Anomalous magnetic behavior of Ba2CoO4 with isolated CoO4 tetrahedra

The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) ${\mathrm{Ba}}₂{\mathrm{CoO}}₄$. In spite of isolated ${\mathrm{CoO}}₄$ tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in the ground state with an anomalously large Curie-Weiss temperature of 110 K compared to ${T}_{\mathrm{N}}=26$ K. More unexpectedly, spin dynamics displays quasi-two-dimensional spin-wave dispersion with an unusually large spin gap, and one-dimensional magnetoelastic coupling. Our results indicate that ${\mathrm{Ba}}₂{\mathrm{CoO}}₄$ is a unique system for exploring the interplay between isolated polyhedra, low-dimensional magnetism, and spin states in oxides.