  Imaging the magnetic fields around a non-magnetic impurity can provide a
clear benchmark for quantifying the degree of magnetic frustration. Focusing on
the strongly frustrated $J_1$-$J_2$ model and the spatially anisotropic
$J_{1a}$-$J_{1b}$-$J_2$ model, very distinct low energy behaviors reflect
different levels of magnetic frustration. In the $J_1$-$J_2$ model, bound
magnons appear trapped near the impurity in the ground state and strongly
reduce the ordered moments for sites proximal to the impurity. In contrast,
local moments in the $J_{1a}$-$J_{1b}$-$J_2$ model are enhanced on the impurity
neighboring sites. These theoretical predictions can be probed by experiments
such as nuclear magnetic resonance and scanning tunneling microscopy, and the
results can elucidate the role of frustration in antiferromagnets and help
narrow the possible models to understand magnetism in the iron pnictdies.
