Study On The Magneto-resistance Effects Of SrTiO_3δ Films And WTe₂ Crystals

Magneto-resistance (MR) effect is very important in the field of physical electronics. With its unique properties, magneto-resistance materials are widely used in the magnetic field measurement, magnetic head sensors, current sensors and many kinds of motion detection. It is one of current research focuses of physics scholars. In this paper, the magneto-resistance effects of SrTiO3-sδthin films and WTe2 crystals are mainly studied.In the paper, the SrTiO3-δ thin films were grown by the pulsed laser deposition (PLD) technique at different growth temperatures. Measurements on the thin films showed that the X-ray diffraction signal was the strongest and the root mean square roughness was the lowest of the sample grown in 680 ℃. The optimal growth conditions of SrTiO-δ thin films are as followings:the laser frequency of 4 Hz, the growth temperature of 680 ℃, the oxygen pressure of 2×10-4 Pa, the growth time of 15 min, the laser energy of 200 mJ, the film thickness of 120 nm. PPMS was used to test the SrTiO3-δ thin films’electro-magnetic properties. It was found that the higher the growth temperature is, the smaller the room temperature resistance, which may be caused by the increasing oxygen vacancies with the raise of the growth temperature. For the sample grown in 680℃, the resistance decreases with the decreases of temperature, and the residual resistance ratio reaches 983.26. With the increases of the magnetic field, the magneto-resistance increases linearly, and there is no sign of saturation. At 5 K and 7 T, the MR=129.5%. With the increases of temperature, the magneto-resistance effect is gradually weakened.The MR in WTe2 features a "turn on" temperature, below which an extremely large MR has been observed. The MR reaches 2628% in 9 T at 2 K, but deceases drastically when the temperature enhances above the "turn on" temperature. The MR in WTe2 is believed to arise from the perfect compensation of the electron and hole populations, which gives rise to a quadratic field-dependent MR. While the MR slightly deviates from a H2-dependent MR in the high temperature region, it approaches to a pure H2 dependence as the temperature is reduced, suggesting that the compensation between electrons and holes improves upon cooling. The improvement of the electron-hole compensation with decreasing temperature may account for the "turn on" effect of the MR at low temperatures. Our observations support the electron-hole compensation mechanism as the origin of the extremely large MR in this material.