  We study static vacancies in the collinear magnetic phase of a frustrated
Heisenberg $J_1$-$J_2$ model. It is found that vacancies can rapidly suppress
the collinear antiferromagnetic state (CAFM)and generate a new magnetic phase,
an anti-collinear magnetic phase (A-CAFM), due to magnetic frustration. We
investigate the quantum phase transition between these two states by studying a
variety of vacancy superlattices. We argue that the anti-collinear magnetic
phase can exist in iron-based superconductors in the absence of any preceding
structural transitions and an observation of this novel phase will
unambiguously resolve the relation between the magnetic and structural
transitions in these materials.

Strongly Correlated Electrons