Effective ferromagnetic coupling between a superconductor and a ferromagnet inLaCaMnO∕Nbhybrids

Abstract

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...

Effects of Nb buffer layer on superconducting and magnetic behavior of IrMn/NiFe/Nb/NiFe spin-valves

“…Furthermore, the magnetic moment of S Ta increases when the temperature is reduced from 6 K to 2 K. In agreement with previous reports [4,32,33] this suggests that the switching effect occurs in low applied magnetic field for S Ta . This is related to the response of Nb to the stray field of the NiFe layers, as discussed in these reports.…”

Magnetic measurements in thin film specimens: Rejecting the contribution of the substrate

“…Probably one of the most widely studied category of such artificial specimens is in the form of thin films . Sputtering [5,6,[8][9][10][12][13][14][17][18][19][20], laser ablation [15,16,33,[36][37][38], molecular beam epitaxy [4,11] or other techniques are usually employed for their preparation, so that the produced samples to exhibit the desired properties.…”

“…Decoupling phenomena observed in the area of superconductivity, enhancement of the magnetic anisotropy or modulation of magnetic domains may be observed when the artificial samples are prepared only in bi-layer or multi-layer thin film form [9,[14][15][16][32][33][34][35][36][37][38] or when patterned in stripes of width lower than some critical length scale [17]. Another category of films widely studied today is the one of films that host dots, squares or other type of shapes with dimensions in the nanometer scale [4,7,8,12,13,[21][22][23][24][25][26][27].…”

“…In any case the magnetic particles should be dispersed on the surface of a substrate, either by means of a controlled photolithographic or ebeam lithographic technique (particles in periodic structures [21][22][23][24][25][26][27]) or by an annealing or chemical process (randomly distributed particles [28][29][30][31]). Also, hybrid magnetic/superconducting structures, consisting of magnetic nanoparticles embedded in superconducting films [21][22][23][24][25][26][27][28][29][30][31] or in the form of bilayers or multilayers consisting of magnetic and superconducting layers [32][33][34][35][36][37][38] are important for the study of basic physics [39] and may be used for the development of various practical applications (for instance, magnetoresistive memory devices [40] or superconducting spin switches [41]) in the near future.…”

Growth and characterization of NbN/SmCo5bilayers: The influence of magnetic stray field on the upper critical field of the superconductor

“…Most of the published literature on this topic considers soft-fm/sc heterostructures. Regarding hard-fm/sc combinations only a few publications [9][10][11][12] exist. In this manuscript we report on heterostructures of hard magnetic SmCo 5 and superconducting NbN.…”