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Abstract:

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The breakdown of antiferromagnetism in the limit of strong disorder is studied in Sr3(Ir1&minus;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 . Long-range antiferromagnetism however persists on the Mn sites to higher Mn concentrations (x&thinsp;&thinsp;&gt;&thinsp;&thinsp;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 antiferromagnetism and the survival of percolating antiferromagnetic order on Mn-sites demonstrates a complex 3d&ndash;5d exchange process that surprisingly enables minority Mn spins to order far below the conventional percolation threshold for a bilayer square lattice.
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