2-DIMENSIONAL AND 3-DIMENSIONAL MAGNETIC ORDER OF ER IN SUPERCONDUCTING ERBA2CU3O7

Neutron diffraction has been used to study the magnetic fluctuations and long-range order of the Er ions in single crystals of superconducting ErBa2Cu3O7. The temperature dependence of the rod of scattering, characteristic of 2D behavior, has been measured above and below the 3D Neel temperature (T(N) congruent-to 0.62 K). This rod intensity is observed to increase as the temperature decreases until T(N) is reached, and then the intensity decreases rapidly below T(N). The 2D magnetic correlation length, which is obtained from measurements of the width of the rod, grows continuously with decreasing temperature, then reaches a resolution-limited maximum at the Neel temperature when long-range magnetic order sets in. Below T(N) the sublattice magnetization, which is a measure of the thermodynamic order parameter, follows Onsager's exact solution of the 2D S = 1/2 Ising model. At low T, a single type of simple 3D antiferromagnetic structure is found, characterized by a wave vector of (1/2 0 1/2).