  Compounds with two-dimensional (2D) layers of magnetic ions weakly connected
by van der Waals bonding offer routes to enhance quantum behavior, stimulating
both fundamental and applied interest. CrPS4 is one such magnetic van der Waals
material, however, it has undergone only limited investigation. Here we present
a comprehensive series of neutron scattering measurements to determine the
magnetic structure and exchange interactions. The observed magnetic excitations
allow a high degree of constraint on the model parameters not normally
associated with measurements on a powder sample. The results demonstrate the 2D
nature of the magnetic interactions, while also revealing the importance of
interactions along 1D chains within the layers. The subtle role of competing
interactions is observed, which manifest in a non-trivial magnetic transition
and a tunable magnetic structure in a small applied magnetic field through a
spin-flop transition. Our results on the bulk compound provide insights that
can be applied to an understanding of the behavior of reduced layer CrPS4.
