.

Coupling of ferromagnetic component to primary spin order is forbidden in TlMnO3.

Abstract

Crystal and magnetic structures of the high-pressure stabilized perovskite phase of TlMnO3 have been studied by neutron powder diffraction. The crystal structure involves two types of primary structural distortions: a+b-b-octahedral tilting and antiferrodistortive type of orbital ordering, whose common action reduces the symmetry down to triclinic P -1. The orbital pattern and the way it is combined with the octahedral tilting are different from the family of LnMnO3 (Ln = lanthanide or Y) manganites who share with TlMnO3 the same tilting scheme. The experimentally determined magnetic structure with the k = (1/2,0,1/2) propagation vector and P_S-1 symmetry implies anisotropic exchange interactions with a ferromagnetic coupling within the (1,0,-1) planes and an antiferromagnetic one between them (A type). The spins in the primary magnetic mode were found to be confined close to the (1,0,-1) plane, which underlines the predominant role of the single ion anisotropy with the local easy axes of Mn3+ following the Jahn-Teller distortions of the octahedra. In spite of the same octahedral tilting scheme in the perovskite structures of both LnMnO3 and TlMnO3 manganites, a coupling of the secondary ferromagnetic component to the primary A-type spin configuration through antisymmetric exchange interaction is allowed in the former and forbidden in the latter cases.