Tuning magnetic and transport properties in quasi-2D (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ single crystals

We report an optimized chemical vapor transport method to grow single crystals of (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ where x = 0, 0.3, 0.5, 0.7&1. Single crystals up to 4\,mm\,$\times$\,3\,mm\,$\times$\,200\,$\mu$m were obtained by this method. As-grown crystals 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 crystallographic principal axes. Overall, the N\'eel temperature TN is non-monotonous, with increasing $Ni^{2+}$ doping the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn$_2$P$_2$S$_6$ (x = 0) up to x = 0.5, and then increases again to 155 K for pure Ni$_2$P$_2$S$_6$ (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ series. Transport studies under hydrostatic pressure on the parent compound Mn$_2$P$_2$S$_6$ evidence an insulator-metal transition at an applied critical pressure of ~22 GPa


Introduction
In recent years, research of functional two-dimensional (2D) materials has stimulated activities, aimed at synthesis of new materials and studies of their functionalities. This class has drawn a great deal of interest and attention chiefly because of their unique electronic [1][2][3], magnetic [4][5][6][7][8] and optical properties [10][11][12] to the bulk counterpart. Layered van der Waals (vdW) materials dominate among the current 2D materials. The vdW interaction only weakly couples the atomic layers together along the c axis, even in a bulk vdW material, and as a result, the electron confinement in the 2D lattice often leads to some specific properties, such as anisotropic magnetic behavior [13,14] and an anisotropic conductivity [15][16][17].
The transition metal phosphorus trichalcogenides (TM2P2Ch6 TM=transition metal; Ch=chalcogen) family belongs to the layered van der Waals materials class [23]. All of these materials have very similar crystal structures, a monoclinic unit cell with the space group of C2/m but different magnetic properties depending on the underlying 3d transition metal [24].
The TM2P2Ch6 compounds are all semiconductors at ambient pressure [23], with band gaps for most of them larger than 1 eV as determined by optical measurements [25] with extremely high room-temperature resistivities.
Band structure calculations [26,27] lead to the conclusions that TM2P2Ch6 are Mott insulators, and by applying external pressure, these materials could be driven to an insulatormetal or Mott transition [28]. These results further stimulated substantial interest to tune the parameters of the system to an intermediate state to access the physics that is not yet fully understood. Additionally, many superconducting materials are low dimensional and lie close to antiferromagnetic Mott-insulator phases in phase diagrams. Theoretical calculations [29] show that these states increase the possibility of introducing superconductivity. Thus, tunable 2D antiferromagnetic Mott insulators provide a promising playground to investigate the basic mechanisms of several unsolved problems in condensed matter physics.
Field-effect transistors based on bulk and few-layer Mn2P2S6 have been fabricated [30].
These devices show the potential for good ultraviolet photodetectors. Bulk and few-layer Mn2P2S6 on top of indium titanium oxide (ITO) coated Si substrate also shows tunneling transport phenomena [31]. The estimated barrier height of thin Mn2P2S6 flakes is 1.31 eV (±0.01) [31]. These investigations bring the magnetic van der Waals material Mn2P2S6 very close to various applications such as field effect transistors and magnetic tunnel junction fabrication.
Mn2P2S6 and Ni2P2S6 are well-known members of the transition metal phosphorus trichalcogenides family. They present interesting anisotropic antiferromagnetism below the Néel temperatures of 80 K for Mn2P2S6 and 155 K for Ni2P2S6 [32,33], which is not well understood. The main difference between these two pristine compounds and the key ingredient in the TM2P2Ch6 family is the underlying anisotropy that dictates the long-range magnetic order down to the monolayer, which eventually depends on the 3d transition metal ions [34][35][36][37]. This anisotropy of the system can be tuned systematically by several methods, for example with chemical substitution or with applied pressure. Motivated by this, here we present two different ways of tuning the ground state of Mn2P2S6 single crystals.
In this work we report the optimized synthesis and crystal growth conditions of (Mn1−xNix)2P2S6. We have investigated the crystal structure and magnetic properties of the series of single crystals. We observe two different effects on the magnetic properties namely a shift of the long-range order temperature TN and a broad anomaly at higher temperatures above TN, which becomes very sharp for selected compositions. Additional transport studies under hydrostatic pressure evidence an insulator-metal transition at a critical pressure of ~ 22 GPa.
The present article is organized in the following order. In the first section, we present the optimized methods for crystal growth for our series of crystals, (Mn1−xNix)2P2S6. In the later sections results of scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDX), x-ray diffraction (XRD) are shown. Finally, detailed magnetic and electrical transport measurements are presented.

