Static and dynamic magnetic properties of the spin- 52 triangle lattice antiferromagnet Na3Fe(PO4)2 studied by 31P NMR

Abstract

Abstract 31P nuclear magnetic resonance (NMR) measurements have been carried out to investigate the magnetic properties and spin dynamics of Fe3+ (S = 5/2) spins in the two-dimensional triangular lattice (TL) compound Na3Fe(PO4)2. The temperature (T) dependence of nuclear spin-lattice relaxation rates ( 1 / T 1 ) shows a clear peak around Néel temperature, T N = 10.9 K, corresponding to an antiferromagnetic (AFM) transition. From the temperature dependence of NMR shift (K) above T N , an exchange coupling between Fe3+ spins was estimated to be J / k B ≃ 1.9 K using the spin-5/2 Heisenberg isotropic-TL model. The temperature dependence of 1 / T 1 T divided by the magnetic susceptibility (χ), 1 / T 1 T χ , above T N proves the AFM nature of spin fluctuations below ∼ 50   K in the paramagnetic state. In the magnetically ordered state below T N , 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 Na3Fe(PO4)2 is a conventional 3D AFM state, and no obvious spin frustration effects were detected in its ground state.