Structural phase transition and magnetism in hexagonal srmno

TL;DR

This study investigates the structural and magnetic phase transitions in hexagonal SrMnO3, revealing a subtle symmetry change at 350K and antiferromagnetic ordering below 300K, highlighting the role of Mn-Mn interactions.

Contribution

The paper provides detailed insights into the structural phase transition and magnetic ordering in hexagonal SrMnO3 using combined diffraction and magnetization techniques.

Findings

Structural transition from hexagonal to orthorhombic symmetry at 350K.

Onset of antiferromagnetic order below 300K.

Reduced magnetic moments indicating direct Mn-Mn exchange.

Abstract

The structural and magnetic properties of the hexagonal four-layer form of SrMnO$_3$ have been investigated by combining magnetization measurements, electron diffraction and high-resolution synchrotron X-ray and neutron powder diffraction. Below 350K, there is subtle structural phase transition from hexagonal symmetry (space group $P6_3/mmc$) to orthorhombic symmetry (space group $C222_1$) where the hexagonal metric is preserved. The second-order phase transition involves a slight tilting of the corner-sharing Mn$_{2}$O$_{9}$ units composed of 2 face-sharing MnO$_6$ octahedra and the associated displacement of Sr$^{2+}$ cations. The phase transition is described in terms of symmetry-adapted displacement modes of the high symmetry phase. Upon further cooling, long range magnetic order with propagation vector $\mathbf{k}=(0,0,0)$ sets in below 300K. The antiferromagnetic structure, analyzed using representation theory, shows a considerably reduced magnetic moment indicating the crucial role played by direct exchange between Mn centers of the Mn$_{2}$O$_{9}$ units.