Seminár centra excelentnosti SAV

„Centrum výskumu elektronických a elektrotechnických súčiastok

novej generácie“, CENG

Smolenice, 25. a 26.9.2006

Vlastnosti tenkých vrstiev MgB2 pripravených na rôznych substrátoch

R. Durný a P. Valko

Katedra fyziky FEI STU v Bratislave

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. DurnyPhysica C 412-414, (2004) 201-205

Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fieldsA. Nishida, C. Taka, S. Chromik, and R. DurnyPhysica C 426-431, (2005) 340-344

Investigation of critical behaviour of MgB2 thin films on SiC/Si substrateA. Nishida, C. Taka, S. Chromik, and R. DurnyJ. Phys. Conf. 43 (2006) 293-296

Investigation of lower critical fields of MgB2 thin films on SiC/Si substrate with parallel magnetic fieldsA. Nishida, C. Taka, S. Chromik, and R. DurnyPhysica C 435, (2006) 74-77.

Investigation of magnetic properties of MgB2 thin films on NbN/Si substrateAkihiko 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 M of magnetization from perfect diamagnetism is gradual due to flux pinning, (M)1/2 was plotted against the external field, and Hc1 was deduced from linear extrapolation of (M)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.

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.

Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate

Plots of the deviation M from the linear diamagnetism and (M)1/2 as a function of applied field H. While M starts to increase tangentially with field increase, (M)1/2 varies linearly with H and linear extrapolation to (M)1/2 =0 gives the lower critical field.

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 (0) ~ 224 nm.

Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate

Magnetization hysteresis M as a function of the field H in different temperatures.

Irreversibility fields Hirr at respective temperatures, which are smaller than those reported for bulk materials.

Investigation of magnetic properties of MgB2 thin films on NbN/Si substrate

In summary, MgB2 films were prepared by the sequential evaporation of boron and magnesium on NbN buffered Si substrate followed by in situ annealing. Magnetic properties of the film were measured with parallel magnetic field to the film surface. With the initial (virgin) DCmagnetization and the careful analysis, we have obtained rather small value of the lower critical field as Hc1(0) = 131 Oe. The small irreversibility field Hirr inferred that the present film has less impurities and defects.

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.

Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields

Onsets of the AC diamagnetic susceptibility ’ upon cooling under various magnetic fields perpendicular to the film surface as indicated by arrows.

Upper critical field Hc2 as a function of temperature T under perpendicular (solid circles) and parallel (open circles) fields with respect to the film surface.

Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields

Critical current density Jc at respective temperatures as a function of the applied field perpendicular to the film surface.

Irreversibility field Hirr for the perpendicular (solid circles) and parallel (open circles) fields. The least square fitting to the scaling law: Hirr~(1 -t2)n, resulted in the exponent n = 2.92 for the perpendicular field and n = 2.17 for the parallel field.

Critical properties of MgB2 thin films on NbN/Si substrate under perpendicular magnetic fields

In summary, AC and DC magnetic measurements were performed in the MgB2 thin film on NbN/Si substrate. The upper critical fields indicated similar variations for perpendicular and parallel magnetic fields with temperature derivative of about 5 kOe/K, which is close to that of the single crystal along the ab-plane. Critical current densities were evaluated to be more than 1 MA/cm2 below 14 K (self field), indicating the rigid current flow along the film. Stronger irreversibility and larger n value for the perpendicular field than that for the parallel field suggested additional flux pinning mechanism such as grain boundary effects. Intentional search and incorporation of fine grain structures or impurity phases effective for flux pinning should be pursued to further improve tolerance of superconductivity against higher magnetic fields.

Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate

A Nishida, C Taka, S Chromik and 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.

Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate

Onsets of the AC diamagnetic susceptibility χ in MgB2/SiC/Si under various magnetic fields perpendicular to the film surface as indicated by arrows.

Upper critical field Hc2(T) under parallel (◦ ) and perpendicular (• ) fields for MgB2/SiC/Si, which is lager than average Hc2 for MgB2/NbN/Si (- - - -).

Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate

Critical current density Jc at respective temperatures as a function of the magnetic field H perpendicular to the film surface in MgB2 on SiC-buffered Si substrate.

Irreversibility field Hirr plotted for the perpendicular (• ) and parallel (◦ ) fields in MgB2/SiC/Si. The straight lines indicate the scaling law : Hirr~(1 − t2)n, with exponents n of 2, 3 and 6.

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 and compared with MgB2/NbN/Si. The upper critical field in MgB2/SiC/Si was larger than that in MgB2/NbN/Si, reflecting the shorter coherence length probably due to impurities yielded by the higher reaction temperature. The temperature derivative, |dHc2/dT | of around 8 kOe/K was among the highest ever reported. On the other hand, although Jc approached 1 MA/cm2 at the lowest temperatures, the maximum value was smaller than that in MgB2/NbN/Si. The irreversibility field Hirr also inferred some effects of weak links. According to these investigations, it will be necessary to further search for appropriate impurity phases that can enhance Hc2 and act as the flux pinning centers but do not reduce the intergrain coupling.

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.

Investigation of lower critical fields of MgB2 thin films on SiC/Si substrate with parallel magnetic fields

Initial (virgin) magnetization M in MgB2/SiC/Si as a function of the applied field H after zero-field cooling. Deviation δM from the perfect diamagnetism (straight line) starts around 60 Oe.

Plot of the deviation δM (closed circles) from the linear diamagnetism and |δM|1/2 (open circles) as a function of H. Extrapolation to the minimal (noise level as indicated by the horizontal dash-dotted line) provide estimate of the lower critical field Hc1.

Investigation of lower critical fields of MgB2 thin films on SiC/Si substrate with parallel magnetic fields

Temperature dependence of the evaluated Hc1 in MgB2/SiC/Si (closed circles) and the linear fit to the data (solid line). Uncertainty in the field determination is considered to be about ±10 Oe. The temperature derivative |dHc1/dT | of about 2.4 Oe/K in MgB2/SiC/Si is smaller than that of about 5.3 Oe/K (dashed line) in MgB2/NbN/Si.

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 films deposited on SiC-buffered Si substrate and the lower critical fields Hc1 were evaluated and compared with those in MgB2 on NbN-buffered Si. Together with the investigations of the upper critical field Hc2, the smaller Hc1 for the SiC buffer was explained in terms of the larger GL parameter κ, which was estimated to be one of the largest ever reported. Higher annealing temperature with SiC buffer probably resulted in some impurity phases and enhanced the κ values. We also obtained the estimate of the thermodynamic critical field Hc(0) = 0.21 T which did not seem to vary with buffer layers. Effects of the buffer layers yielding such variance and invariance in the critical properties should be further investigated.