Highly sensitive spin-flop transition in antiferromagnetic van der Waals material $M\mathrm{P}{\mathrm{S}}_3$ ($M=\mathrm{Ni}$ and Mn)

Recent developments in two-dimensional (2D) magnetism have motivated the search for van der Waals (vdW) magnetic materials to explore magnetic phenomena in the 2D limit. Metal thiophosphates, $M\mathrm{P}{X}_3$, are a class of magnetic vdW materials with antiferromagnetic ordering persisting down to the atomically thin limit. The magnetism in this material family has been found to be highly dependent on the choice of transition metal $M$. In this work, we have synthesized the intermediate compounds ${\mathrm{Ni}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ ($0\le x\le 1$) and investigated their magnetic properties. Our study reveals that the variation of Ni and Mn content in ${\mathrm{Ni}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ can efficiently tune the spin-flop transition, likely due to the modulation of the magnetic anisotropy. Such effective tuning offers a promising candidate to engineer 2D magnetism for future device applications.

Figure 1

(a) Single-crystal x-ray diffraction pattern for $\mathrm{N}{\mathrm{i}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ showing the ($00L$) reflections. Right panels show (001) diffraction peak. Inset: Optical microscope images of the single crystals. (b) Crystal structure of $M\mathrm{P}{\mathrm{S}}_3$ ($M=\mathrm{Ni}/\mathrm{Mn}$).

Figure 2

Field induced spin-flop transitions in pristine $\mathrm{MnP}{\mathrm{S}}_3$ and $\mathrm{NiP}{\mathrm{S}}_3$. (a), (b) Isothermal magnetization of $\mathrm{MnP}{\mathrm{S}}_3$ at different temperatures under (a) $H\perp ab$ and (b) $H\parallel ab$. Inset in (b): magnetic structure of $\mathrm{MnP}{\mathrm{S}}_3$. (c), (d) Isothermal magnetization of $\mathrm{NiP}{\mathrm{S}}_3$ at different temperatures under (c) $H\perp ab$ and (d) $H\parallel ab$. Upper inset in (c): 3D view of magnetic structure of $\mathrm{NiP}{\mathrm{S}}_3.$ Lower inset in (c): Top view of magnetic structure of $\mathrm{NiP}{\mathrm{S}}_3$. Inset in (d): Isothermal magnetization at 50–140 K measured with $H\parallel ab$. The same color code is used to indicate temperatures for (a)–(d). (e) Isothermal magnetization at 2 K under $H\parallel ab$ with different in-plane field orientations. Inset: Optical microscope image of $\mathrm{NiP}{\mathrm{S}}_3$ single crystal with arrows pointing to the applied field direction. (f) Temperature dependence of susceptibility of $\mathrm{NiP}{\mathrm{S}}_3$ measure with 1, 3, 5, 7, 8, and 9 T fields, measured with $H\parallel ab$. Inset: Temperature dependence of susceptibility at 9 T under $H\parallel ab$. The solid and dashed lines represent zero-field cooled (ZFC) and field cooled (FC) data, respectively.

Figure 3

(a), (b) Isothermal magnetization of $\mathrm{N}{\mathrm{i}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ samples ($0\le x\le 1$) at $T=2\mathrm{K}$ under (a) $H\parallel ab$ and (b) $H\perp ab$. The same color code is used to indicate temperatures for (a), (b). (c) Trigonal distortion in $M\mathrm{P}{\mathrm{S}}_3$ compounds; $z$ axis is the trigonal axis. (d) Isothermal magnetization of $\mathrm{N}{\mathrm{i}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ ($x=0$ and 0.05) at $T=2\mathrm{K}$ under $H\parallel ab$ (Left panel) and $H\perp ab$ (Right panel). Inset: zoom-in view of the low-field magnetization. (e) Isothermal magnetization of $\mathrm{N}{\mathrm{i}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ ($x=0.95$ and 1) at $T=2\mathrm{K}$ under $H\parallel ab$ (Left panel) and $H\perp ab$ (Right panel). Inset: zoom in of the low-field magnetization.

Figure 4

(a), (c) Temperature dependence of out of plane ($H\perp ab$, red, upper panels) and in-plane ($H\parallel ab$, black, lower panels) susceptibilities of (a) ${\mathrm{Ni}}_{0.95}{\mathrm{Mn}}_{0.05}\mathrm{P}{\mathrm{S}}_3$, (b) ${\mathrm{Ni}}_{0.1}{\mathrm{Mn}}_{0.9}\mathrm{P}{\mathrm{S}}_3$, and (c) ${\mathrm{Ni}}_{0.05}{\mathrm{Mn}}_{0.95}\mathrm{P}{\mathrm{S}}_3$ samples. The dashed lines denote $T_{\mathrm{N}}$. (d) Temperature dependence of heat capacity of ${\mathrm{Ni}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ samples with $x=0$, 0.1, 0.2, 0.8, and 1. The black triangles denote $T_{\mathrm{N}}$. Data are shifted for clarity. (e) Magnetic phase diagram of $\mathrm{N}{\mathrm{i}}_{1\text{−}x}{\mathrm{Mn}}_x\mathrm{P}{\mathrm{S}}_3$ ($0\le x\le 1$). The transition temperatures are determined by susceptibility measurements on single ($\chi$-single crystals) and polycrystals ($\chi$-polycrystals), and heat capacity ($C$).