Experimental methods and crystal growth
The composition of our crystals was determined using energy dispersive x-ray spectroscopy (EDX), with an accelerating voltage 30 kV. Scanning electron microscope was used to obtain electron microscopic images using two types of signals: the backscattered electrons (BSE) for chemical contrast and the secondary electrons (SE) for topographic contrast. X-ray powder diffraction data were collected using a STOE powder laboratory diffractometer in transmission geometry with Cu-1 radiation (the wavelength () is 1.540560 Å) from a curved Ge (111) single crystal monochromator and detected by a MYTHEN 1K 12.5 • linear position sensitive detector manufactured by DECTRIS. An XRD pattern of a polycrystalline sample was obtained by grinding as-grown single crystals.
Temperature and field dependent magnetization studies were performed using a Quantum Design Superconducting Quantum Interference Device Vibrating Sample Magnetometer (SQUID-VSM). The measurements were performed for field-cooled (FC) conditions and for two crystallographic directions, i.e., parallel and perpendicular to the growth direction of the crystal.
High-pressure transport experiments were performed in a screw-pressure-type DAC made of CuBe alloy. A pair of anvil culets of 300 μm was used. A mixture of epoxy and fine cubic boron nitride (c-BN) powder was compressed firmly to insulate the electrodes from the steel gasket. A single-crystal flake with dimensions of ~120 × 45 × 10 μm 3 was loaded together with NaCl fine powder and ruby powder. A five-probe configuration was utilized to measure the resistance and Hall resistance of the flake, where the external magnetic field was perpendicular to the surface of the flake. The ruby fluorescence shift was used to calibrate the pressure at room temperature in all experiments [38].
Single crystals of (Mn1−xNix)2P2S6 were grown by the chemical vapor transport technique. All preparation steps were performed in a glovebox, before sealing the ampule.
The starting material was loaded in a quartz ampoule (6 mm inner diameter, 2 mm wall thickness) and then was cooled by liquid nitrogen to avoid significant losses of the transport agent during the evacuation process to a residual pressure of 10 −8 bar. The ampoule was sealed under static pressure at a length of approximately 12 cm by the oxy-hydrogen flame. Then, the closed ampules were heated in a two-zone furnace. A similar approach was used by us for the crystal growth in the closely related sister compounds such as (Fe1-xNix)2P2S6 and AgCrP2S6. [39][40].
The optimized temperatures for Ni2P2S6 [36,39] were chosen as 750 • C and 700 • C for the hot and the cold zone, respectively, and for Mn2P2S6 [40][41] as 680 • C and 630 • C. For the substituted (Mn1−xNix)2P2S6 samples, due to the absence of published information, the following temperature profile has been optimized by us. Initially, the furnace was heated homogeneously to 300 • C with 100 • C/h and dwelled for 24 hours to provide a pre-reaction of P and S with the transition elements. After that, an inverse transport gradient is applied to transport particles adhering to the walls to the one side of the ampoule which is the charge region to avoid the formation of random nucleation centers. Later, the charge region was heated to 720 • C in 4 h with a dwelling time of 336 h, whereas, the sink region was initially heated up to 770 • C in 4 h, and dwelled at this temperature for 24 h. Later, the temperature in the growth zone was gradually reduced during one day to 670 • C to slowly form the transport temperature gradient for controlling nucleation and held at this temperature for 289 h. As a result, the temperature gradient was set for vapor transport between 720 • C (charge) and 670 • C (sink) for 12 days. In the final stage, the charge region was cooled to the sink temperature in 1 hour before both regions were furnace-cooled to room temperature.
Thin lustrous black and green in the Mn2P2S6 case plate-like crystals perpendicular to the c axis in the size of up to 4 mm × 3 mm × 200 μm were obtained (as shown in Fig. 2). All as-grown crystals show a layered morphology and they are easily exfoliated by scotch tape.

