Temperature-dependent magnetization enhancement of the ferromagnetic layers in the high-Tc-superconductor/ferromagnetic-oxide superlattice system

In this study, we investigate the competition between superconductivity and ferromagnetism at the interface of a YBa₂Cu₃O₇₋δ (YBCO, ~12 nm) / La₀.₆₇Sr₀.₃₃MnO₃ (LSMO, ~5 nm) superlattice. Polarized neutron reflectivity (PNR) was employed to probe the magnetic profile of each layer. The best fit (χ² ≈ 2.0) reveals ~1 nm magnetic dead layers in the LSMO near the interfaces, likely due to the superconducting proximity effect, where spin-singlet Cooper pairs penetrate into LSMO and suppress its magnetism. Interestingly, upon cooling from 95 K to 8 K, the magnetization of the first and third LSMO layers increases by ~5%, while the second (middle) layer shows an ~11% enhancement. For comparison, a single-layer LSMO film of the same thickness shows only ~4% increase under the same temperature change, suggesting an additional interfacial effect is involved in the superlattice. This unusual behavior resembles the paramagnetic Meissner effect (PME), though PME typically occurs under much weaker magnetic fields. Since our measurements were performed under a 0.6 T field, significantly higher than PME conditions, an alternative explanation could be the emergence of spin-triplet superconductivity, where parallel-spin Cooper pairs (2μB) align with Mn moments and enhance the M of the LSMO layer."Correspondent Author: hchou@mail.nsysu.edu.tw"