Ferroelectric and Multiferroic Tunnel Junctions
Ferroelectric and Multiferroic Tunnel Junctions
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DOI：
10.5772/10499
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发表时间：
2010-12
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影响因子：
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通讯作者：
Tianyi Cai;S. Ju;Jian Wang;Zhen-ya Li
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文献类型：
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作者：
Tianyi Cai;S. Ju;Jian Wang;Zhen-ya Li
自量子力学出现以来，电子隧穿现象已经为人所知，但它继续丰富我们对许多物理领域的理解，并为开发有用的设备提供了一条途径。隧道结由两个金属电极组成，电极之间隔着一层纳米厚的绝缘势垒层，其中电子可以越过超过电子能量的势垒。因此，在势垒的另一侧发现电子的概率是有限的。在20世纪70年代，Tedrow和Meservey(1)(2)观察到自旋依赖的电子从铁磁性金属电极穿过非晶Al2O3薄膜。基于这一发现，Julliere提出并证明了在磁性隧道结中，隧道电流取决于两个铁磁电极的相对磁化方向(3)。这种现象现在被称为隧道磁阻（TMR）(4)。磁隧道结在自旋电子学器件（如磁场传感器和磁随机存储器）的各种技术应用中非常有用。其它绝缘子也用于隧道屏障。例如，De Teresa等人研究了外延钙钛矿SrTiO3势垒，以证明界面在自旋相关隧道中的重要性(5)。Ikeda等人发现，在具有MgO势垒的隧道结中，室温下的磁阻高达604%，5k时的磁阻高达1144%(6)，接近Butler等人(7)和Mathon等人(8)的理论预测。尽管用作隧道结势垒的材料多种多样，但其共同的特点是几乎所有的势垒都是非极性介质。另一方面，磁性绝缘子EuO， EuS和EuSe用于隧道屏障。Moodera等人(9)首先讨论了在这些结中观察到自旋滤波。在1988年。他们观察到Au/EuS/Al结中的隧道电流具有高达80%的自旋极化。并将其归因于电子穿过自旋相关势垒的隧穿（图1）。后来，他们报道了穿过Ag/EuSe/Al结的隧道电流增强了自旋极化，达到97%（10）。最近，Santos等人利用居里温度（69 K）高于EuS （16.7 K）和EuSe （4.6 K）的EuO，获得了29%的自旋极化隧穿电流（11）。当然，如果电极不是普通金属，而是铁磁性材料，则可以观察到TMR和自旋过滤效应（图2）（12）。另一个重要的概念是铁电隧道结（FTJ）（13）（14）（15），它利用铁电作为势垒材料。铁电体具有自发的2
The phenomenon of electron tunneling has been known since the advent of quantum mechanics, but it continues to enrich our understanding of many fields of physics, as well as offering a route toward useful devices. A tunnel junction consists of two metal electrodes separated by a nanometer-thick insulating barrier layer, in which an electron is allowed to transverse a potential barrier exceeding the electron’s energy. The electron therefore has a finite probability of being found on the opposite side of the barrier. In the 1970’s, spin-dependent electron tunneling from ferromagnetic metal electrodes across an amorphous Al2O3 film was observed by Tedrow and Meservey(1)(2). Based on this discovery, Julliere proposed and demonstrated that in a magnetic tunnel junction tunnel current depends on the relativemagnetization orientation of the two ferromagnetic electrodes(3). Such a phenomenon nowadays is known as tunneling magnetoresistance(TMR)(4). Magnetic tunnel junctions may be very useful for various technological applications in spintronics devices such as magnetic field sensors and magnetic random access memories. Other insulators are also used for tunnel barriers. For example, epitaxial perovskite SrTiO3 barriers were studied by De Teresa et al. to demonstrate the importance of interfaces in spin-dependent tunneling(5). In tunnel junctions with MgO barriers, Ikeda et al. found large magnetoresitance as high as 604% at room temperature and 1144% at 5 K(6), which approaches the theoretical predictions of Butler et al.(7) and Mathon et al.(8). Despite the diversity of materials used as the barrier of the tunnel junctions, the common feature is that almost all the barriers are nonpolar dielectrics. On the other hand, magnetic insulators, i.e, EuO, EuS and EuSe, are used for tunnel barriers. Spin filtering has been observed in these junctions as were first discussed by Moodera et al.(9). in 1988. They observed that the tunneling current in Au/EuS/Al junction has a spin polarization with the magnitude as high as 80%. and attributed it to the electron tunneling across the spin-dependent barriers (Fig.1). Later, they reported that the tunneling current across Ag/EuSe/Al junctions has an enhanced spin-polarization reaching 97%(10). Recently, using EuO with a higher Curier temperature (69 K) than EuS (16.7 K) and EuSe (4.6 K), Santos et al. obtained 29% spin-polarized tunneling current(11). Naturally, if electrodes are not normal metals, but ferromagnetic materials, both TMR and spin filter effects can be observed(Fig.2)(12). Another important concept is the ferroelectric tunnel junction (FTJ)(13)(14)(15), which take advantage of a ferroelectric as the barrier material. Ferroelectrics possess a spontaneous 2