A variable temperature study of the crystal and magnetic structures of the giant magnetoresistant materials LnMnAsO (Ln = La, Nd)

TL;DR

This study investigates the magnetic and crystal structures of LnMnAsO compounds across temperatures, revealing spin reorientation in NdMnAsO and coupling between electronic and lattice properties without structural transitions.

Contribution

It provides detailed temperature-dependent magnetic and structural data on LnMnAsO materials, highlighting spin reorientation and electronic-lattice coupling phenomena.

Findings

NdMnAsO exhibits spin reorientation below TN.

No magnetic structure change with temperature in LaMnAsO.

Discontinuities in lattice parameters at ~150 K indicate electronic-lattice coupling.

Abstract

A variable temperature neutron and synchrotron diffraction study have been performed on the giant magnetoresistant oxypnictides LnMnAsO (Ln = La, Nd). The low temperature magnetic structures have been studied and results show a spin reorientation of the Mn2+ spins below TN (Nd) for NdMnAsO. The Mn2+ spins rotate from alignment along c to alignment into the basal plane and the Mn2+ and Nd3+ moments refine to 3.54(4) \mu B and 1.93(4) \mu B respectively at 2 K. In contrast there is no change in magnetic structure with temperature for LaMnAsO. There is no evidence of a structural transition down to 2 K, however discontinuities in the cell volume, Ln-O and Mn-As bond lengths are detected at \sim 150 K for both materials. This temperature coincides with the electronic transition previously reported and suggests a coupling between electronic and lattice degrees of freedom.