Collinear order in the spin- 52 triangular-lattice antiferromagnet Na3Fe(PO4)2

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

“…In this paper, we report the results of 31 P NMR measurement on the polycrystalline Na 3 Fe(PO 4 ) 2 sample in the temperature range of T = 1.5 − 300 K. The NMR spectra, nuclear spin-lattice relaxation rate 1/T 1 and nuclear spin-spin relaxation rate 1/T 2 measurements show the AFM long-range order below T N ∼ 11 K. In the paramagnetic state above T N , from the analysis of NMR shift K(T) using the high-temperature series expansion, J/k B was estimated to be ≃1.9 K, which is consistent with the previously reported value of J/k B ≃ 1.8 K. From the temperature dependences of 1/T 1 and 1/T 2 in the paramagnetic state above T N , AFM spin fluctuations were found to be developed below ∼50 K. In the AFM ordered state, we observed a rectangular shape of the spectrum, evidencing a commensurate AFM state. However, additional peaks in the spectra in the AFM state were observed above 3 T, which are probably due to the spin flop reported previously [11]. The temperature dependences of the sublattice magnetization and 1/T 1 in the AFM state were well explained by spin waves for a conventional three-dimensional (3D) antiferromagnet with a finite spin-anisotropy gap ∼5.7 K, indicating that the AFM state is rigid without any obvious effects of spin frustrations in its ground state.…”

“…Polycrystalline Na 3 Fe(PO 4 ) 2 sample was synthesized using the conventional solid-state method [11]. NMR measurements of 31 P (I = 1 2 , γN 2π = 17.237 MHz/T) nuclei were conducted using a laboratory-built phase-coherent spin-echo pulse spectrometer.…”

“…Na 3 Fe(PO 4 ) 2 crystallizes in a monoclinic space group (C2/c) with the lattice parameters of a = 9.0714(2) Å, b = 5.032(1) Å, and c = 13.8683(3) Å at room temperature [9,10]. Fe 3+ ions (S = 5/2) in distorted FeO 6 octahedra form an S = 5/2 anisotropic TL via corner-sharing with PO 4 tetrahedra in the ab plane as shown in figure 1, where the triangles are slightly distorted due to the two different Fe-Fe distances of 5.182 Å and 5.028 Å [11]. Recently, Sebastian et al reported that Na 3 Fe(PO 4 ) 2 exhibits an antiferromagnetic (AFM) ordering with a Néel temperature of T N = 10.6 K with a reduced Fe ordered magnetic moment of ∼1.52µ B at 1.6 K in zero magnetic field [11].…”

“…Fe 3+ ions (S = 5/2) in distorted FeO 6 octahedra form an S = 5/2 anisotropic TL via corner-sharing with PO 4 tetrahedra in the ab plane as shown in figure 1, where the triangles are slightly distorted due to the two different Fe-Fe distances of 5.182 Å and 5.028 Å [11]. Recently, Sebastian et al reported that Na 3 Fe(PO 4 ) 2 exhibits an antiferromagnetic (AFM) ordering with a Néel temperature of T N = 10.6 K with a reduced Fe ordered magnetic moment of ∼1.52µ B at 1.6 K in zero magnetic field [11]. The average exchange coupling between Fe 3+ spins was estimated to be J/k B ≃ 1.8 K by fitting magnetic susceptibility χ(T) data using the hightemperature series expansion for a spin-5/2 Heisenberg TL model.…”

“…The average exchange coupling between Fe 3+ spins was estimated to be J/k B ≃ 1.8 K by fitting magnetic susceptibility χ(T) data using the hightemperature series expansion for a spin-5/2 Heisenberg TL model. The authors also determined, from neutron diffraction measurements, a collinear AFM structure with a propagation vector q = (1, 0, 0) where the Fe spins are coupled ferromagnetically along the b axis or in the [100] plane [11]. However, the detailed magnetic properties, especially magnetic fluctuations, of Na 3 Fe(PO 4 ) 2 have still not been investigated.…”