Emergence of Scaling in Random Networks
Emergence of Scaling in Random Networks
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DOI：
10.1515/9781400841356.349
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
B. McInnes;Jannene S. McBride;N. Evans;David D. Lambert;A. Andrew
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文献类型：
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作者：
B. McInnes;Jannene S. McBride;N. Evans;David D. Lambert;A. Andrew
当LSMO的费米能级近似于Co的自旋2 DOS的最大值时，预计会出现Co水平和逆TMR最大值。这与Co/STO/LSMO结在20.4 V时观察到的逆TMR最大值一致（图3A）。对于正偏置，当LSMO的费米能级下降到Co的大部分自旋d带的能量范围时，TMR预计会改变符号，在1 V以上变为正常。这在图3A中也可以观察到。对于ALO和ALO/STO势垒，主要的s-特征电子隧穿（见图2B中的箭头）是正极化的通常解释（6-8）。随着偏置的快速下降（图3B）与在大多数具有ALO势垒的结中观察到的情况相似，与主要由d-特征电子穿隧时所获得的结果完全不同（图3A）。TMR在相对较小的偏置下迅速下降的原因从未得到清楚的解释。这与Nguyen-Mahn等人(8)计算Co-ALO界面时sp-d成键效应对ALO第一原子层诱导的DOS的能量依赖性大致一致。但Zhang等人（13）也表明，TMR下降的很大一部分可归因于自旋波的激发。本文和最近几篇论文（3,4）中报道的实验证明了金属-氧化物界面的电子结构在决定隧道电子的自旋极化方面的重要作用。Co-STO界面的负极化归因于Al和Ti(4)之间的d-d键合效应。这种解释类似于从sp-d键的角度解释Co-ALO界面上的正极化(8)。然而，对于co的实验结果，目前还没有普遍的理论预测，即与d元素的氧化物（STO， CLO, Ta2O5）呈负极化，而仅存在s态和p态时呈正极化（ALO）。自旋极化很可能也取决于费米能级相对于绝缘体间隙上下每个字符的电子能级的位置。此外，由于绝缘体中的倏逝波是具有虚波矢量的布洛赫波，因此可以预期不同性质的布洛赫波的衰减长度不同。这意味着最终的极化也可能取决于势垒的厚度，正如MacLaren等人对Fe/ZnSe/Fe结的计算所示（14）。势垒对自旋极化的影响为TMR的形成和优化开辟了新的途径。通过选择d电子并探测d- dos的精细结构，可以获得有趣的偏置依赖关系，如图3A所示。d波段的DOS也可以很容易地通过合金化（例如，通过引入虚拟束缚态）来产生特定的偏置依赖。虽然这里我们关注的是Co电极的自旋极化问题，并将强自旋极化LSMO仅视为有用的自旋分析仪，但Co和LSMO电极结合获得的大TMR比（50%与STO势垒）也是一个有趣的结果。LSMO的低居里温度（350 K）的缺点是室温下的TMR降低，Co/STO/ LSMO在300 K时下降到约5%(4)。然而，双钙钛矿家族的其他类型的氧化物（例如，Sr2FeMoO6）结合了与锰矿相似的电子性质，并且具有明显更高的居里温度（15）。它们在磁性隧道结中的应用为室温应用的新一代具有非常高磁阻的隧道结提供了希望。
level of Co and a maximum of inverse TMR is expected when the Fermi level of LSMO is approximately at the maximum of the spin2 DOS of Co. This is consistent with the maximum of inverse TMR observed at 20.4 V for Co/STO/LSMO junctions (Fig. 3A). For a positive bias, the TMR is expected to change sign and become normal above 1 V when the Fermi level of LSMO goes down into the energy range of the majority spin d-band of Co. This is also observed in Fig. 3A. For ALO and ALO/STO barriers, a predominant tunneling of s-character electrons (see arrow in Fig. 2B) is the usual explanation of the positive polarization (6–8). The rapid drop with bias (Fig. 3B) is similar to what has been observed in most junctions with ALO barriers, and completely different from what is obtained when the tunneling is predominantly by d-character electrons (Fig. 3A). The origin of this rapid decrease of the TMR at relatively small bias has never been clearly explained. This is roughly consistent with the energy dependence of the DOS induced by sp-d bonding effects on the first atomic layer of ALO in the calculation of Nguyen-Mahn et al. (8) for the Co-ALO interface. But Zhang et al. (13) have also shown that a large part of the TMR drop can be attributed to the excitation of spin waves. The experiments reported here and in several recent publications (3, 4) demonstrate the important role of the electronic structure of the metal-oxide interface in determining the spin polarization of the tunneling electrons. The negative polarization for the Co-STO interface has been ascribed to d-d bonding effects between Al and Ti (4). This interpretation is similar to that proposed to explain, in terms of sp-d bonding, the positive polarization at the Co-ALO interface (8). However, there is no general theory predicting the trend of the experimental results for Co—that is, a negative polarization with oxides of d elements (STO, CLO, Ta2O5) and a positive one when there are only s and p states (ALO). It is likely that the spin polarization should also depend on the position of the Fermi level with respect to the electronic levels of each character above and below the gap of the insulator. In addition, as an evanescent wave in an insulator is a Bloch wave with an imaginary wave vector, one can expect different decay lengths for Bloch waves of different character. This means that the final polarization could also depend on the thickness of the barrier, as illustrated by the calculations of MacLaren et al. for Fe/ZnSe/Fe junctions (14). The influence of the barrier on the spin polarization opens new ways to shape and optimize the TMR. Interesting bias dependencies can be obtained with barriers selecting the d electrons and probing the fine structure of the d-DOS, as in Fig. 3A. The DOS of a d-band can also be easily tailored by alloying (for example, by introduction of virtual bound states) to produce specific bias dependencies. Although here we concentrated on the problem of the spin polarization of the Co electrode and regarded the strongly spin-polarized LSMO only as a useful spin analyzer, the large TMR ratios obtained by combining Co and LSMO electrodes (50% with a STO barrier) are also an interesting result. The drawback arising from the low Curie temperature of LSMO (;350 K) is the reduction of the TMR at room temperature, down to about 5% at 300 K in Co/STO/ LSMO (4). However, other types of oxides of the double-perovskite family (for example, Sr2FeMoO6) combine electronic properties similar to those of manganites with a definitely higher Curie temperature (15). Their use in magnetic tunnel junctions is promising for a new generation of tunnel junctions with very high magnetoresistance for room-temperature applications.