Preferential quenching of 5d antiferromagnetic order in Sr₃(Ir_1-x Mn _x )₂O₇

The breakdown of [Formula: see text] antiferromagnetism in the limit of strong disorder is studied in Sr₃(Ir_1-x Mn _x )₂O₇. Upon Mn-substitution, antiferromagnetic ordering of the Ir cations becomes increasingly two-dimensional, resulting in the complete suppression of long-range Ir magnetic order above [Formula: see text]. Long-range antiferromagnetism however persists on the Mn sites to higher Mn concentrations (x  >  0.25) and is necessarily mediated via a random network of majority Ir sites. Our data suggest a shift in the Mn valence from Mn⁴⁺ to Mn³⁺ at intermediate doping levels, which in turn generates nonmagnetic Ir⁵⁺ sites and suppresses long-range order within the Ir network. The collapse of long-range [Formula: see text] antiferromagnetism and the survival of percolating antiferromagnetic order on Mn-sites demonstrates a complex 3d-5d exchange process that surprisingly enables minority Mn spins to order far below the conventional percolation threshold for a bilayer square lattice.