  We use neutron scattering to study the lattice and magnetic structure of the
layered half-doped manganite Pr$_{0.5}$Ca$_{1.5}$MnO$_4$. On cooling from high
temperature, the system first becomes charge- and orbital- ordered (CO/OO) near
$T_{CO}=300$ K and then develops checkerboard-like antiferromagnetic (AF) order
below $T_{N}=130$ K. At temperatures above $T_{N}$ but below $T_{CO}$
($T_N<T<T_{CO}$), the appearance of short-range AF spin correlations suppresses
the CO/OO induced orthorhombic strain, contrasting with other half-doped
manganites, where AF order has no observable effect on the lattice distortion.
These results suggest that a strong spin-lattice coupling and the competition
between AF exchange and CO/OO ordering ultimately determines the
low-temperature properties of the system.
