Superconductivity and 2-dimensional magnetism in orthorhombic and tetragonal GdBa₂Cu₃O_7-δ

Susceptibility, specific heat and resistance measurements have been performed on GdBa₂Cu₃O_7-δ, both in the orthorhombic phase which is a superconductor with a transition temperature of 93.0K, and in the tetragonal phase which is a semiconductor. We find that the large 7μ_B Gd moments have essentially no influence on the superconductivity. Because the tetragonal phase is non-superconducting we were able to study the magnetic properties at low temperatures without the complications of superconducting shielding currents. Our ac susceptibility and specific heat studies reveal a similar magnetic transition in both phases at T_N=2.24K to an antiferromagnetic order of the Gd moments with strong 2-dimensional Ising character.