In this work we present magnetization data on hybrids consisting of multilayers (MLs) of manganites [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3]15 in contact with a low-Tc Nb superconductor (SC). Although a pure SC should behave diamagnetically in respect to the external magnetic field in our ML-SC hybrids we observed that the magnetization of the SC follows that of the ML. Our intriguing experimental results show that the SC below its T SC c becomes ferromagnetically coupled to the ML. As a result in the regime where diamagnetic behaviour of the SC was expected its bulk magnetization switches only whenever the coercive field of the ML is exceeded. By employing specific experiments where the ML was selectively biased or not we demonstrate that the ML inflicts its magnetic properties on the whole hybrid. Possible explanations are discussed in connection to recent theoretical proposals and experimental findings that were obtained in relative hybrids.PACS numbers: 74.25. Ha, 74.80.Dm In recent years hybrids comprised of superconducting (SC) and ferromagnetic (FM) ingredients have attracted much interest due to the fascinating properties that they exhibit depending on the choice of materials, their specific structure etc. Among others, basic structures of such artificial hybrids are bilayers (BLs), trilayers (TLs) and even multilayers (MLs) of homogenous films but also distinct ferromagnetic nanoparticles (FNs) placed on top of or embedded in a SC layer. 1,2,3,4,5,6 For the case of ordered arrays of FNs it has been shown that the magnetoresistance of the SC presents distinct periodic minima at specific values of the applied magnetic field.1,2,3 When the FNs are randomly distributed in the SC, periodic minima are not observed in the magnetoresistance but a clear enhancement in the surface-superconductivity critical field occurs when their saturation field is exceeded.
5,6Referring to homogenous layered hybrids recently spinvalve devices comprised of FM-SC-FM TLs are studied intensively. 7,8,9 In Refs.7,9 the studied TLs employed Nb and CuNi as the SC and FM constituents, while in Ref.8 it was studied high-T c SC YBa 2 Cu 3 O 7 in contact with FM La 0.7 Ca 0.3 MnO 3 . While in TLs comprised of low-T c Nb 7,9 the superconducting transition was increased (decreased) when the FM layers where antiparallel (parallel), in the TLs consisting of high-T c YBa 2 Cu 3 O 7 8 the opposite behaviour was observed.The present work offers experimental results on hybrids consisting of MLs of manganites in direct contact with a layer of low-T c Nb SC. We chose MLs of manganites for the constructed hybrids since we wanted to examine how the mechanism of exchange biasing 10,11 could influence the SC. In addition, the layered structure of the ML offers enhanced coercivity which is of practical importance for the design of functional apparatus. In ‡ Author to whom correspondence should be addressed (densta@ims.demokritos.gr) all samples a relatively thick FM buffer layer has been used since we expected that this should act as a main reservoir for...
A series of [La2/3Ca1/3MnO3/La1/3Ca2/3MnO3]15 multilayers, with bilayer thicknesses between 2 and 32 nm, has been prepared by pulsed laser deposition. The study of their magnetic and magnetotransport properties reveals, for the first time in this category of materials, the presence of an exchange biasing mechanism at low temperatures. Zero-field-cooling and field-cooling magnetic measurements reveal a blocking temperature around 70 K that is independent of the bilayer thickness, whereas the average film magnetization becomes zero at 250 K.
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