${}^{31}$P NMR investigation of quasi-two-dimensional magnetic correlations in $T_2$P${}_2$S${}_6$ ($T$ = Mn & Ni)

Abstract

We report the anomalous breakdown in the scaling of the microscopic magnetic susceptibility, as measured via the ${}^{31}$P nuclear magnetic resonance (NMR) shift $K$, with the bulk magnetic susceptibility $\chi$ in the paramagnetic state of Mn${}_2$P${}_2$S${}_6$. This anomaly occurs near $T_{\max }\sim 117$ K the maximum in $\chi (T)$ and is therefore associated with the onset of quasi-two-dimensional (quasi-2D) magnetic correlations. The spin-lattice relaxation rate divided by temperature $(T_{1}T{)}^{-1}$ in Mn${}_2$P${}_2$S${}_6$ exhibits broad peak-like behavior as a function of temperature, qualitatively following $\chi$, but displaying no evidence of critical slowing down above the Néel temperature $T_N$. In the magnetic state of Mn${}_2$P${}_2$S${}_6$, NMR spectra provide good evidence for 60 degree rotation of stacking-fault-induced magnetic domains, as well as observation of the spin-flop transition that onsets at 4 T. The temperature-dependent critical behavior of the internal hyperfine field at the P site in Mn${}_2$P${}_2$S${}_6$ is consistent with previous measurements and the two-dimensional anisotropic Heisenberg model. In a sample of Ni${}_2$P${}_2$S${}_6$, we observe only two magnetically split resonances in the magnetic state, demonstrating that the multiple-peaked NMR spectra previously associated with 60 degree rotation of stacking faults is sample dependent. Finally, we report the observation of a spin-flop-induced splitting of the NMR spectra in Ni${}_2$P${}_2$S${}_6$, with an onset spin-flop field of $H_{\mathrm{sf}}=14$ T.