Paramagnetic phase of the infinite-layer antiferromagnet Ca0.85Sr0.15CuO2 as seen by Cu NMR

We report a nuclear magnetic resonance study of the Cu nuclei in the paramagnetic phase of Ca0.85Sr0.15CuO2. By measuring the temperature dependence of the magnetic and quadrupolar shift and of the spin-lattice relaxation rate, we have obtained the following major results. With decreasing temperature, Ca0.85Sr0.15CuO2 undergoes two successive crossovers: a dimensionality crossover from a two-dimensional to a three-dimensional (3D) Heisenberg antiferromagnet followed, very close to T-N, by a spin-degree crossover from a 3D Heisenberg to a 3D XY antiferromagnet. There is no evidence for any structural changes accompanying the magnetic phase transition. The values of the hyperfine coupling constants A(c), A(ab), and B have been determined.