YBa_1-xSr_xCuFeO_5 layered perovskites: exploring the magnetic order beyond the paramagnetic-collinear-spiral triple point

Layered perovskites of general formula AA'CuFeO_5 are one of the few examples of cycloidal spiral magnets where the ordering temperatures T_spiral can be tuned far beyond room temperature by introducing modest amounts of Cu/Fe chemical disorder in the crystal structure. This rare property makes these materials prominent candidates to host multiferroicity and magnetoelectric coupling at room temperature. Moreover, it has been proposed that the highest T_spiral value that can be reached in this structural family (∼ 400 K) corresponds to a paramagnetic-collinear-spiral triple point with potential to show exotic physics. Since generating high amounts of Cu/Fe disorder is experimentally difficult, the phase diagram region beyond the triple point has been barely explored. To fill this gap we investigate here the YBa_1-xSr_xCuFeO_5 solid solutions (0 ≤ x ≤ 1), where we replace Ba with Sr with the aim of enhancing the impact of the experimentally available Cu/Fe disorder. Using a combination of bulk magnetization, synchrotron X-ray and neutron powder diffraction we show that the spiral state is destabilized beyond a critical degree of Cu/Fe disorder, being replaced by a non-frustrated, fully antiferromagnetic state with propagation vector k_c2 = (1/2, 1/2, 0) and ordering temperature T_coll2 ≥ T_spiral, which is progressively stabilized beyond the triple point. Interestingly, T_spiral and T_coll2 increase with x at the same rate. This suggests a common, disorder-driven origin, consistent with theoretical predictions.