Non-volatile magnetic memory devices based on field-free spin-orbit torque switching of perpendicularly polarized magnets.

Exploitation of spin degree of freedom in solid state systems is key to realizing ultrafast, energy efficient, and densely packed magnetic data storage devices. In these magnetic memory devices, the information is encoded in the magnetization direction of a bistable ferromagnetic thin film layer. To achieve thermally stable and closely packed magnetic bits, a perpendicularly magnetized ferromagnetic layer is preferred.

Spin orbit torques, generated by applying a charge current to an adjacent spin source material, has emerged as an efficient means to manipulate or switch the perpendicularly magnetized ferromagnetic layer. However, an external magnetic field is required to achieve deterministic spin orbit torque switching of a perpendicularly magnetized ferromagnetic layer because the charge-current induced spin current in conventional spin-source materials is polarized in the film plane due to the crystal symmetry.

This fundamental limitation in conventional spin-source materials has prevented the use of spin orbit torque to achieve field-free deterministic switching of ferromagnets with perpendicular magnetic anisotropy. To overcome this longstanding hurdle in the field of spintronics, this research program proposes to explore low-symmetry crystal structure and topological electronic structure in a special class of materials, namely Weyl semimetals, to demonstrate field-free deterministic magnetization switching of variety of perpendicularly magnetized ferromagnets.

A successful outcome of this research program will lead to next generation energy efficient magnetic memory and spin-logic devices. This research program will support the education and training of a graduate student and undergraduate students. Through proposed research activities, the graduate student and undergraduate students will be trained in experimental techniques and skill sets for spin orbit torques for nonvolatile magnetic memory devices.

Educational outreach activities and events will be planned to kindle scientific interest and provide interactions and mentorship for middle school and high school students, especially those in southwestern Pennsylvania. Moreover, hands-on experimental demos to explain spin and magnetism-related phenomena will be developed for outreach activities.

Utilization of topology and crystal-symmetry in emergent quantum materials, to obtain large current induced spin orbit torques for an energy efficient and field-free manipulation of the magnetization in FM materials, is promising for spintronic device applications. In this context, Weyl semimetals provide a distinct opportunity to obtain highly efficient and unconventional charge to spin conversion owing to strong spin-orbit coupling, symmetry breaking, and topology-based phenomena.

The goal of this research program is to demonstrate field-free operation of spin orbit torque-based prototype magnetic memory devices that exploit the interplay of low-symmetry crystal structure and topological electronic structure in Weyl semimetal candidate materials to generate spin current with out-of-plane spin polarization. The main research objectives of this program are to use electric field induced out-of-plane oriented spin current in Weyl semimetals to demonstrate field-free magnetization manipulation in three different classes of ferromagnets with perpendicular magnetic anisotropy, namely: (1) van der Waals based semi-metallic ferromagnets; (2) van der Waals based semiconducting ferromagnets; and (3) a ferromagnetic insulator.

In proposed devices, integrated electrostatic gates will be used to tune the charge carrier density in Weyl semimetal thin films, to control magnetism in semiconducting ferromagnets for enhanced spin orbit torque-based device functionalities, and to probe electric-field dependent switching phase diagram to map out the optimal parameter space for low-energy spin orbit torque switching for magnetic memory applications.

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.