Study of one-dimensional nature of S=1/2 (Sr,Ba)2Cu(PO4)2 and BaCuP2O7 via 31P NMR -: art. no. 174436

The magnetic behavior of the low-dimensional phosphates (Sr,Ba)(2)Cu(PO4)(2) and BaCuP2O7 was investigated by means of magnetic susceptibility and P-31 nuclear magnetic resonance (NMR) measurements. We present here the NMR shift, the spin-lattice (1/T-1), and spin-spin (1/T-2) relaxation-rate data over a wide temperature range 0.02 K <= T <= 300 K. The temperature dependence of the NMR shift K(T) is well described by the S = 1/2 Heisenberg antiferromagnetic chain model [D. C. Johnston, R. K. Kremer, M. Troyer, X. Wang, A. Klumper, S. L. Bud'ko, A. F. Panchula, and P. C. Canfield, Phys. Rev. B 61, 9558 (2000)] with an intrachain exchange of J/k(B) similar or equal to 165, 15 1, and 108 K in Sr2Cu(PO4)(2), Ba2Cu(PO4)(2), and BaCuP2O7, respectively. Deviations from Johnston's expression are seen for all these compounds in the T dependence of K(T) at low temperatures. P-31 is located symmetrically between the Cu ions and fluctuations of the staggered susceptibility at q = pi/a should be filtered out due to vanishing of the geometrical form factor. However, the qualitative temperature dependence of our P-31 NMR 1/T-1 indicates that relaxation due to fluctuations around q = pi/t (but not equal pi/a) have the same T dependence as those at q = pi/a and apparently dominate. Our measurements suggest the presence of magnetic ordering at 0.85 K in BaCuP2O7 (J/k(B) 108 K) and a clear indication of a phase transition (divergence) in 1/T-1 (T), 1/T-2(T), and a change of the line shape is observed. This enables us to investigate the one-dimensional (1D) behavior over a wide temperature range. We find that 1/T-1 is nearly T independent at low temperatures (1 K <= T <= 10 K), which is theoretically expected for 1D chains when relaxation is dominated by fluctuations of the staggered Susceptibility. At high temperatures, 1/T-1 varies nearly linearly with temperature, which accounts for contribution of the uniform susceptibility.