Voltage controlled antiferromagnetism in magnetic tunnel junctions

非技术摘要：影响薄膜磁性的一种有效方法是使用电场，这可能会在未来几代电子器件中带来许多新的功能。在大多数情况下，这种类型的磁序电控制是在铁磁(FM)系统中研究的。例如，Fe或Co薄膜的优选磁化方向和磁化大小都可以通过施加在相邻介质层上的电压来改变。事实上，这种效应已经成功地用于操纵磁隧道结(MTJ)的电阻，MTJ是一种夹层结构，其中电子跨越绝缘势垒的隧道几率取决于绝缘体两侧两个FM膜的磁化的相对取向，以获得低于10fJ的小开关能量。在这个项目中，主要研究人员探索了反铁磁(AF)系统中的压控磁性。与FM相比，反铁磁体具有许多优势：它们没有净磁化，因此可以被包装成极高的密度，而不会相互影响；出于同样的原因，它们不会受到外部磁场的影响；由于反铁磁体中自旋的交错排列，自旋电流可能会穿透更深的地方；最重要的是，反铁磁体的固有磁化切换频率可以在THz区，有望实现超高速操作，同时降低开关能量。该项目将研究生/本科生和高中生的教育和培训纳入研究的所有阶段，特别是那些属于代表性不足群体的学生。此外，还将通过物理Phun之夜和物理开放日等活动向公众进行宣传。技术摘要：该项目旨在探索具有AF势垒的MTJ中的压控反铁磁性，以及具有自旋过滤隧道效应的MTJ中的磁阻由AF/FM耦合驱动。在第一个研究课题中，制备了具有AF势垒的垂直MTJ，例如氧化铬(III)，其中绝缘层既用作隧道势垒又用作AF材料。通过测量不同偏置电压下的垂直隧道磁阻曲线的突变，可以非常灵敏地检测到AF材料的交换偏置和压控的AF有序。生长具有极小粗糙度的高质量薄膜异质结构的能力，以及横向尺寸小于100 nm的垂直MTJ样品的制备，将首次能够在较少与晶界和AF层缺陷相关的寄生效应的情况下研究许多固有特性。在第二个研究课题中，将探索以新发现的二维(2D)材料为势垒的自旋过滤隧道效应。将不同的2D材料与不同的非磁性电极结合在一起，以便于理解超大型TMR。这项工作将集中在2D势垒中的压控AF/FM耦合，这可能会导致非常有效的MTJ开关。这些研究活动不仅将显著增加我们对电压控制反铁磁性的理解，而且还将有益于MRAM、自旋逻辑、自旋电子振荡器和神经形态处理器等应用。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical Abstract:An effective way to influence the magnetic properties of a thin film is by using electric fields, which may lead to many new functionalities in future generations of electronic devices. In most cases, this type of electric control of magnetic order is investigated in ferromagnetic (FM) systems. For example, both the preferred direction of magnetization and the size of magnetization of a Fe or Co thin film can be modified by voltages applied on adjacent dielectric layers. Indeed, this effect has been successfully used to manipulate the resistance of a magnetic tunnel junctions (MTJ), a sandwich structure where the tunneling probability of electrons across an insulting barrier depends on the relative orientations of the magnetizations of the two FM films on both sides of the insulator, to achieve a small switching energy that is below 10 fJ. In this project, the principal investigator explores the voltage-controlled magnetic properties in antiferromagnetic (AF) systems. Antiferromagnets have a number of advantages compared with the their FM counterparts: they have no net magnetization therefore AF cells can be packed into extremely high density without affecting each other; for the same reason, they are immune to external magnetic fields; due to the staggered arrangement of spins in antiferromagnets, the spin currents can possibly penetrate much deeper; and most importantly, the intrinsic magnetization switching frequency of antiferromagnets can be in the THz region, promising ultra-high speed operations as well as reduced switching energy. This project incorporates the education and training of graduate/undergraduate students and high school students, especially those belonging to underrepresented groups, in all stages of the research. In addition, outreach to general public will be carried out through activities such as Physics Phun Night and Physics Open House.Technical Abstract:This project aims to explore voltage-controlled antiferromagnetism in MTJs with AF barriers, and in MTJs with the spin-filtering tunneling effect where the magnetoresistance is driven by the AF/FM coupling. In the first research topic, perpendicular MTJs with AF barriers such as Chromium(III) oxide are fabricated, where the insulating layer serves as both the tunnel barrier and the AF material. The exchange bias of the AF materials and the voltage-controlled AF order can be very sensitively detected by the sharp transitions of the perpendicular tunneling magnetoresistance curves measured under different bias voltages. The ability to grow high quality thinfilm heterostructure with extremely small roughness, as well as the fabrication of perpendicular MTJ samples with sub-100nm lateral dimension will enable, for the first time, the investigation of many intrinsic properties with less parasitic effects associated with grain boundaries and defects in the AF layer. In the second research topic, the spin-filtering tunneling effect with the newly discovered 2 dimensional (2D) materials as barriers will be explored. Different 2D materials are incorporated with various nonmagnetic electrodes to facilitate the understanding of the very large TMR. The effort will be focused on voltage-controlled AF/FM coupling in 2D barriers, which can potentially lead to very efficient switching of MTJs. These research activities will not only add significantly to our understanding of voltage-controlled antiferromagnetism, but also benefit applications such as MRAM, spin logic, spintronic oscillators, and neuromorphic processors.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.