Spin dynamics and antiferromagnetic order in PrBa2Cu4O8 studied by Cu   nuclear respnance
  Results of the nuclear resonance experiments for the planar Cu sites in PrBa2Cu4O8 are presented. The NMR spectrum at 1.5 K in zero magnetic field revealed an internal field of 6.1 T, providing evidence for an antiferromagnetic order of the planar Cu spins. This confirms that the CuO2 planes are insulating, therefore, the metallic conduction in this material is entirely due to the one-dimensional zigzag Cu2O2 chains. The results of the spin-lattice relaxation rates measured by zero field NQR above 245 K in the paramagnetic state are explained by the theory for a Heisenberg model on a square lattice. 
S. Fujiyama, M. Takigawa, T. Suzuki, N. Yamada, S. Horii, and Y.   Yamada