  We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR)
field $H_r$, found in a square lattice of circular Permalloy dots when the
interdot distance $a$ gets comparable to the dot diameter $d$. The minimum
$H_r$, along the lattice $<11>$ axes, and the maximum, along the $<10>$ axes,
differ by $\sim$ 50 Oe at $a/d$ = 1.1. This anisotropy, not expected in
uniformly magnetized dots, is explained by a non-uniform magnetization
$\bm(\br)$ in a dot in response to dipolar forces in the patterned magnetic
structure. It is well described by an iterative solution of a continuous
variational procedure.
