Energy-efficient phase-locked arrays of spin torque nano-oscillators based on current-induced torques in magnetic metals

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Further advances in energy-efficiency and speed of computing require new revolutionary approaches such as brain-inspired (neuromorphic) signal processing. Neuromorphic devices based on conventional semiconductor components are power-hungry. Therefore, new types of components are needed for neuromorphic signal processing. One such promising component is a spin torque nano-oscillator that consists of a magnetic and a non-magnetic metallic layer, electric current flowing in the non-magnetic layer generates microwave electric signal in the magnetic layer. The goal of this research project is to significantly improve energy efficiency of spin torque nano-oscillator via demonstration of a new method of microwave signal generation by electric current that flows in the magnetic layer rather than the non-magnetic layer. It is expected that the proposed method will reduce power consumption of spin torque nano-oscillators by over 90 percent thereby enabling the advancement in computer technology. Graduate and undergraduate students, including persons from underrepresented groups, will be trained in device nanofabrication and characterization techniques. This will contribute to training of the qualified workforce needed for reestablishing the United States leadership in semiconductor chip development and manufacturing. The PI will participate in middle school outreach activity held twice a year. As a part of this activity, middle school students will visit the PI’s lab and participate in hands-on experiments on magnetism, superconductivity, vacuum technology and plasma physics.Spin-orbit torques are at the core of several promising spintronic technologies, including non-volatile magnetic memory, spin torque nano-oscillators for non-Boolean signal processing and ultrasensitive microwave detectors. Fundamental understanding of spin-orbit torques is crucial for the development of these devices. In this project, a new class of spin-orbit torques – universal self-generated spin-orbit torques in ferromagnetic metals will be studied. These torques are self-generated because they are applied to magnetization of the ferromagnet in which they are produced, and are universal because their presence is expected in all types of ferromagnetic metals independent of their crystal structure. The torques will be measured by spin-torque ferromagnetic resonance in nanowire- and nanocross-based structures prepared using e-beam lithography from epitaxial and polycrystalline magnetic multilayers deposited by magnetron sputtering. Two major types of self-generated torques, anomalous Hall torque and planar Hall torque, will be studied, and ferromagnetic systems maximizing the magnitude of these torques for technological applications will be determined. To illustrate the transformative potential of the novel torques, highly energy-efficient phase-locked arrays of spin torque nano-oscillators will be demonstrated. Potential of these devices for neuromorphic signal processing will be evaluated. Operation of such spin torque oscillator arrays in zero magnetic field will be demonstrated in structures with interfacial and strain-induced magnetic anisotropy. Energy efficiency of spin torque nano-oscillators driven by universal self-generated spin-orbit torques will be compared to that of spin torque nano-oscillators driven by spin-orbit torques from topological insulators and Weyl semimetals.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.