The magnetic structures of the anisotropic inter-metallic compounds Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$

TL;DR

This study investigates the magnetic structures of Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$ using neutron diffraction, revealing different antiferromagnetic arrangements despite structural similarities, and discusses the role of crystal electric field effects.

Contribution

The paper provides the first detailed neutron diffraction analysis of these compounds, identifying distinct magnetic structures and their relation to crystal electric field effects.

Findings

Er$_2$CoGa$_8$ orders antiferromagnetically at 3.0 K

Tm$_2$CoGa$_8$ orders antiferromagnetically at 2.0 K

Different magnetic space groups were determined for the two compounds

Abstract

Two members of the iso-structural R$_2$CoGa$_8$ inter-metallic series, Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$, have been studied by powder neutron diffraction. Antiferromagnetic ordering of the rare-earth sublattices was confirmed to occur at 3.0 K and 2.0 K, respectively. Furthermore, determination of the critical exponent showed Er$_2$CoGa$_8$ to adopt a 3D universality class. In spite of a common magnetic easy axis and similar structural characteristics, the antiferromagnetic structures were found to be different for the erbium and thulium based compounds. The corresponding magnetic space groups were determined to be $P_{2a}mmm'$ and $P_Cmmm$. The difference in magnetic structures is discussed based on crystal electric field effects that are known to be prevalent in such materials.