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.
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.
