Smolenice, 25. a 26.9.2006

1. Seminár centra excelentnosti SAV„Centrum výskumu elektronických a elektrotechnických súčiastok novej generácie“, CENG Vlastnosti tenkých vrstiev MgB2 pripravených na rôznych substrátochR. Durný a P. ValkoKatedra fyziky FEI STU v Bratislave Smolenice, 25. a 26.9.2006

2. Doteraz publikované články v zahraničných vedeckých periodikách (časopisoch) • Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate A. Nishida, C. Taka, S. Chromik, and R. Durny Physica C 412-414, (2004) 201-205 • Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields A. Nishida, C. Taka, S. Chromik, and R. Durny PhysicaC426-431, (2005) 340-344 • Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate A. Nishida, C. Taka, S. Chromik, and R. Durny J. Phys. Conf.43 (2006) 293-296 • Investigation of lower critical fields of MgB2 thin filmson SiC/Si substrate with parallel magnetic fields A. Nishida, C. Taka, S. Chromik, and R. Durny PhysicaC435, (2006) 74-77.

3. Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate Akihiko Nishida, Chihiro Taka, Stefan Chromik, Rudolf Durny Magnetic properties of MgB2 thin films grown on NbN/Si substrate were investigated. MgB2 films were prepared by sequential evaporation of boron and magnesium on NbN buffered Si substrate followed by in situ annealing. DC magnetizations were measured with magnetic field parallel to the film surface in temperatures between 2 and 45 K. From the initial (virgin) DCmagnetization, the lower critical field Hc1 was estimated. Since deviation dM of magnetization from perfect diamagnetism is gradual due to flux pinning, (dM)1/2 was plotted against the external field, and Hc1 was deduced from linear extrapolation of (dM)1/2 to 0. Irreversibility field was evaluated from magnetization hysteresis The quality and superconductivity of the film were examined with the obtained characteristic fields.

4. Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate Typical results of DCmagnetization versus magnetic field (M–H) curves. Temperatures for magnetization isotherms are indicated at the left of the curves from up to center for respective curves from the outermost to the innermost curves. Lower-field dependence of the initial magnetization at 14 K. The diamagnetic moment seems to grow linearly until around 80 Oe as indicated by the solid straight line, which should correspond to the perfect diamagnetism.

5. Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate Temperature dependence of the deduced lower critical field Hc1, which varies almost linearly except for near Tc. Linear extrapolation (dashed straight line) provides Hc1(0)= 131 Oe and l(0) ~ 224 nm. Plots of the deviation dM from the linear diamagnetismand (dM)1/2 as a function of applied field H. While dM starts toincrease tangentially with field increase, (dM)1/2 varieslinearlywith H and linear extrapolation to (dM)1/2=0gives the lowercritical field.

6. Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate Magnetization hysteresis DM as a functionof the field Hin different temperatures. Irreversibility fields Hirr at respectivetemperatures,which are smaller than thosereported for bulk materials.

7. Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate In summary, MgB2 films were prepared by thesequential evaporation of boron and magnesiumon NbN buffered Si substrate followed by in situannealing. Magnetic properties of the film weremeasured with parallel magnetic field to the filmsurface. With the initial (virgin) DCmagnetizationand the careful analysis, we have obtained rathersmall value of the lower critical field asHc1(0) = 131Oe. The small irreversibility field Hirrinferred that the present film has less impuritiesand defects.

8. Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields Akihiko Nishida, Chihiro Taka, Stefan Chromik, Rudolf Durny Critical magnetic properties of MgB2 thin films grown on NbN/Si substrate were investigated under magnetic fields perpendicular to the film surface. Polycrystalline MgB2 films were prepared by sequential evaporation of boron and magnesium on NbN buffered Si substrate followed by an in situ annealing. AC and DC magnetizations were measured by the PPMS system. From the onset of AC diamagnetic susceptibilities, the upper critical fields were estimated resulting in the temperature derivative of about 5 kOe/K at lower temperatures. Critical current densities were evaluated from DC magnetization hysteresis to be more than 1 MA/cm2 below 14 K (self field). Critical properties including irreversibility fields were examined in comparison with those under parallel magnetic fields.

9. Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields Onsets of the AC diamagnetic susceptibility ’ uponcooling under various magnetic fields perpendicularto the filmsurface as indicated by arrows. Upper critical field Hc2 as a function of temperatureTunder perpendicular (solid circles) and parallel(open circles)fields with respect to the film surface.

10. Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields Irreversibility field Hirr for the perpendicular (solid circles) and parallel(opencircles) fields. The least square fitting to thescaling law:Hirr~(1 -t2)n, resulted in the exponent n= 2.92 for theperpendicular field and n = 2.17 for theparallel field. Critical current density Jc at respective temperaturesas a function of the applied field perpendicular to thefilm surface.

11. Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields In summary, AC and DC magnetic measurementswere performed in the MgB2 thin film onNbN/Si substrate. The upper critical fields indicatedsimilar variations for perpendicular andparallelmagnetic fields with temperature derivative ofabout 5kOe/K, which is close to that of the singlecrystal along the ab-plane. Criticalcurrent densitieswere evaluated to be more than 1 MA/cm2below 14 K (selffield), indicating the rigid currentflow along the film. Stronger irreversibility andlarger n value forthe perpendicular field than thatfor the parallel field suggested additionalflux pinningmechanism such as grain boundary effects.Intentional searchand incorporation of fine grainstructures or impurity phases effective forfluxpinning should be pursued to further improvetolerance ofsuperconductivity against higher magneticfields.

12. Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate A Nishida, C Taka, S Chromikand R Durny Critical magnetic behaviour of MgB2 thin films grown on SiC-buffered Si substrate was investigated in comparison with MgB2 films on NbN-buffered Si substrate. MgB2 films were prepared by sequential evaporation of boron and excess magnesium followed by in situ annealing in an Ar atmosphere. The upper critical field Hc2 estimated from the onset of AC diamagnetic susceptibility was larger in films with SiC buffer than those with NbN buffer. The temperature dependence was almost linear with |dHc2/dT| up to 8 kOe/K, especially under parallel magnetic field to the film surface. Although the critical current density evaluated from DC magnetization hysteresis approached 1 MA/cm2 at the lowest temperatures, the maximum value was smaller than that in MgB2/NbN/Si. The irreversibility field also inferred effects of weak links, possibly due to some impurity phases.

13. Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate Onsets of the AC diamagneticsusceptibility χ in MgB2/SiC/Si under variousmagnetic fields perpendicular to the film surfaceas indicated by arrows. Upper critical field Hc2(T) underparallel (◦ ) and perpendicular (• ) fields forMgB2/SiC/Si,which is lager than average Hc2 forMgB2/NbN/Si (- - - -).

14. Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate Critical current density Jc atrespective temperatures as a function of themagnetic field H perpendicular to the film surfacein MgB2 on SiC-buffered Si substrate. Irreversibility field Hirr plottedfor the perpendicular(• ) andparallel (◦ ) fields in MgB2/SiC/Si. Thestraight lines indicate the scaling law : Hirr~(1 − t2)n,with exponents n of 2, 3 and 6.

15. Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate In conclusion, AC and DC magnetic measurements were performed in MgB2/SiC/Si andcompared with MgB2/NbN/Si. The upper critical field in MgB2/SiC/Si waslarger than thatin MgB2/NbN/Si, reflecting theshorter coherence length probably due to impurities yielded bythe higherreaction temperature. The temperature derivative, |dHc2/dT | of around 8kOe/Kwas among the highest ever reported. On the other hand, althoughJc approached 1 MA/cm2at the lowest temperatures, the maximum valuewas smaller than that in MgB2/NbN/Si. Theirreversibility field Hirr alsoinferred some effects of weak links. According to these investigations,itwill be necessary to further search for appropriate impurity phases thatcan enhance Hc2 andact as the flux pinning centers but do not reduce theintergrain coupling.

16. Investigation of lower critical fields of MgB2 thin films on SiC/Si substrate with parallel magnetic fields Akihiko Nishida, Chihiro Taka, Stefan Chromik, Rudolf Durny Lower critical fields Hc1 of MgB2 thin films grown on SiC-buffered Si substrate were investigated with magnetic fields applied parallel to the film surface. MgB2 films were prepared by sequential evaporation of boron and magnesium on SiC/Si substrate. The amount of supplied boron was adjusted so as to result in 200 nm stoichiometric MgB2 film after reaction with the excess Mg top layer. In order to estimate Hc1, the film was zero-field cooled, and the initial (virgin) magnetization was measured with increasing field. Square root of magnetic deviation from the perfect diamagnetism was plotted against the applied field, and Hc1 was evaluated from extrapolation analyses. The temperature variation can be fitted by the linear dependence with the temperature derivative |dHc1/dT | of 2.4 Oe/K in MgB2/SiC/Si which is smaller than that of 5.2 Oe/K in MgB2/NbN/Si. The difference was examined in relation to the Ginzburg-Landau (GL) parameter, parameter, which was estimated to be about 86 with the SiC buffer under the parallel field.

17. Investigation of lower critical fields of MgB2 thin films on SiC/Si substrate with parallel magnetic fields Plot of the deviation δM (closed circles) from thelinear diamagnetismand |δM|1/2 (open circles) as afunction of H. Extrapolation to the minimal(noiselevel as indicated by the horizontal dash-dotted line)provideestimate of the lower critical field Hc1. Initial (virgin) magnetization M in MgB2/SiC/Si as a function of theapplied field H after zero-fieldcooling. Deviation δM from the perfectdiamagnetism (straight line) starts around 60 Oe.

18. Investigation of lower critical fields of MgB2 thin films on SiC/Si substrate with parallel magnetic fields Temperature dependence of theevaluated Hc1 in MgB2/SiC/Si(closedcircles) and the linear fitto the data (solid line).Uncertainty in thefielddetermination is considered to beabout ±10 Oe. The temperaturederivative |dHc1/dT | of about2.4 Oe/K in MgB2/SiC/Si issmaller thanthat of about 5.3Oe/K (dashed line) inMgB2/NbN/Si.

19. Investigation of lower critical fields of MgB2 thin films on SiC/Si substrate with parallel magnetic fields In conclusion, the virgin magnetization was measured in MgB2 thin filmsdepositedon SiC-buffered Si substrate and the lower critical fields Hc1wereevaluated and compared with those in MgB2 on NbN-buffered Si.Togetherwith the investigations of the upper critical field Hc2, thesmaller Hc1 for theSiC buffer was explained in terms of the larger GL parameter κ, which wasestimated to be one of the largest ever reported. Higher annealing temperaturewith SiC buffer probably resulted in some impurity phases and enhanced theκ values. We also obtained the estimate of the thermodynamic critical fieldHc(0) = 0.21 T which did not seem to vary with buffer layers. Effects of thebuffer layers yielding suchvariance and invariance in the critical propertiesshould be further investigated.