Theoretical study on temperature-bias phase diagram of MgO-based magnetic tunnel junctions
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摘要: MgO基磁性隧道结是自旋电子器件研究的热点问题, 其温度特性和偏压特性在实际应用中极其重要. 因此, 亟需在理论上计算得到MgO基磁性隧道结的温度-偏压相图. 本文构建了适用于单晶势垒层磁性隧道结的理论. 该理论将单晶势垒层视作周期性光栅, 利用光学衍射理论处理势垒层对隧穿电子的衍射, 因此可以很好地计入隧穿电子波的相干性. 根据此理论, 同时计入温度和偏压的影响计算了MgO基磁性隧道结的温度-偏压相图. 理论结果表明, 通过调节MgO基磁性隧道结的铁磁电极半交换劈裂能Δ、化学势μ以及势垒层周期势v( K h)可以优化其温度特性和偏压特性. 该结果为MgO基磁性隧道结的应用提供了坚实的理论基础.Abstract: MgO-based magnetic tunnel junction is a hot issue in the field of spin electronic devices, and its temperature and bias voltage play quite an important role in practical applications. Therefore, it is desiderated to obtain the temperature-bias phase diagram of MgO-based magnetic tunnel junction. This paper develops a theory which is suitable for magnetic tunnel junctions with single crystal barrier. In this theory, the single crystal barrier is regarded as a periodic grating, and the tunneling process is treated by optical diffraction theory, so the coherence of the tunneling electron can be well taken into account. Most importantly, the theory can handle both the temperature effect and bias effect of MgO-based magnetic tunnel junctions. According to the present theory, the temperature-bias phase diagram of MgO-based magnetic tunnel junctions is calculated under different half the exchange splittings, chemical potentials and periodic potentials. The theoretical results show that the extreme phase point of tunneling magnetoresistance (TMR) can move to high temperature region through regulating half the exchange splitting Δ of ferromagnetic electrode of MgO-based magnetic tunnel junction. This will be beneficial to the applications of magnetic tunnel junctions at room temperature. Moreover, the chemical potential μ can change the bias corresponding to the maximum phase point of TMR. As is well known, the chemical potential will vary with the material of ferromagnetic electrode. Therefore, if the material of ferromagnetic electrode is chosen with a proper chemical potential, we can obtain a large TMR under high bias voltage. In other words, the output voltage can be considerably increased. This will be favorable for the preparation of high power devices. In addition, it is found that the phase diagram of TMR is significantly dependent on periodic potential v( K h). As a result, the effects of temperature and bias voltage in the MgO-based magnetic tunnel junctions can be optimized by regulating half the exchange splitting Δ, chemical potential μ, and periodic potential v( K h). The present work provides a solid theoretical foundation for the applications of MgO-based magnetic tunnel junctions.
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图 2 不同铁磁电极半交换劈裂能Δ下的温度-偏压相图 (a) Δ = 8 eV; (b) Δ = 9 eV; (c) Δ = 10 eV
Figure 2. Phase diagram of temperature and bias with variation of half the exchange splitting of the ferromagnetic electrodes Δ: (a) Δ = 8 eV; (b) Δ = 9 eV; (c) Δ = 10 eV.
图 3 不同的化学势μ下的温度-偏压相图 (a) μ = 10 eV; (b) μ = 11 eV; (c) μ = 12 eV
Figure 3. Phase diagram of temperature and bias with variation of chemical potential μ: (a) μ = 10 eV; (b) μ = 11 eV; (c) μ = 12 eV.
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