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Collinear to Anti-collinear Quantum Phase Transition by Vacancies

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.