Electrical nonuniformity and non-ohmic current-voltage characteristics in Co2FeSi film

The full Heusler compound Co2FeSi is one kind of half metal as its 100% spin polarization with Curie temperature as high as 1100K. It is interesting to investigate electrical property of Co2FeSi film for applications in magnetoelectric devices. The electrical characteristics of Co2FeSi film were investigated. The Co2FeSi film was 100nm and was prepared by magnetron sputtering at room-temperature on silicon. Resistance was investigated by four-probe method. The results from figure.1 showed that the Co2FeSi film represents low resistance metal conductor behavior at temperature T > 280K. However, with temperature decreasing down to T <; 280K, the resistance of the film increases sharply by about two magnitudes which representing semiconductor or insulator behavior. Interestingly, the resistance variation at low temperature (T <; 220K) is quite different at different currents. These results indicate that the film changed from electrical disorderd metal conductor phase to orderd insulator phase with temperature decreasing. And the film is electrical nonuniform at low temperature. However, the I-V characteristics from figure.2 show that it has non-ohmic behavior at temperature 260K <; T <; 320K. This non-ohmic behavior is opposite to the I-V characteristics of semiconductor. Comparing figure.1 and figure.2, we know that the film is not true metal conductor at T > 280K. Perhaps it is result from lattice defects in the film.