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

TL;DR

This study uses ${}^{31}$P NMR to explore quasi-2D magnetic correlations and domain behaviors in $T_2$P$_2$S$_6$ ($T$ = Mn, Ni), revealing anomalies, spin-flop transitions, and domain rotations related to magnetic ordering.

Contribution

The paper provides new insights into magnetic correlations, domain dynamics, and spin-flop phenomena in $T_2$P$_2$S$_6$ using ${}^{31}$P NMR, highlighting differences between Mn and Ni compounds.

Findings

Anomalous breakdown in ${}^{31}$P NMR susceptibility scaling near 117 K in Mn$_2$P$_2$S$_6$

Observation of spin-flop transition at 4 T in Mn$_2$P$_2$S$_6$

Detection of spin-flop-induced NMR spectral splitting at 14 T in Ni$_2$P$_2$S$_6$

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 $T_\mathrm{max} \sim 117$ 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 $(T_1T)^{-1}$ 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 N\'{e}el 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.