Degenerate ground state in the classical pyrochlore antiferromagnet Na$_3$Mn(CO$_3$)$_2$Cl

TL;DR

This study identifies Na$_3$Mn(CO$_3$)$_2$Cl as a classical pyrochlore antiferromagnet exhibiting high ground state degeneracy, with no magnetic order or spin-glass transition, highlighting its potential for studying frustrated magnetism.

Contribution

The paper reports the synthesis and characterization of Na$_3$Mn(CO$_3$)$_2$Cl as a new classical pyrochlore antiferromagnet with high ground state degeneracy and no magnetic order, expanding the understanding of frustrated magnetic systems.

Findings

No magnetic order or spin-glass transition detected.

Presence of antiferromagnetic short-range order.

High ground state degeneracy supported by magnetic entropy.

Abstract

In an ideal classical pyrochlore antiferromagnet without perturbations, an infinite degeneracy at a ground state leads to absence of a magnetic order and spin-glass transition. Here we present Na$_3$Mn(CO$_3$)$_2$Cl as a new candidate compound where classical spins are coupled antiferromagnetically on the pyrochlore lattice, and report its structural and magnetic properties.The temperature dependences of the magnetic susceptibility and heat capacity, and the magnetization curve are consistent with those of an $S$ = 5/2 pyrochlore lattice antiferromagnet with nearest-neighbor interactions of 2 K. Neither an apparent signature of a spin-glass transition nor a magnetic order is detected in magnetization and heat capacity measurements, or powder neutron diffraction experiments. On the other hand, an antiferromagnetic short-range order from the nearest neighbors is evidenced by the $Q$-dependence of the diffuse scattering which develops around 0.85 \AA$^{-1}$. A high degeneracy near the ground state in Na$_3$Mn(CO$_3$)$_2$Cl is supported by the magnetic entropy estimated as almost 4 J K$^{-2}$ mol$^{-1}$ at 0.5 K.