November 07, 2022
Minneapolis, United States
Spin Hall oscillators based on ferromagnetic metal/lithium aluminum ferrite bilayers with enhanced output power and quality factor
Spin-Hall oscillators (SHOs) are promising spintronic devices to realize dc controlled GHz frequency signals in nanoscale devices for a variety of applications1,2, including for neuromorphic computing3. However, traditional SHOs have high auto-oscillation threshold currents and those based on magnetic tunnel junctions require a complex etching process. Here, we demonstrate that by combining a metallic ferromagnetic nanowire with low damping and high-quality ferromagnetic insulators, specifically epitaxial lithium aluminum ferrite thin films4, SHO characteristics are dramatically improved. Our magnetic bilayers SHOs are composed of Pt(5nm)/Py(5nm)/(Li0.5Al1.0Fe1.5O4 (LAFO) or Li0.5Al0.5Fe2O4 (LFO)) (x nm) layers with varied thicknesses x (including x=0, i.e. just Pt/Py layers) and two different compositions of insulator. The Pt/Py layers are patterned into 400 nm wide x 400 nm long nanowires between two Au contact pads as shown in Fig. 1a. Ferromagnetic resonance (FMR) measurements of the unpatterned heterostructure and spin-torque ferromagnetic resonance (ST-FMR) of micron-scale wires show that Py and LAFO are strongly ferromagnetically coupled and thus precess coherently. In the nanowires, we observe two prominent auto-oscillation modes in all samples, which we associate with bulk and edge spin-wave modes. Increased oscillator quality factor and maximum emission power (Fig. 1b) as well as lower threshold currents are obtained in samples containing LAFO and LFO. In addition, the maximum emission power and quality factor increases with the LAFO thickness. The performance can be further enhanced by replacing LAFO with LFO which possesses higher saturation magnetization. The improved characteristics are associated with the excitation of larger spin-precession angles and volumes in LAFO and LFO samples. We further find that the presence of LAFO enhances the auto-oscillation amplitude of spin-wave edge modes, consistent with our micromagnetic modeling.
1 V.E. Demidov, S. Urazhdin, H. Ulrichs, V. Tiberkevich, A. Slavin, D. Baither, G. Schmitz, and S.O. Demokritov, Nat. Mater. 11, 1028 (2012). 2 Z. Duan, A. Smith, L. Yang, B. Youngblood, J. Lindner, V.E. Demidov, S.O. Demokritov, and I.N. Krivorotov, Nat. Commun. 5, 1 (2014). 3 J. Grollier, D. Querlioz, K. Y. Camsari, K. Everschor-Sitte, S. Fukami, and M. D. Stiles, Nature Electronics 3, 360 (2020) 4 X.Y. Zheng, L.J. Riddiford, J.J. Wisser, S. Emori, and Y. Suzuki, Appl. Phys. Lett. 117, 1 (2020).