Revealing the degree of magnetic frustration in iron pnictides

Proposed theories for collinear antiferromagnetism in iron pnictides vary greatly in the amount of magnetic frustration and proximity to a quantum critical point. We discuss how imaging the magnetic fields around a non-magnetic impurity can quantify the degree of magnetic frustration and thereby distinguish various theoretical proposals. It is found that in a frustrated J1-J2 model a non-magnetic impurity strongly reduces its neighboring local moments, and overturned dynamical spins appear close to zero energy. In contrast, the spatially anisotropic J1a-J1b-J2 model produces enhanced local moments on sites neighboring the impurity. In both cases, the disturbance in the magnon local density of states exhibits an anisotropic stripe pattern. These predictions can be tested by experiments such NMR or spin-resolved STM measurements. The results can elucidate the role of frustration in antiferromagnets and help narrow the possible models for the iron-pnictide superconductors.