2D and 3D magnetic behavior of Er in ${\mathrm{ErBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Neutron scattering has been used to study the magnetic order of the Er ions in superconducting ${\mathrm{ErBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. Above the 3D Néel temperature (${\mathrm{T}}_{\mathrm{N}}$=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 ${\mathrm{T}}_{\mathrm{N}}$ 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 (1/2,0,0), and the other by (1/2,0,1/2).