Short- and long-range magnetic order in LaMnAsO

TL;DR

This study investigates the magnetic ordering in LaMnAsO, revealing a Neel temperature of 360 K, short-range magnetic correlations above this temperature, and a spin gap of 3.5 meV, highlighting complex magnetic behavior in layered Mn-based oxypnictides.

Contribution

The paper provides detailed neutron scattering and magnetization analysis of LaMnAsO, demonstrating the coexistence of long-range and short-range magnetic order and their temperature dependence.

Findings

Neel temperature T_N = 360 K identified

Short-range magnetic order persists up to 650 K

Spin gap of 3.5 meV observed in the ordered state

Abstract

The magnetic properties of the layered oxypnictide LaMnAsO have been revisited using neutron scattering and magnetization measurements. The present measurements identify the N\'{e}el temperature $T_N$ = 360(1) K. Below $T_N$ the critical exponent describing the magnetic order parameter is $\beta$ = 0.33$-$0.35, consistent with a three dimensional Heisenberg model. Above this temperature, diffuse magnetic scattering indicative of short-range magnetic order is observed, and this scattering persists up to $T_{SRO}$ = 650(10) K. The magnetic susceptibility shows a weak anomaly at $T_{SRO}$ and no anomaly at $T_N$. Analysis of the diffuse scattering data using a reverse Monte Carlo algorithm indicates that above $T_N$ nearly two- dimensional, short-range magnetic order is present with a correlation length of 9.3(3) {\AA} within the Mn layers at 400 K. The inelastic scattering data reveal a spin-gap of 3.5 meV in the long-range ordered state, and strong, low-energy (quasi-elastic) magnetic excitations emerging in the short-range ordered state. Comparison with other related compounds correlates the distortion of the Mn coordination tetrahedra to the sign of the magnetic exchange along the layer-stacking direction, and suggests that short-range order above $T_N$ is a common feature in the magnetic behavior of layered Mn-based pnictides and oxypnictides.