Linear Magnetoelectric Phase in Ultrathin ${\mathrm{MnPS}}_3$ Probed by Optical Second Harmonic Generation

The transition metal thiophosphates $M{\mathrm{PS}}_3$ ($M=\mathrm{Mn}$, Fe, Ni) are a class of van der Waals stacked insulating antiferromagnets that can be exfoliated down to the ultrathin limit. ${\mathrm{MnPS}}_3$ is particularly interesting because its Néel ordered state breaks both spatial-inversion and time-reversal symmetries, allowing for a linear magnetoelectric phase that is rare among van der Waals materials. However, it is unknown whether this unique magnetic structure of bulk ${\mathrm{MnPS}}_3$ remains stable in the ultrathin limit. Using optical second harmonic generation rotational anisotropy, we show that long-range linear magnetoelectric type Néel order in ${\mathrm{MnPS}}_3$ persists down to at least 5.3 nm thickness. However an unusual mirror symmetry breaking develops in ultrathin samples on ${\mathrm{SiO}}_2$ substrates that is absent in bulk materials, which is likely related to substrate induced strain.

Figure 1

Crystal and magnetic structure of $M{\mathrm{PS}}_3$. $M{\mathrm{PS}}_3$ lattice viewed along the (a) $c$ and (b) $b$ axis. Adjacent $ab$ planes are displaced by $a/3$ along the $\widehat{a}$ direction. (c) AF structures of $M{\mathrm{PS}}_3$. Arrows denote spin orientation. Star denotes an inversion center of the AF structure. The inset shows the in-plane orientation of the incident electric field.

Figure 2

SHG-RA patterns and long-range Néel order in ${\mathrm{MnPS}}_3$. (a) SHG-RA patterns of $M{\mathrm{PS}}_3$ above and (b) below their respective AF ordering temperatures: $T_{\mathrm{AF}}=78$ (${\mathrm{MnPS}}_3$), 123 (${\mathrm{FePS}}_3$), and 155 K (${\mathrm{NiPS}}_3$). Solid circles are experimental data and the solid lines are best fits to the phenomenological model described in the main text. For $T>T_{\mathrm{AF}}$, all data were fit using only an EQ term. For $T<T_{\mathrm{AF}}$, the ${\mathrm{FePS}}_3$ and ${\mathrm{NiPS}}_3$ data were fit using only an EQ term, whereas the ${\mathrm{MnPS}}_3$ data were fit using a coherent sum of an EQ and ED term. (c) Temperature dependence of the SHG intensity along the $\varphi =60°$ direction. Solid line on the ${\mathrm{MnPS}}_3$ data is a best fit to the power law function described in the main text, which accounts for a constant EQ term and a temperature dependent ED term.

Figure 3

${\mathrm{MnPS}}_3$ nanodevice for probing long-range Néel order. (a) Optical image of exfoliated ${\mathrm{MnPS}}_3$ flakes on ${\mathrm{SiO}}_2$ with gold ring and electrode on top to improve cooling efficiency. The 5.3 and 12.5 nm flakes are found within the white box. (b) Atomic force microscopy scan of the area bounded by the white box in (a). Green lines indicate the positions of line scans, with corresponding magnified line profiles shown in white. (c) SHG-RA patterns from various regions of the device at 10 K.

Figure 4

Long-range Néel order in few-layer ${\mathrm{MnPS}}_3$. (a)–(c) Temperature dependent SHG-RA patterns from ${\mathrm{MnPS}}_3$ flakes of different thicknesses. (d) Normalized temperature dependence of SHG Intensity along the $\varphi =0°$ direction.