Voltage-Induced Interface Reconstruction and Electrical Instability of the Ferromagnet-Semiconductor Device
Shu-Jui Chang1, Po-Chun Chang2, Wen-Chin Lin2, Shao-Hua Lo1, Liang-Chun Chang3, Shan-Fang Lee3, Yuan-Chieh Tseng1*
1Materials Science & Engineering, National Chiao Tung University, Taiwan
2Physics, National Taiwan Normal University, Taiwan
3Institute of Physics, Academia Sinica, Taiwan
* presenting author:Yuan-Chieh Tseng, email:yctseng21@mail.nctu.edu.tw
Using x-ray magnetic spectroscopy with in-situ electrical characterizations, we investigated the effects of external voltage on the spin-electronic and transport properties at the interface of a Fe/ZnO device. Layer-, element-, and spin-resolved information of the device was obtained by cross-tuning of the x-ray mode and the photon energy, when the voltage was applied during the operation. We found that at the early stage of the operation, the device exhibited a low-resistance state featuring robust Fe-O bonds. However, the Fe-O bonds were broken with increasing applied voltage, where the Fe layer underwent an irreversible oxidized→metallic transition. Breaking of the Fe-O bonds caused the formation of oxygen vacancies at the Fe/ZnO interface and resulted in a high-resistance state. Such interface reconstruction was coupled to a charge-transfer effect via Fe 3d-O 2p hybridization, which suppressed/enhanced the magnetization/coercivity of Fe electronically. Nevertheless, the interface would become stabilized with the metallic phase if the device was continuously polarized. During this stage, the spin-polarization of Fe was enhanced whereas the coercivity was lowered by the applied voltage, but changes of both characteristics were reversible. This stage is desirable for spintronic device applications, owing to a different voltage-induced electronic transition compared to the first stage. Our results enabled a straightforward detection of the spin-electronic state at the interface in relation to the transport and reversal properties of the versatile ferromagnet-semiconductor systems during the operation process of the device. The key questions for the development of spintronics based on similar material combinations are: How to engineer the interface properties by the external voltage, i.e., how to mitigate the interface reconstruction (1st stage); and how to turn the device into a state of high spin-polarization and fast magnetic switching (low coercivity) state (2nd stage).

Keywords: x-ray absorption spectroscopy, spintronics