The magnetic behavior of the low-dimensional phosphates ͑Sr, Ba͒ 2 Cu͑PO 4 ͒ 2 and BaCuP 2 O 7 was investigated by means of magnetic susceptibility and 31 P nuclear magnetic resonance ͑NMR͒ 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 ഛ T ഛ 300 K. The temperature dependence of the NMR shift K͑T͒ is well described by the S =1/2 Heisenberg antiferromagnetic chain model ͓D. C. Johnston, R. K. Kremer, M. Troyer, X. Wang, A. Klümper, S. L. Bud'ko, A. F. Panchula, and P. C. Canfield, Phys. Rev. B 61, 9558 ͑2000͔͒ with an intrachain exchange of J / k B Ӎ 165, 151, and 108 K in Sr 2 Cu͑PO 4 ͒ 2 , Ba 2 Cu͑PO 4 ͒ 2 , and BaCuP 2 O 7 , respectively. Deviations from Johnston's expression are seen for all these compounds in the T dependence of K͑T͒ at low temperatures. 31 P is located symmetrically between the Cu ions and fluctuations of the staggered susceptibility at q = / a should be filtered out due to vanishing of the geometrical form factor. However, the qualitative temperature dependence of our 31 P NMR 1 / T 1 indicates that relaxation due to fluctuations around q = / a ͑but / a͒ have the same T dependence as those at q = / a and apparently dominate. Our measurements suggest the presence of magnetic ordering at 0.85 K in BaCuP 2 O 7 ͑J / k B Ӎ 108 K͒ and a clear indication of a phase transition ͑divergence͒ in 1 / T 1 ͑T͒, 1/T 2 ͑T͒, and a change of the line shape is observed. This enables us to investigate the one-dimensional ͑1D͒ behavior over a wide temperature range. We find that 1 / T 1 is nearly T independent at low temperatures ͑1 Kഛ T ഛ 10 K͒, 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.

We have studied the magnetic behaviour of PrAu2(Si1-xGex)2 by means of magnetic susceptibility, resistivity, and heat capacity measurements, and x-ray and neutron powder diffraction. All compounds are isostructural and crystallize in the well known ThCr2Si2-type structure. PrAu2Si2 shows the characteristic features of a canonical spin glass with a freezing temperature of TF = 3 K. Completely unexpectedly, on introducing/increasing the atomic disorder by alloying with Ge, the spin-glass transition is suppressed as is first evident from a slight decrease of the freezing temperature TF up to x = 0.10. Long-range magnetic order sets in for x0.12. Within the range 0.15 x1, all compounds show the same simple AF-I-type antiferromagnetic structure with a monotonic increase of both the Néel temperature and the ordered magnetic moment for increasing Ge concentration. The magnetic phase diagram of PrAu2(Si1-xGex)2 is explained by the presence of weak disorder at a constant level at the Au sites. Clear indications of crystal-field effects have been observed.

We report on 45 Sc NMR experiments in the geometrically frustrated spinels FeSc 2 S 4 and MnSc 2 S 4 . The temperature dependences of line shift K͑T͒, linewidth ⌬͑T͒, spin-spin relaxation rate 1 / T 2 ͑T͒, and spin-lattice relaxation rate 1 / T 1 ͑T͒ are deduced. The manganese compound reveals the behavior of a long-range ordered antiferromagnet, but with the onset of critical spin fluctuations at unusually high temperatures of 40 T N . K͑T͒ of FeSc 2 S 4 exhibits a cusplike maximum, a hallmark of spin frustration. Orbital fluctuations strongly enhance 1 / T 1 ͑T͒ at elevated temperatures. Toward lowest temperatures these fluctuations slow down resulting in an increase of 1 / T 2 ͑T͒. The exponential decrease of 1 / T 1 ͑T͒ results from the opening of a spin gap ⌬ = 0.2 meV, a characteristic feature of a spin liquid.

The magnetic properties of Na 2 CuP 2 O 7 were investigated by means of 31 P nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity measurements. We report the 31 P NMR shift, the spin-lattice (1/T 1 ), and spin-spin (1/T 2 ) relaxation-rate data as a function of temperature T . The temperature dependence of the NMR shift K(T ) is well described by the S = 1/2 square lattice Heisenberg antiferromagnetic (HAF) model with an intraplanar exchange of J/k B ≃ (18 ± 2) K and a hyperfine coupling A = (3533 ± 185) Oe/µ B . The 31 P NMR spectrum was found to broaden abruptly below T ∼ 10 K signifying some kind of transition. However, no anomaly was noticed in the bulk susceptibility data down to 1.8 K. The heat capacity appears to have a weak maximum around 10 K. With decrease in temperatures, the spin-lattice relaxation rate 1/T 1 decreases monotonically and appears to agree well with the high temperature series expansion expression for a S =1/2 2D square lattice.

NMR measurements were performed on single-crystalline samples of the one-dimensional Heisenberg antiferromagnet ͑1D HAF͒ LiCuVO 4 . We investigated 7 Li and 51 V NMR spectra, deduced the respective lineshifts K͑T͒, spin-lattice relaxation rates 1 / T 1 , and spin-spin relaxation rates 1 / T 2 and 1 / T 2G . The results are compared to theoretical predictions for the relaxation rates 1 / T 1 ͑T͒ and 1 / T 2G ͑T͒ deduced from the dynamic susceptibility ͑q , ͒ of a 1D HAF spin chain as proposed by Sachdev ͓Phys. Rev. B 50, 13006 ͑1994͔͒. We document a correspondence between the anisotropic relaxation rates derived from electron paramagnetic resonance linewidth ⌬H͑T͒ and nuclear magnetic resonance 1 / T 1 ͑T͒, respectively, the former being well described in the framework of symmetric anisotropic exchange interactions for LiCuVO 4 by Krug von Nidda et al. ͓Phys. Rev. B 65, 134445 ͑2002͔͒.