Samanta Kartik, Tsymbal Evgeny Y
Department of Physics and Astronomy & Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588, United States of America.
J Phys Condens Matter. 2024 Sep 11;36(49). doi: 10.1088/1361-648X/ad765f.
Magnetic tunnel junctions (MTJs), that consist of two ferromagnetic electrodes separated by an insulating barrier layer, have non-trivial fundamental properties associated with spin-dependent tunneling. Especially interesting are fully crystalline MTJs where spin-dependent tunneling is controlled by the symmetry group of wave vector. In this work, using first-principles quantum-transport calculations, we explore spin-dependent tunneling in fully crystalline SrRuO/SrTiO/SrRuO(001) MTJs and predict tunneling magnetoresistance (TMR) of nearly 3000%. We demonstrate that this giant TMR effect is driven by symmetry matching (mismatching) of the incoming and outcoming Bloch states in the SrRuO(001) electrodes and evanescent states in the SrTiO(001) barrier. We argue that under the conditions of symmetry-controlled transport, spin polarization, whatever definition is used, is not a relevant measure of spin-dependent tunneling. In the presence of diffuse scattering, however, e.g. due to localized states in the band gap of the tunnel barrier, symmetry matching is no longer valid and TMR in SrRuO/SrTiO/SrRuO(001) MTJs is strongly reduced. Under these conditions, the spin polarization of the interface transmission function becomes a valid measure of TMR. These results provide an important insight into understanding and optimizing TMR in all-oxide MTJs.
磁隧道结（MTJs）由两个被绝缘势垒层隔开的铁磁电极组成，具有与自旋相关隧穿相关的重要基本特性。特别有趣的是完全结晶的MTJs，其中自旋相关隧穿由波矢对称群控制。在这项工作中，我们使用第一性原理量子输运计算，探索完全结晶的SrRuO/SrTiO/SrRuO(001) MTJs中的自旋相关隧穿，并预测隧穿磁电阻（TMR）接近3000%。我们证明，这种巨大的TMR效应是由SrRuO(001)电极中入射和出射布洛赫态与SrTiO(001)势垒中倏逝态的对称匹配（失配）驱动的。我们认为，在对称控制输运的条件下，无论使用何种定义，自旋极化都不是自旋相关隧穿的相关量度。然而，在存在漫散射的情况下，例如由于隧道势垒带隙中的局域态，对称匹配不再有效，SrRuO/SrTiO/SrRuO(001) MTJs中的TMR会大幅降低。在这些条件下，界面传输函数的自旋极化成为TMR的有效量度。这些结果为理解和优化全氧化物MTJs中的TMR提供了重要见解。
