Static and dynamic magnetic properties of the spin-52triangle lattice antiferromagnet Na 3 Fe(PO 4 ) 2 studied by 31 P NMR.

31 P nuclear magnetic resonance (NMR) measurements have been carried out to investigate the magnetic properties and spin dynamics of Fe 3+ ( S = 5/2) spins in the two-dimensional triangular lattice (TL) compound Na 3 Fe(PO 4 ) 2 . The temperature ( T ) dependence of nuclear spin-lattice relaxation rates (1/T1) shows a clear peak around Néel temperature,TN=10.9K, corresponding to an antiferromagnetic (AFM) transition. From the temperature dependence of NMR shift ( K ) aboveTN, an exchange coupling between Fe 3+ spins was estimated to beJ/kB≃1.9K using the spin-5/2 Heisenberg isotropic-TL model. The temperature dependence of1/T1Tdivided by the magnetic susceptibility ( χ ),1/T1Tχ, aboveTNproves the AFM nature of spin fluctuations below∼50 Kin the paramagnetic state. In the magnetically ordered state belowTN, the characteristic rectangular shape of the NMR spectra is observed, indicative of a commensurate AFM state in its ground state. The strong temperature dependence of 1/ T 1 in the AFM state is well explained by the two-magnon (Raman) process of the spin waves in a 3D antiferromagnet with a spin-anisotropy energy gap of 5.7 K. The temperature dependence of sublattice magnetization is also well reproduced by the spin waves. Those results indicate that the magnetically ordered state of Na 3 Fe(PO 4 ) 2 is a conventional 3D AFM state, and no obvious spin frustration effects were detected in its ground state.