Toward two-dimensional ionic crystals with intrinsic ferromagnetism

Highlights: • The existence of 2D ionic crystals is explored. • Our model predicts the existence of 2D ferromagnetic semiconductors. • The mechanism of magnetic anisotropy in maintaining the magnetic order is revealed. Modern advanced technologies have been employed to synthesize new two-dimensional (2D) materials of various compositions. Since ionic bonds have no orientation, the ability of 2D ionic crystals to resist thermal fluctuation perpendicular to the 2D surface is worth studying. In this paper, we propose a theoretical analysis on the existence of 2D ionic EuS crystals. Although the Mermin-Wagner theorem denies the possibility of long-range magnetic order in 2D systems, our theoretical model predicts that 2D EuS crystals are ferromagnetic semiconductors. The mechanism of magnetic anisotropy in maintaining the magnetic order is uncovered. The model indicates the effect of the low-energy gap in the spin-wave spectrum on preventing thermal disturbance from destroying the ferromagnetism. A new type of magnetic interaction in 2D EuS crystals is revealed, which leads to the enhancement of Curie temperature by applying a gate voltage. Our theoretical research can benefit future research of other magnetic 2D crystals.