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

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, which was estimated to be about 86 with the SiC buffer under the parallel field.