  The magnetic anisotropy of thin (~ 200 nm) and thick (~ 2 $\mu$m) films and
of polycrystalline (diameters ~ 60 nm) powders of the Prussian blue analogue
Rb$_{0.7}$Ni$_{4.0}$[Cr(CN)$_6$]$_{2.9} \cdot n$H$_2$O, a ferromagnetic
material with $T_c \sim 70$ K, have been investigated by magnetization, ESR at
50 GHz and 116 GHz, and variable-temperature x-ray diffraction (XRD). The
origin of the anisotropic magnetic response cannot be attributed to the direct
influence of the solid support, but the film growth protocol that preserves an
organized two-dimensional film is important. In addition, the anisotropy does
not arise from an anisotropic g-tensor nor from magneto-lattice variations
above and below $T_c$. By considering effects due to magnetic domains and
demagnetization factors, the analysis provides reasonable descriptions of the
low and high field data, thereby identifying the origin of the magnetic
anisotropy.
