Abstract
In the layered perovskites YBaCuFeO5 (YBCFO) magnetic frustration stabilizes incommensurate spiral order up to unexpectedly high temperatures (TS). The level of frustration and hence TS depend directly on the fraction (nd) of B-site (Cu/Fe) chemical disorder in the samples. We report a neutron investigation on the magnetic transitions in two YBaCuFeO5 single crystals (one pure and the second slightly doped with Mn), grown by floating-zone methods, that present very low Fe/Cu cation disorder (nd→0). The low density of antiferromagnetic Fe2O9 bipyramid units in these crystals precludes the stabilization of a frustrated topological network, which is at the origin of the non-collinear (helicoidal) magnetic order. Instead, the ground state is substituted by two competing commensurate collinear magnetic phases, the majority one (k1) attributed to the perfectly ordered YBCFO perovskite (nd=0). A second type of collinear magnetic domains (k3) develop for samples with disorder below a critical threshold (0<nd< n0). Only in YBCFO samples with Cu/Fe disorder above the critical threshold a long-range helicoidal or spiral magnetic state can be formed.
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