2D and 3D magnetic behavior of Er in ErBa2Cu3O7.

Neutron scattering has been used to study the magnetic order of the Er ions in superconducting ErBa{sub 2}Cu{sub 3}O{sub 7}. Above the 3D Neel temperature ({ital T}{sub {ital N}}=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 {ital T}{sub {ital N}} the order parameter follows the exact Onsager solution for a 2D Ising antiferromagnet. At low {ital T}, two separate types of simple 3D antiferromagnetic structures are found, one characterized by a wave vector of (1/2,0,0), and the other by (1/2,0,1/2).