Two-dimensional
(2D) ferromagnetic materials with high spin polarization
are highly desirable for spintronic devices. 2D Janus materials exhibit
novel properties due to their broken symmetry. However, the electronic
structure and magnetic properties of 2D Janus magnetic materials with
high spin polarization are still unclear. Inspired by the successful
synthesis of a ferromagnetic FeCl2 monolayer and 2D Janus
MoSSe and WSSe, we systematically study the electronic structure and
magnetic properties of Janus FeXY (X, Y = Cl, Br, and I, X ≠
Y) monolayers. Based on the Goodenough–Kanamori–Anderson
theory, the ferromagnetism stems from the superexchange interaction
mediated by Fe–X/Y–Fe bonds. The band gaps of spin-up
channels are large enough (>4 eV) to prevent spin flipping, which
is beneficial for spintronic devices. Additionally, the sizable magnetocrystalline
anisotropy energy (MAE) indicates that Janus FeXY monolayers are suitable
for information storage. More importantly, the half-metallic character
is still kept in Janus FeXY monolayers, and their magnetic properties
are enhanced by the biaxial compressive strain. The MAE of FeClI and
FeBrI increases by 1 order of magnitude, and the Curie temperature
of FeXY monolayers enhances by 100%. These results provide an example
of the 2D Janus half-metallic materials and enrich the 2D magnetic
material library.