Revisiting the antiferromagnetic structure of $\rm Tb_{14}Ag_{51}$. The importance of distinguishing alternative symmetries for a multidimensional order parameter

TL;DR

This study revisits the antiferromagnetic structure of b714Ag51, proposing a new magnetic model based on symmetry analysis that better fits experimental data and reveals complex cycloidal magnetic ordering with harmonic contributions.

Contribution

The paper introduces a new magnetic structure model for b714Ag51 using symmetry constraints, improving upon previous models and highlighting the role of third harmonic components.

Findings

New magnetic structure fits experimental data better

Identification of a cycloidal magnetic order with harmonic contributions

Refined magnetic moments of 8.48 b5_B per Tb atom

Abstract

We revisit the antiferromagnetic structure of $\rm Tb_{14}Ag_{51}$ [P. Fischer., $et\, al$. (2005). PRB, 72 134413] with the propagation vector $[{1\over3},{1\over3},{0}] $ and parent space group $P6/m$ using both magnetic symmetry and irreducible representation arguments. We have found a new magnetic structure under the hexagonal Shubnikov magnetic space group $P\bar{6}'$, which fits much better the experimental data. This new solution was obtained by constraining the spin arrangement to one of the three possible magnetic space groups of maximal symmetry that can be realised by a magnetic ordering transforming according to the 4-dimensional physically irreducible representation that is known to be relevant in this magnetic phase. The refined model, parameterised under $P\bar{6}'$, implicitly includes the presence of a third harmonic with the propagation vector at the gamma point $[0,0,0]$, which has an important weight in the final result. The structure consists of 13 symmetry-independent Tb magnetic moments with the same size of $8.48(2)\mu_B$, propagating cycloidally in the $ab$-plane. The modulation has a substantial deviation from being purely sinusoidal due to the contribution of the mentioned third harmonic.