Static and dynamic magnetic properties of the spin-5/2 triangle lattice antiferromagnet Na3Fe(PO4)2 studied by 31P NMR

Abstract

${}^{31}$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 ($1/T_1$) shows a clear peak around Néel temperature, $T_{\mathrm{N}}=10.9$~K, corresponding to an antiferromagnetic (AFM) transition. From the temparature dependence of NMR shift ($K$) above $T_{\mathrm{N}}$, an exchange coupling between Fe${}^{3+}$ spins was estimated to be $J/k_{\mathrm{B}}\simeq 1.9$~K using the spin-5/2 Heisenberg isotropic-TL model. The temperature dependence of $1/T_{1}T$ divided by the magnetic susceptibility ($\chi$), $1/T_{1}T\chi$, above $T_{\mathrm{N}}$ proves the AFM nature of spin fluctuations below $\sim$ 50 K in the paramagnetic state. In the magnetically ordered state below $T_{\mathrm{N}}$, 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 Na${}_3$Fe(PO${}_4$)${}_2$ is a conventional 3D AFM state, and no obvious spin frustration effects were detected in its ground state.

arXiv comment17 pages, 9 figures

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Cited by in corpus (1)

• Short-range Spin Freezing State in the Double Trillium Lattice Spin-Liquid Candidate KSrFe${}_2$(PO${}_4$)${}_3$ Revealed via ${}^{31}$P NMR