【Member News】Xianggang team of Sichuan University: Performance Enhancement and In Situ Observation of Resistive Switching and Magnetic Modulation by a Tunable Two-Level System of Mn Dopants in α-Gallium Oxide-based Memristor

      The research team of Professor Xiang Gang from the School of Physics of Sichuan University recently developed a gallium oxide memristors based on manganese doping, which can greatly improve the switching ratio through the manganese two-level system, and realize the double modulation of resistance and magnetic moment. The results were recently published in the journal Advanced Functional Materials.

      As one of the four passive electrical components, memristor was predicted by Professor CAI Shaotang at the University of California, Berkeley in 1971 and first prepared by HP Laboratory in 2008, and has been continuously updated and iterated. Memristor realizes the memory function of flowing through its charge through nonlinear resistance value, which can be used for high-density data storage and low-energy neuron simulation, and has great application potential in the field of information technology and artificial intelligence.

     As an emerging ultra-wide band gap conductor, gallium oxide has attracted the attention of researchers and developers because of its excellent photoelectric properties, thermal stability, chemical stability and tolerance to extreme environments. In recent years, gallium oxide has become an important candidate for memristors. A key parameter of the memristor is the switch ratio, which is decisive for whether the stored information can be effectively recognized by the peripheral circuits in a changing environment. However, the current switch ratio of pure gallium oxide memristors is still low, usually between 1~10². That is why it is important to improve the switching ratio of gallium oxide memristors. Previous research work by Professor Xiang's team showed that the electrical and magnetic properties of manganese-doped crystalline gallium oxide films can be modulated by oxygen vacancy concentration and manganese doping concentration（Journal of the American Ceramic Society 106 （2023）374）. And manganese-doped amorphous gallium oxide films show stronger room temperature ferromagnetism due to more oxygen vacancies (Scripta Materialia 220（2022）114919）. Interestingly, the manganese doping provides a tunable two-level system:The Mn³⁺ and Mn²⁺ ions exist in neutral and p-type dopants, respectively, and the conversion between the two can be achieved by changing the oxygen vacancy concentration.

      Based on the above facts, the research team of Professor Xiang Gang from Sichuan University designed and prepared a high-performance manganese-mixed amorphous gallium oxide memristor. The switch ratio of this memristor can be increased to 10³, which is 10~500 times that of those previously reported for pure gallium oxide memristor, which ensures a low data read / write error rate. At the same time, the memristor realizes the dual modulation of the resistance and the magnetic moment, which can be used for the new multivariate information memory devices.

      The team revealed the physical mechanism by which the gallium oxide memristor device has a high switch ratio. In the low resistance state, more oxygen vacancies rich in amorphous gallium oxide makes the resistance of the induced oxygen vacancy conductive wire lower. While in the high resistance state, Mn²⁺ ions can capture the electrons provided by oxygen vacancies, resulting in a higher resistance. Therefore, memdevices based on manganese doped amorphous gallium oxide have higher switching ratios. Furthermore, the stability cycles and durability of the device reaches or near the highest level previously reported.

Figure 1. (a) Bipolar resistance curve of a memristor with different manganese doping concentrations;(b) Oxygen vacancy content in amorphous gallium oxide thin films with different manganese doping concentrations;(c) the I-V curves of the memristor SET and RESET processes;(d) the relationship between the resistance of the high resistance and the low resistance and the device area;(e) memostor durability cycle test;(f) memristor persistence test

      For the first time, the research team used an adjustable manganese two-level system to detect the resistance and magnetic moment modulation behavior in the device. The ferromagnetism in the films comes from bound magnetic polarons generated by the coupling of oxygen vacancies and manganese ions. When high resistance is switched to low resistance, more oxygen vacancies are produced. This is not only conducive to the formation of oxygen vacancy conducting filaments, but also enables the bound magnetic polarons to occupy a larger total volume and overlap more manganese ions, thus enhancing the ferromagnetism. Meanwhile, part of the Mn³⁺ ions change to Mn²⁺ ions due to increased oxygen vacancies, which would slightly reduce the local spin in the enlarged bound magnetic polaron. The competitive synergies of the two mechanisms increase the saturated magnetization in Goxide from high resistance to low resistance. And vice versa. The test shows that the gallium oxide memristor can achieve up to ~200% magnetic moment modulation in the resistance process.

Figure 2.In the high and low resistance states, gallium oxide memristors’ (a) hysteresis loop, (b) Mn2p graph, (c) 01s graph and (d) EPR spectrum.

      This work reports a high-performance gallium oxide memristors with high switching ratio and resistance magnetic moment double modulation, and shows that the manganese two-level system can be used as a new tool to detect resistance and magnetic moment changes, providing a useful reference for related memristor and electronic magnetic devices.

      This work was supported by the National Key Research and Development Program and the National Natural Science Foundation of China.