O. Morán

Depression of the superconducting critical temperature and finite-size scaling relation in YBa 2 Cu 3 O 7-δ /La 2/3 Ca 1/3 MnO 3 bilayers

High-quality YBa2Cu3O7-δ/La2/3Ca 1/3MnO3 (YBCO/LCMO) bilayers were fabricated on (001)-oriented SrTiO3 (STO) substrates by dc-sputtering technique. Bottom layer was always LCMO since it grows better on STO than on YBCO. The thickness of the ferromagnetic layer varied between 5 and 35 monolayers (~2-13nm) and that of the top YBCO was fixed at 10 monolayers (~12nm). The transport properties of the YBCO layers as well as the magnetic properties of the LCMO counterparts were studied as a function of the LCMO layer thickness. A sizeable depression of the Curie temperature (TC) of the LCMO layers from the bulk to lower temperatures is observed when decreasing their layer thickness dLCMO, which might be ascribed to intrinsic dimensionality effects or strain-induced phenomena. On the contrary, the superconducting critical temperature of the YBCO layer TS displays a sudden strong decrease at a critical LCMO thickness of ~12nm. Since no dramatic change of the structural and morphological quality of the YBCO top layers with increasing dLCMO is observed, the suppression of TS of the YBCO layers should take place via proximity effect due to the increasing magnetization strength in the LCMO layer. However, extrinsic factors like interface strain, interdiffusion of cations between YBCO and LCMO or injection of spin-polarized carriers from the magnetic into the superconducting layer could also play an important role or even to be directly responsible for the observed depressed TS.

Magnetic anisotropic behavior in epitaxial La2/3Ca1/3MnO3/YBa2Cu3O7-δ/ La2/3Ca1/3MnO3 and La2/3Ca1/3MnO3/La1/3Ca2/3MnO3/YBa2Cu3O7-δ trilayered structures

The temperature dependence of the magnetization along with magnetization hysteresis loops were measured on dc sputtered La2∕3Ca1∕3MnO3[ferromagnetic(F)]∕YBa2Cu3O7−δ[superconductor(S)]∕La2∕3Ca1∕3MnO3(F) and La2∕3Ca1∕3MnO3(F)∕La1∕3Ca2∕3MnO3[antiferromagnetic(AF)]∕YBa2Cu3O7δ(S) trilayered structures. The magnetic fields were applied both parallel (in-plane) and perpendicular (out-of-plane) to the film plane. Magnetization hysteresis loops, recorded at 5K, shown a clear central peak around zero field when the magnetic field was applied perpendicular to the film plane and a typical ferromagnetic behavior when applied parallel. Additionally, the dependence of the magnetization on the temperature, performed under the zero-field-cooling (ZFC) and field-cooling (FC) regimens, evidenced the active presence of the diamagnetic and ferromagnetic phases both in the F∕S∕F and F∕AF∕S trilayers. Thus, the diamagnetic phase was clearly observed under the ZFC regimen, whereas a possible paramagnetic Meissner effect was obse...

Ferromagnetic response of multiferroic TbMnO3 films mediated by epitaxial strain and chemical pressure

High quality Tb1−xAlxMnO3 (xu2009=u20090, 0.3) films have been grown under different values of compressive/tensile strain using (001)-oriented SrTiO3 and MgO substrates. The films were grown by means of rf sputtering at substrate temperature of 800u2009u2009°C. X-ray diffraction analysis shows that films are single phase, preferentially oriented in the (111) and (122) directions for films deposited on SrTiO3 and MgO substrates, respectively. Although the TbMnO3 target shows antiferromagnetic order, the films deposited on both substrates show weak ferromagnetic phase at low temperature coexisting with the antiferromagnetic phase. The introduction of Al in the films clearly enhances their ferromagnetic behavior, improving the magnetic performance of this material. Indeed, M(H) measurements at 5u2009K show a well-defined hysteresis for films grown on both substrates. However, a stronger magnetic signal (larger values of remanence and coercive field) is observed for films deposited on MgO substrates. The chemical pressure genera...