P31 NMR investigation of quasi-two-dimensional magnetic correlations in T2P2S6 ( T =Mn, Ni)

Abstract

We report the anomalous breakdown in the scaling of the microscopic magnetic susceptibility---as measured via the $^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) shift $K$---with the bulk magnetic susceptibility $\ensuremath{\chi}$ in the paramagnetic state of ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$. This anomaly occurs near ${T}_{\mathrm{max}}\ensuremath{\sim}117$ K the maximum in $\ensuremath{\chi}(T)$ and is therefore associated with the onset of quasi-two-dimensional (quasi-2D) magnetic correlations. The spin-lattice relaxation rate divided by temperature ${({T}_{1}T)}^{\ensuremath{-}1}$ in ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ exhibits broad peaklike behavior as a function of temperature, qualitatively following $\ensuremath{\chi}$, but displaying no evidence of critical slowing down above the N\'eel temperature ${T}_{N}$. In the magnetic state of ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, NMR spectra provide good evidence for ${60}^{\ensuremath{\circ}}$ 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 ${\mathrm{Mn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ is consistent with previous measurements and the two-dimensional anisotropic Heisenberg model. In a sample of ${\mathrm{Ni}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, we observe only two magnetically split resonances in the magnetic state, demonstrating that the multiple-peaked NMR spectra previously associated with ${60}^{\ensuremath{\circ}}$ rotation of stacking faults is sample-dependent. Finally, we report the observation of a spin-flop-induced splitting of the NMR spectra in ${\mathrm{Ni}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$, with an onset spin-flop field of ${\ensuremath{\mu}}_{0}{H}_{\mathrm{sf}}=14$ T.