An ultrasharp photoluminescence line intimately related to antiferromagnetic order has been found in ${\mathrm{NiPS}}_3$, a correlated van der Waals material, opening prospects for magneto-optical coupling schemes and spintronic applications. Here we unambiguously clarify the singlet origin of this excitation, confirming its roots in the spin structure. Based on a comprehensive investigation of the electronic structure using angle-resolved photoemission and $q$-dependent electron energy loss spectroscopy as experimental tools we develop, in a first step, an adequate theoretical understanding using density functional theory (DFT). In a second step the DFT is used as input for a dedicated multiplet theory by which we achieve excellent agreement with available multiplet spectroscopy. Our Letter connects the understanding of the electronic structure and of optical processes in ${\mathrm{NiPS}}_3$ and related materials as a prerequisite for further progress of the field.

We report the crystal growth by chemical vapor transport together with a thorough structural and magnetic characterization of the quasi-2D magnets ${\left({\mathrm{Fe}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{P}}_2{\mathrm{S}}_6$ with $x_{\text{Ni}}=\text{0,}\text{0.3,}\text{0.5,}\text{0.7,}\text{0.9}$, and 1. As-grown crystals exhibit a layered morphology with weak van der Waals interactions between layers parallel to the crystallographic $ab$ plane of the monoclinic structure in the space group $C2/m$ (No. 12). Magnetization measurements reveal an antiferromagnetic ground state for all grown crystals. In the ordered state, the magnetization along different crystallographic directions is in agreement with an Ising anisotropy for $0\le x\le 0.9$ and only for ${\mathrm{Ni}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ a different behavior is observed which is in line with an anisotropic Heisenberg or $XXZ$ model description. This striking abrupt change of anisotropy at a critical Ni concentration is coupled to a change in the width of the maximum in the magnetization around $T_{\text{max}}$. Furthermore, all intermediate compounds $0<x\le 0.9$ exhibit an anisotropic magnetization already in the paramagnetic state similar to the parent compound ${\mathrm{Fe}}_2{\mathrm{P}}_2{\mathrm{S}}_6$.

We report an optimized chemical vapor transport method to grow single crystals of (Mn_1−xNi_x)₂P₂S₆ where x = 0, 0.3, 0.5, 0.7, and 1. Single crystals up to 4 mm × 3 mm × 200 μm were obtained by this method. As-grown crystals are characterized by means of scanning electron microscopy and powder X-ray diffraction measurements. The structural characterization shows that all crystals crystallize in monoclinic symmetry with the space group C2/m (No. 12). We have further investigated the magnetic properties of this series of single crystals. The magnetic measurements of the all as-grown single crystals show long-range antiferromagnetic order along all principal crystallographic axes. Overall, the Néel temperature T_N is non-monotonous; with increasing Ni²⁺ doping, the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn₂P₂S₆ (x = 0) up to x = 0.5 and then increases again to 155 K for pure Ni₂P₂S₆ (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn_1−xNi_x)₂P₂S₆ series. Transport studies under hydrostatic pressure on the parent compound Mn₂P₂S₆ evidence an insulator-metal transition at an applied critical pressure of ~22 GPa.

We report optimized crystal growth conditions for the quaternary compound ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive x-ray spectroscopy and powder x-ray diffraction. ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ is structurally closely related to the $M_2{\mathrm{P}}_2{\mathrm{S}}_6$ family ($M$: transition metal), which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a platelike, layered morphology as well as a hexagonal habitus. We present successful exfoliation of such as-grown crystals down to thicknesses of 2.6 nm corresponding to four layers. ${\mathrm{CuCrP}}_2{\mathrm{S}}_6$ crystallizes in the monoclinic space group $C2/c$. Magnetization measurements reveal an antiferromagnetic ground state with $T_{\text{N}}\approx 30$ K and a positive Curie-Weiss temperature in agreement with dominant ferromagnetic intralayer coupling. Specific heat measurements confirm this magnetic phase transition and the magnetic order is suppressed in an external magnetic field of about 6 T (8 T) applied parallel (perpendicular) to the $ab$ plane. At higher temperatures between 140–200 K, additional broad anomalies associated with structural changes accompanying antiferroelectric ordering are detected in our specific heat studies.

The quasi-two-dimensional antiferromagnet ${\mathrm{Ni}}_{\text{2}}{\mathrm{P}}_{\text{2}}{\mathrm{S}}_{\text{6}}$ belongs to the family of magnetic van der Waals compounds that provides a rich material base for the realization of fundamental models of quantum magnetism in low dimensions. Here, we report high-frequency/high-magnetic field electron spin resonance measurements on single crystals of ${\mathrm{Ni}}_{\text{2}}{\mathrm{P}}_{\text{2}}{\mathrm{S}}_{\text{6}}$. The results enable one to reliably determine the positive, “easy”-plane type of the single ion anisotropy of the ${\mathrm{Ni}}^{2+}$ ions with the upper limit of its magnitude $\lesssim 0.7\mathrm{meV}$. The resonance response reveals strongly anisotropic spin fluctuations setting in shortly above the Néel temperature $T_{\mathrm{N}}=158\mathrm{K}$ and extending in the antiferromagnetically ordered state down to low temperatures. There, a low-energy magnon excitation gapped from the ground state by only $\sim 1\mathrm{meV}$ was found. This magnon mode may explain unusual low temperature relaxation processes observed in the latest nuclear magnetic resonance experiments and together with the estimate of the single ion anisotropy they should enable setting up a realistic spin model for an accurate description of magnetic properties of ${\mathrm{Ni}}_{\text{2}}{\mathrm{P}}_{\text{2}}{\mathrm{S}}_{\text{6}}$.

We report optimized crystal growth conditions for the quarternary compound AgCrP${}_2$S${}_6$ by chemical vapor transport. Compositional and structural characterization of the obtained crystals were carried out by means of energy-dispersive X-ray spectroscopy and powder X-ray diffraction. AgCrP${}_2$S${}_6$ is structurally closely related to the $M_2$P${}_2$S${}_6$ family, which contains several compounds that are under investigation as 2D magnets. As-grown crystals exhibit a plate-like, layered morphology as well as a hexagonal habitus. AgCrP${}_2$S${}_6$ crystallizes in monoclinic symmetry in the space group $P2/a$ (No. 13). The successful growth of large high-quality single crystals paves the way for further investigations of low dimensional magnetism and its anisotropies in the future and may further allow for the manufacturing of few-layer (or even monolayer) samples by exfoliation.