YBa_1−𝑥⁢Sr_𝑥⁢CuFeO₅ 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 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 temperatures suitable for applications. 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 eleven YBa_1−𝑥⁢Sr_𝑥⁢CuFeO₅ 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=(1/2,1/2,1/2±𝑞) is destabilized beyond a critical Sr content, being replaced by a fully antiferromagnetic state with ordering temperature 𝑇_coll⁢2≥𝑇_spiral and propagation vector 𝐤_𝑐⁢2=(1/2,1/2,0). Interestingly, both 𝑇_spiral and 𝑇_coll⁢2 increase with 𝑥 with comparable rates. This suggests a common, disorder-driven origin for both magnetic phases, consistent with theoretical predictions.