It was recently demonstrated in bilayers of permalloy and platinum, that by combining spin torques arising from the spin Hall effect with Oersted field-like torques, magnetization dynamics can be induced with a directional preference.1 This “unidirectional” magnetization dynamic effect is made possible by exploiting the different even and odd symmetry that damping-like and field-like torques respectively have when magnetization is reversed. The experimental method used to demonstrate this effect was the spin-torque ferromagnetic (ST-FMR) resonance technique; a popular tool used in the phenomenological quantification of a myriad of damping-like and field-like torques. In this report, we review the phenomenology which is used to describe and analyze the unidirectional magnetization dynamic effect in ST-FMR measurements. We will focus on how the asymmetry in the dynamics also is present in the phase angle of the magnetization precession. We conclude by demonstrating a utility of this directional effect; we will outline an improved experimental method that can be used to distinguish a phase-shifted field-like torque in a ST-FMR experiment from a combination of field-like and damping-like torques.
J. Sklenar, W. Zhang, M. B. Jungfleisch, and A. Hoffmann, "Unidirectional response in spin-torque driven magnetization dynamics," Proc. SPIE 10732, Spintronics XI, 1073212 (Presented at SPIE Nanoscience + Engineering: August 20, 2018; Published: 20 September 2018); https://doi.org/10.1117/12.2321259.
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