Study of Multiferroic Tunnel Junctions

本奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。多铁性隧道结是指采用铁电绝缘体阻挡层的铁磁性隧道结。理论计算表明，铁电极化反转将导致隧道结的电阻开关行为。此外，铁电层和铁磁层之间的相互作用会影响势垒的电极化、界面处的电子结构和磁性能，以及隧道电流的自旋极化。因此，这种类型的结可以显示新的功能，如四元或八进制逻辑状态，它们可以被磁场和电场编码。它还可以开辟新的方法来潜在地控制自旋极化。然而，在夹在两个铁磁层之间的纳米厚铁电层中保持铁电性是极具挑战性的，而且以前还没有实现可靠的多铁结。该个人研究者奖支持了一项研究计划，该计划旨在系统地研究锰矿/Ba1-xSrxTiO3/锰矿和锰矿/Ba1-xSrxTiO3/Co结构中的多铁性隧道结。该项目的目标是首先建立这些结在电场和磁场下的行为，然后系统地研究电场和磁场对不同状态的操纵。该项目将为研究生和本科生（包括女性和少数族裔）提供一个刺激的环境，让他们在物理、材料科学、设备制造和测试方面获得跨学科的经验。该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。磁隧道结的最新进展使得在计算机读头、磁传感器和非易失性磁随机存取存储器中使用这些器件成为可能。隧道结由两个金属层和一个纳米厚的绝缘层或屏障组成。理论上预测，通过使用非常薄的铁电绝缘体（一种具有自发电荷极化的材料，可以通过电压切换）作为屏障，可以实现新的功能。例如，它可以显示可由磁场和电场编码的四元逻辑状态，而不是目前磁性隧道结中的二进制状态。这对于量子信息处理非常重要。铁磁层和铁电层之间的强相互作用也可以导致对自旋极化的电控制，这是实现功能性自旋电子器件的关键进展，这是一种依赖自旋极化输运而不是传统电子输运的新器件概念。然而，在纳米厚的材料中实现电荷极化是极具挑战性的，而且以前还没有制造过这样的结。该奖项支持一项研究计划，该计划使用锰矿作为电极，钛酸钡锶作为绝缘体来制造和系统地研究几种这样的结结构。该项目的目标是首先制造结，然后系统地研究结的性质以及如何利用电场和磁场来操纵不同的状态。该项目将为研究生和本科生（包括女性和少数族裔）提供一个刺激的环境，让他们在物理、材料科学、设备制造和测试方面获得跨学科的经验。

Technical AbstractThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Multiferroic tunnel junction refers to a ferromagnetic tunnel junction using a ferroelectric insulator barrier. Theoretical calculations have shown that the ferroelectric polarization reversal will result in resistance switching behavior in a tunnel junction. Furthermore, the interplay between the ferroelectric and ferromagnetic layers can affect the electric polarization of the barrier, the electronic structures and the magnetic properties at the interface, and the spin polarization of the tunneling current. This type of junctions can thus display new functionalities, such as quaternary or octal logic state, which can be encoded by both magnetic and electric fields. It can also open up new ways to potentially control spin polarization electrically. However, to preserve ferroelectricity in a nanometer thick ferroelectric layer sandwiched between two ferromagnetic layers is extremely challenging and reliable multiferroic junctions have not been achieved previously. This individual investigator award supports a research plan to fabricate and systematically study the multiferroic tunnel junctions in manganite/Ba1-xSrxTiO3/manganite and manganite/Ba1-xSrxTiO3/Co structures. The objective of the project is to first establish the behaviors of such junctions under electric and magnetic fields, and then to systematically study the manipulation of the different states by both electric and magnetic fields. The project will provide both graduate and undergraduate students, including women and underrepresented minorities, with a stimulating environment for interdisciplinary experience in physics, materials sciences, and device fabrication and testing. Non- Technical AbstractThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Recent advances in magnetic tunnel junctions have made it possible to use these devices in computer read head, magnetic sensors, and non-volatile magnetic random access memories. A tunnel junction consists of two metal layers and a nanometer thick insulator layer or barrier in between. It was predicted theoretically that by using a very thin ferroelectric insulator--a material with a spontaneous charge polarization which can be switched by an electrical voltage--as the barrier, new functionalities can be achieved. For example, it can display quaternary logic states which can be encoded by both magnetic and electric fields, instead of the binary states in the current magnetic tunnel junctions. This could be very important for quantum information processing. The strong interaction between the ferromagnetic layer and the ferroelectric layer can also lead to control of the spin polarization electrically, which is a critical advance in achieving functional spintronic devices, a new device concept that relies on the spin polarized transport instead of the conventional electron transport. However, to achieve charge polarization in a nanometer thick material is extremely challenging and such junctions have not been made previously. This award supports a research plan to fabricate and systematically study several such junction structures using manganite as the electrode and Barium Strontium titanate as an insulator. The goal of the project is to first fabricate the junctions, and then to systematically study the junction properties and how to manipulate the different states by using electrical and magnetic fields. The project will provide both graduate and undergraduate students, including women and underrepresented minorities, with a stimulating environment for interdisciplinary experience in physics, materials sciences, and device fabrication and testing.