Characterization: compositional and structural analysis
All as-grown single crystals of the series exhibit the typical features of a layered van der Waals structure, which is in line with the pristine compounds (Fig. 1). As an example, crystals are shown in Fig. 1 (a-e) with clear hexagonal layers and a well-defined flat facade with 120° angles, which clearly indicates that the crystals grew along the symmetry axes.
Backscattered electron (BSE) images of our samples have no change of chemical contrast on the surface of the crystals, as shown in Fig. 1 (f-h). This indicates a homogeneous elemental composition on the respective area of the crystal.
To gain further information about the distribution of elements for our (Mn1−xNix)2P2S6 samples, an elemental mapping was performed on an area of the crystal with dimensions of about 150×100μm 2 as shown in Fig. 2 (a -f). The results indicate a homogeneous distribution of Mn, Ni, P and S.
The composition of the as-grown single crystals was determined by energy-dispersive x-ray spectroscopy (EDX). The result of EDX measurements highly depends on the sample topography [42]. EDX was mapped out via measuring different areas and points on the surface       Table 2 summarizes the results of the structural refinement and the lattice parameters.
Note that the presence of a high concentration of stacking faults is a well-known problem in layered vdW compounds and was observed for Ni2P2S6 in Ref. [45 -46]. Also stacking faults in the samples manifest by an asymmetry of the 00l reflections in the pXRD patterns. This might explain the high value of our R-factor from our x-ray analysis.   or Zn 2+ . In that series, however, for x > 0.3 a percolation threshold was observed in Ref. [46][47][48][49]. In our (Fe1-xNix)2P2S6 substitution series [39], we also observed a gradual evolution of the magnetic transition temperature up to x = 1. Note that while TN shifts in both series, (Mn1−xNix)2P2S6 and (Fe1-xNix)2P2S6, a minimum in the TN(x) dependence was only observed for (Mn1-xNix)2P2S6. Also, the magnetic anisotropy as well as the broad maximum at Tmax show two distinctly different behaviors for (Fe1-xNix)2P2S6, one up to xNi = 0.9 which is similar to the magnetic anisotropy of Fe2P2S6 and another one for Ni2P2S6, whereas a gradual but nonmonotonous behavior is observed for Tmax(x) in our (Mn1-xNix)2P2S6 series.
Note that the parent compound Ni2P2S6 has a broad maximum well above TN, which gets more pronounced for Mn substitution and becomes the dominant feature for Mn2P2S6.
The shape of the maximum seems to be determined by the difference between Tmax and the inflection point. The smaller the difference, the sharper the maximum becomes. Ni2P2S6 shows the highest Tmax as well as the highest TN, whereas (Mn0.5Ni0.5)2P2S6 shows the lowest Tmax and TN and which appear very close to each other in contrast to both parent compounds.

Transport properties:
Electrical resistivity, Hall and magnetotransport measurements were performed on single crystals of Mn2P2S6. Figure 10 shows the temperature-dependent resistance R(T) of our Mn2P2S6 single crystal at various pressures up to 59.0 GPa. As shown in Fig. 10a, Mn2P2S6 displays an insulating behavior at 18.8 GPa, similar to the early reports from Wang et al. [51].
With increasing pressure up to 22.2 GPa, the resistance in the whole temperature range decreases remarkably and a metal-insulator transition appears at TMIT~250 K for 22.2 GPa (see Fig. 10b). One can see that TMIT initially decreases up to 25 GPa and then increases in the pressure range 25.0-30.9 GPa. Notably, the sample exhibits a metallic behavior in the whole temperature range up to room temperature at 34.1 GPa. As shown in Fig. 10c, the metallic behavior maintains with increasing pressure up to 59.0 GPa and no traces of superconductivity are detected down to 1.8 K.
High-pressure Hall resistance and magnetoresistance measurements were further performed to detect the evolution of charge carriers in pressurized Mn2P2S6. Figure 11a shows  around zero field, decreases initially with increasing pressure and then increases above 30.9 GPa (see Fig. 11b). Figure

Conclusions
In summary, we have successfully grown single crystals of (Mn1−xNix)2P2S6 via the chemical vapor transport technique using Iodine as a transport agent. The optimized crystal growth conditions yielded a series of (Mn1−xNix)2P2S6 with (x=0, 0.3, 0.5, 0.7, 1) single crystals up to 4 mm × 3 mm × 200 μm. As-grown crystals obtained by this method were The magnetoresistance measured at 10K shows a negative magnetoresistance. The successful growth of high-quality single crystals of our series (Mn1−xNix)2P2S6 opens an opportunity for further anisotropic investigations in the future.