Preferential quenching of 5d antiferromagnetic order in Sr3(Ir1-xMnx)2O7

The breakdown of J(eff) = 1/2 antiferromagnetism in the limit of strong disorder is studied in Sr-3(Ir1-xMnx)(2)O-7. Upon Mn-substitution, antiferromagnetic ordering of the Jr cations becomes increasingly two-dimensional, resulting in the complete suppression of long- range Ir magnetic order above x approximate to 0.25. 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 J(eff) = 1/2 antiferromagnetism and the survival of percolating antiferromagne tic 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.