Asymmetric proximity effect of interlayer exchange coupling in CoFe/NiFe bilayer systems probed by optical mode ferromagnetic resonance

Yi-Kai Liao^1*, Kuan-Chia Chiu¹, Jing Chien¹, Hong-Bin Lu¹, Chin Liao², Pei-hsun Jiang², Minn-Tsong Lin^1,3,4

¹Department of Physics, National Taiwan University, Taipei, Taiwan
²Department of Physics, National Taiwan Normal University, Taipei, Taiwan
³Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
⁴Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan

* Presenter:Yi-Kai Liao, email:liaoyikai1994@gmail.com

We present a systematic study of how individual layer thicknesses influence the interlayer exchange coupling in CoFe/NiFe bilayer films. By independently varying the thickness of either the CoFe or NiFe layer, we uncover distinct thickness dependences in the optical mode of ferromagnetic resonance (FMR), revealing asymmetric behavior that challenges conventional methods for extracting the exchange coupling constant. To resolve this, we establish a thickness-dependent model that attributes the observed asymmetry to the unequal proximity effects of the two ferromagnetic layers. This framework allows reliable determination of the intrinsic interfacial exchange coupling and provides deeper insight into the evolution of optical and acoustic FMR modes with varying layer thickness. Our findings emphasize that thickness-dependent proximity effects play a decisive role in accurately quantifying interlayer exchange interactions in ferromagnetic bilayers and in interpreting their collective spin-dynamical behavior.

We thank G. J. Chen and S. Y. Huang for assistance with the VSM measurement. The work was supported by the National Science and Technology Council in Taiwan through grant No. NSTC 112-2112-M-002-046-MY3.

Keywords: Ferromagnetic resonance, Bilayer systems, Exchange coupling, Optical mode