Degenerate ground state in the classical pyrochlore antiferromagnet Na3Mn(CO3)2Cl

In an ideal classical pyrochlore antiferromagnet without perturbations, an infinite degeneracy in a ground state leads to the absence of magnetic order and a spin-glass transition. Here we present Na3Mn(CO3)2Cl 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 magnetic order is detected in magnetization and heat capacity measurements or powder neutron diffraction experiments. On the other hand, antiferromagnetic short-range order of the nearest neighbors is evidenced by the Q dependence of the diffuse scattering which develops around 0.85Å-1. A high degeneracy near the ground state in Na3Mn(CO3)2Cl is supported by the magnetic entropy, estimated as almost 4JK-2mol-1 at 0.5 K.