Study of one-dimensional nature of S =1/2 ÑSr, BaÖ 2 CuÑPO 4 Ö 2 and BaCuP 2 O 7 via 31 P NMR

The magnetic behavior of the low-dimensional phosphates sSr, Bad2CusPO4d2 and BaCuP2O7 was investigated by means of magnetic susceptibility and 31 P nuclear magnetic resonance sNMRd measurements. We present here the NMR shift, the spin-lattice s 1/ T1d, and spin-spin s 1/ T2d relaxation-rate data over a wide temperature range 0.02 K T 300 K. The temperature dependence of the NMR shift KsTd is well described by the S =1/2 Heisenberg antiferromagnetic chain model fD. 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 s2000dg with an intrachain exchange of J / kB. 165, 151, and 108 K in Sr2CusPO4d2 ,B a 2CusPO4d2, and BaCuP2O7, respectively. Deviations from Johnston’s expression are seen for all these compounds in the T dependence of KsTd at low temperatures. 31 P is located symmetrically between the Cu ions and fluctuations of the staggered susceptibility at q=p / a should be filtered out due to vanishing of the geometrical form factor. However, the qualitative temperature dependence of our 31 P NMR 1 /T1 indicates that relaxation due to fluctuations around q =p / a sbut fip / ad have the same T dependence as those at q= p / a and apparently dominate. Our measurements suggest the presence of magnetic ordering at 0.85 K in BaCuP2O7 sJ / kB. 108 Kd and a clear indication of a phase transition sdivergenced in 1 / T1sTd ,1 /T2sTd, and a change of the line shape is observed. This enables us to investigate the one-dimensional s1Dd behavior over a wide temperature range. We find that 1 /T1 is nearly T independent at low temperatures s 1K T 10 Kd, which is theoretically expected for 1D chains when relaxation is dominated by fluctuations of the staggered susceptibility. At high temperatures, 1 / T1 varies nearly linearly with temperature, which accounts for contribution of the uniform susceptibility.