Incommensurate magnetic ordering in $Cu_2 Te_2 O_5 X_2$ (X=Cl,Br) studied by neutron diffraction

TL;DR

This study reveals incommensurate antiferromagnetic order in Cu2Te2O5X2 (X=Cl, Br) using neutron diffraction, showing complex magnetic structures likely caused by geometrical frustration without structural transitions.

Contribution

First neutron diffraction analysis of Cu2Te2O5X2 revealing incommensurate magnetic order and complex spin arrangements due to frustration.

Findings

Incommensurate magnetic order with specific propagation vectors

Complex helical magnetic structures fit the diffraction data

No structural transition observed at magnetic ordering

Abstract

We present the results of the first neutron powder and single crystal diffraction studies of the coupled spin tetrahedra systems ${\CuTeX}$ (X=Cl, Br). Incommensurate antiferromagnetic order with the propagation vectors ${\bf{k}_{Cl}}\approx[0.150,0.422,\half]$, ${\bf{k}_{Br}}\approx[0.158,0.354,\half]$ sets in below $T_{N}$=18 K for X=Cl and 11 K for X=Br. No simple collinear antiferromagnetic or ferromagnetic arrangements of moments within Cu${}^{2+}$ tetrahedra fit these observations. Fitting the diffraction data to more complex but physically reasonable models with multiple helices leads to a moment of 0.67(1)$\mu_B$/Cu${}^{2+}$ at 1.5 K for the Cl-compound. The reason for such a complex ground state may be geometrical frustration of the spins due to the intra- and inter-tetrahedral couplings having similar strengths. The magnetic moment in the Br- compound, calculated assuming it has the same magnetic structure as the Cl compound, is only 0.51(5)$\mu_B$/Cu${}^{2+}$ at 1.5 K. In neither compound has any evidence for a structural transition accompanying the magnetic ordering been found.