双磁性分子耦合体系的隧穿磁阻研究

发布时间：2018-05-27 04:31

本文选题：分子磁体双子 + 自旋极化　；参考：《山西大学》2016年博士论文

【摘要】：介观系统中的量子输运已经被研究了数十年。在简单的量子点系统中就有很多有趣的效应被发现,例如近藤效应、库伦阻塞、泡利自旋阻塞、负微分电导、隧穿磁阻效应、自旋轨道耦合和热电效应等。最近几年,人们更多关注分子磁体,其中分子磁体与电子之间的自旋交换相互作用都会导致一些新奇的现象。磁隧穿结中由电流不平衡引起的隧穿磁阻效应尤其受到人们的关心。首先,我们用顺序隧穿机制下的率方程方法研究了串联双量子点中的自旋极化输运,其中串联双量子点弱耦合于共线的铁磁电极,而我们主要关注自旋交换相互作用对输运特性的影响。我们在非对称的双量子点输运中发现巨大的负微分电导,它是由两个量子点间的能级差引起的。事实证明,隧穿磁阻与偏压相关,而且散粒噪声对自旋交换相互作用非常敏感,而自旋交换相互作用使得自旋极化电流更加不平衡。电极磁化方向平行时,自旋交换相互作用更倾向于自旋聚束,因此超泊松噪声增大,而当双量子点不对称且电极为反平行结构时,强的反铁磁自旋相互作用会抑制散粒噪声。电压影响隧穿磁阻和散粒噪声这一性质可能对自旋交换相互作用的测量有帮助。然后,我们在顺序隧穿机制下分别研究了铁磁和反铁磁类型的分子磁体双子的自旋极化输运,其中双子串联弱耦合于共线的铁磁电极。我们主要关注自旋交换相互作用对隧穿磁阻(TMR)的影响。当两个分子磁体之间的自旋交换相互作用为反铁磁时,在正偏压区域会出现极大的正隧穿磁阻,而且我们发现会出现隧穿磁阻达到理论值下限的区域。因此,分子磁体双子可以作为一个理想的阀器件去完全阻塞电极平行磁化时的电流。最后,我们研究了隧穿结的量子输运,其中单轴各向异性的分子磁体双子与铁磁电极磁化方向不共线。不同于通常模型,分子磁体的易轴与铁磁电极极化方向不共线。我们主要关心非共线角对自旋极化电流和隧穿磁阻的影响。对于铁磁自旋交换相互作用耦合的双子,正(多数)分量的自旋极化电流随着非共线角的增大而减小,而负分量几乎不变。因此电流极化率在某个特定角度发生反转,而且当各向异性轴与电极磁化方向垂直时,高电流极化和低电流密度同时出现。在反铁磁双子结中,我们也发现了可调节的隧穿磁阻从负值变化到正值。我们的发现可能应用于自旋阀器中的技术。
[Abstract]:Quantum transport in mesoscopic systems has been studied for decades. Many interesting effects have been found in simple quantum dot systems, such as Kondo effect, Coulomb blocking, Pauli spin blocking, negative differential conductance, tunneling magnetoresistive effect, spin orbit coupling and thermoelectric effect. In recent years, more attention has been paid to molecular magnets, in which spin-exchange interactions between molecular magnets and electrons lead to novel phenomena. The tunneling magnetoresistive effect caused by current imbalance in magnetotunneling junction is especially concerned. Firstly, we study the spin polarization transport in series double quantum dots using the rate equation method under sequential tunneling mechanism, in which the series double quantum dots are weakly coupled to the collinear ferromagnetic electrodes. We mainly focus on the effect of spin exchange interaction on transport characteristics. We find a large negative differential conductance in the transport of asymmetric double quantum dots, which is caused by the energy level difference between two quantum dots. It is proved that the tunneling magnetoresistance is related to the bias voltage, and the particle noise is very sensitive to the spin exchange interaction, and the spin exchange interaction makes the spin polarization current more unbalanced. When the magnetization direction of the electrode is parallel, the spin exchange interaction is more inclined to spin bunching, so the superPoisson noise increases, while when the double quantum dot is asymmetric and the electrode is antiparallel, the strong antiferromagnetic spin interaction will suppress the particle noise. The effect of voltage on tunneling magnetoresistance and granular noise may be helpful for the measurement of spin exchange interactions. Then we study the spin-polarized transport of ferromagnetic and antiferromagnetic type molecular magnets under the sequential tunneling mechanism in which the Gemini in series is weakly coupled to the collinear ferromagnetic electrodes. We mainly focus on the effect of spin exchange interaction on tunneling magnetoresistive (TMR). When the spin exchange interaction between two molecular magnets is antiferromagnetic, there will be a great positive tunneling magnetoresistance in the positive bias region, and we find that the tunneling magnetoresistance reaches the lower limit of the theoretical value. Therefore, the molecular magnet binon can be used as an ideal valve device to completely block the current of parallel magnetization of the electrode. Finally, we study the quantum transport of tunneling junctions, in which uniaxial anisotropic molecular magnets are not collinear with ferromagnetic electrodes. Unlike the usual model, the easy axis of the molecular magnet is not coincident with the polarization direction of the ferromagnetic electrode. We are mainly concerned with the influence of noncollinear angles on the spin polarization current and tunneling magnetoresistance. For ferromagnetic spin-exchange coupling binons, the spin polarization current of the positive (majority) component decreases with the increase of the non-collinear angle, but the negative component is almost invariant. Therefore, the current polarizability reverses at a particular angle, and when the anisotropic axis is perpendicular to the magnetic direction of the electrode, both high current polarization and low current density occur simultaneously. In the antiferromagnetic binon junction, we also find that the tunable tunneling magnetoresistance changes from negative to positive. Our findings may be applied to techniques in spin valves.
【学位授予单位】：山西大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O561

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