  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)
Ba2CoO4. In spite of isolated CoO4 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 TN = 26 K. More
unexpectedly, spin dynamics displays quasi-2D spin wave dispersion with an
unusually large spin gap, and 1D magnetoelastic coupling. Our results indicate
that Ba2CoO4 is a unique system for exploring the interplay between isolated
polyhedra, low-dimensional magnetism, and novel spin states in oxides.
