DC magnetization (M) measurements were performed in a superconducting quantum interference device magnetometer (MPMS-XL 7T, Quantum Design). Small pellets (diameter ~ 3 mm, height ~ 1 mm, mass ~ 15 to 25 mg) from the same batches as the samples used for the PND measurements were measured between 2 and 400 K by heating under a magnetic field of μ0H = 0.5 T after being cooled down to 2 K in zero field. The magnetization was further measured between 300 and 600 K using a high-temperature insert. The samples, mounted on transparent drinking straws for the measurements below 400 K, were dismounted and wrapped in aluminum foil as described in (40) for the measurements above this temperature. The signal from the aluminum foil, measured separately, was found to be temperature independent and negligible when compared with the sample’s magnetization. For the samples with magnetic RE3+ cations, the magnetic transitions from the Cu/Fe sublattice are difficult to observe in the magnetization data because of the large paramagnetic contribution of the RE3+ moments, and they have been missed in some previous studies (41). However, they can still be tracked in the first derivatives of the DC inverse susceptibility 1/χDC = H/M (fig. S1). The values of Tcollinear and Tspiral derived from this technique correspond to the midpoint of the step-like anomalies in the 1/χDC derivative, which coincides with the χDC maximum in the YBa1−xSrxCuFeO5 series.

Although the REBaCuFeO5 samples were also synthesized in the past (42, 43), the existence of magnetic transitions has only been reported for a few of them (41, 4447), and there is no apparent systematics between the reported ordering temperatures and the RE ionic radii. A similar situation was observed in the YBa1−xSrxCuFeO5 series, where the presence of spiral phases was either disproven (48) or not demonstrated (49) in previous studies. As shown in Fig. 2C, this is not the case in this study, where the collinear and spiral phases are observed for all samples. Moreover, Tcollinear and Tspiral change in a monotonic way as a function of both the RE ionic radius and the Sr content x.

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