³¹P NMR study of Na₂CuP₂O₇: an S = 1/2 two dimensional Heisenberg antiferromagnetic system

The magnetic properties of Na2CuP2O7 were investigated by means of P-31 nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity measurements. We report the 31P 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 model with an intraplanar exchange of J/k(B) similar or equal to 18 +/- 2 K and a hyperfine coupling A = 3533 +/- 185 Oe/mu B. The P-31 NMR spectrum was found to broaden abruptly below T similar to 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 temperature, 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.