YB a1-xSrxCuFeO5 layered perovskites: An attempt to explore the magnetic order beyond the paramagnetic-collinear-spiral triple point

Layered perovskites of general formula AA'CuFeO5 are characterized by the presence of spiral magnetic phases whose ordering temperatures Tspiral 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 temperatures suitable for applications. Moreover, it has been proposed that the highest Tspiral 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 eleven YBa1-xSrxCuFeO5 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 measurements, synchrotron x-ray and neutron powder diffraction we show that the spiral state with ks=(12,12,12±q) is destabilized beyond a critical Sr content, being replaced by a fully antiferromagnetic state with ordering temperature Tcoll2≥Tspiral and propagation vector kc2=(12,12,0). Interestingly, both Tspiral and Tcoll2 increase with x with comparable rates. This suggests a common, disorder-driven origin for both magnetic phases, consistent with theoretical predictions.