Magnetic proximity effect in oxide-based superconductor/ferromagnet superlattices

The main goal of this thesis is to study the phenomenology of the magnetic proximity effect in YBa2Cu3O7/La2/3Ca1/3MnO3 (YBCO/LCMO) and YBCO/LaMnO3+y(LMO) superlattices grown on La0.3Sr0. 7Al0.65Ta0. 35O3 (LSAT) substrates with almost symmetric layer thicknesses. For both samples the cuprate and manganite layers preserve their respective superconducting and ferromagnetic properties. The superconducting transition onset temperature amounts to TC ≈ 88K and 77K for the YBCO/LCMO and the YBCO/LMO superlattices, respectively. The reduced TC in the latter suggests a small oxygen deficiency in the YBCO layers. The Curie temperature of the LCMO and LMO layers amounts to TCurie ≈ 200K and 135K, respectively. The structural properties of both samples are comparable; the superlattices are strained and adopt in-plane lattice parameters similar to the ones of the substrate. While the LCMO layers exhibit a metal-to-insulator transition at TCurie, the LMO layers remain insulating in the ferromagnetic phase. This different behavior in the ferromagnetic state allows the study of the influence of the electronic properties of the ferromagnetic layers on the magnetic proximity effect. The magnetic depth profile of the superlattices has been studied by means of polarized neutron reflectometry (PNR). The high quality of the superlattices and the stability of the LSAT substrate at low temperatures permit the measurement of the reflectivity curve up to qz values of 0.14 Å−1. The access to this extended qz range allows the identification of the differences between the observations and the simulations based on different magnetic configurations at the interface. The analysis of the neutron reflectivity curves measured at room temperature reveals that the interface roughness for both the YBCO/LMCO and YBCO/LMO superlattices is similar and small. In both cases the even order superlattice Bragg peaks (SLBPs) are strongly suppressed as expected for the similar thickness of the cuprate and manganite layers. The PNR curves measured in the ferromagnetic state of the YBCO/LCMO superlattice exhibit prominent even order SLBP which suggests a mismatch between the magnetic and structural depth profiles. The observed features of the reflectivity curves are properly reproduced only when a so-called depleted layer, in which the ferromagnetic moment is strongly suppressed, is introduced on the LCMO side of the interface. These depleted layers extend over several LCMO monolayers from the interfaces and their thickness is larger than the interface roughness. This suggests that they are of intrinsic rather than structural origin. Furthermore, the temperature evolution of the reflectivity curves suggests that the thickness of the depleted layers exhibits a rather sudden change in the vicinity of the YBCO superconducting transition temperature TC. Whether this is evidence of superconductivity-induced changes in the magnetic-depth profile needs to be further explored. In contrast, the PNR curves measured on the YBCO/LMO sample in the ferromagnetic state do not show a significant increase in the intensity of the even order SLBPs. The magnetic contributions to the neutron potential are thus nearly symmetric. The quantitative analysis of the PNR curves shows that the depleted layers in this case are bound to the first LMO monolayer at the interface. Since the structural and interface properties of the YBCO/LCMO and YBCO/LMO samples are comparable, the large difference in the strength of the depleted layers appears to be related to the electronic properties of the manganite layers. The intrinsic origin of the depleted layers is thus confirmed. The electric and magnetic properties of the Cu ions on the YBCO side of the interface have been studied with x-ray absorption spectroscopy measurements with linear and circular