Linear Magnetoelectric Phase in Ultrathin MnPS3 Probed by Optical Second Harmonic Generation

The transition metal thiophosphates MPS3 (M = Mn, Fe, Ni) are a class of van der Waals stacked insulating antiferromagnets that can be exfoliated down to the ultrathin limit. MPS3 is particularly interesting because its Neel 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 MPS(3 )remains stable in the ultrathin limit. Using optical second harmonic generation rotational anisotropy, we show that long-range linear magnetoelectric type Neel order in MPS3 persists down to at least 5.3 nm thickness. However an unusual mirror symmetry breaking develops in ultrathin samples on SiO2 substrates that is absent in bulk materials, which is likely related to substrate induced strain.