Neutron scattering has been used to study the magnetic order of the Er ions in superconducting ErBa2Cu307. Above the 3D Neel temperature (7"^^ =0.618 K) a rod of scattering characteristic of 2D behavior is unambiguously observed, showing that the magnetic interactions of the rare-earth ions are highly anisotropic; below TN the order parameter follows the exact Onsager solution for a 2D Ising antiferromagnet. At low T, two separate types of simple 3D antiferromagnetic structures are found, one characterized by a wave vector of (y ,0,0), and the other by (y ,0, y ).

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