  Ferromagnetism in two-dimensional materials presents a promising platform for
the development of ultrathin spintronic devices with advanced functionalities.
Recently discovered ferromagnetic van der Waals crystals such as CrI$_{3}$,
readily isolated two-dimensional crystals, are highly tunable through external
fields or structural modifications. However, there remains a challenge because
of material instability under air exposure. Here, we report the observation of
an air stable and layer dependent ferromagnetic (FM) van der Waals crystal,
CrPS$_{4}$, using magneto-optic Kerr effect microscopy. In contrast to the
antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in
the monolayer crystal. Furthermore, alternating AFM and FM properties observed
in even and odd layers suggest robust antiferromagnetic exchange interactions
between layers. The observed ferromagnetism in these crystals remains resilient
even after the air exposure of about a day, providing possibilities for the
practical applications of van der Waals spintronics.
