Exploring the Limits of RKKY Coupling for Synthetic Antiferromagnets

Synthetic antiferromagnets (SAFs), made from ferromagnet/non-magnetic metal/ferromagnet exchange-coupled heterostructures, represent a crucial component of magnetic tunnel junctions and giant magnetoresistance spin valves. Yet, beyond their usage for pinning the magnetization in these multilayer, ultra-thin-film stacks, the limits of their reach in thicker films and other applications have not been given much attention. The widespread practice is to grow SAFs with a non-magnetic metal spacer layer thickness optimized to achieve the first oscillatory peak for antiferromagnetic interlayer exchange coupling. However, recent work by Waring et al. suggests that when Ru is used as the metal spacer material, the second oscillatory antiferromagnetic exchange coupling peak may, in fact, achieve better antiferromagnetic anisotropy resulting from the fact that the slightly thicker Ru layer provides better uniformity of the grown material film [1]. This increased interlayer exchange coupling manifests itself as a narrower resonance peak with fewer damping losses in the SAF structure, thereby translating to more efficient, low-power operation. Delving deeper into this question of efficiency, the present research work explores the variation in magnetic anisotropy in Co₄₃Fe₄₃B₁₄/Ru/Co₄₃Fe₄₃B₁₄ exchange-coupled SAF trilayers. Thicknesses of both the non-magnetic and magnetic layers are experimentally varied to observe the change in RKKY coupling strength.<br/><br/>Varying the thickness of the Ru non-magnetic metal spacer between the two ferromagnetic layers produced the signature oscillatory behavior between ferromagnetic and antiferromagnetic interlayer exchange coupling. In the first set of experiments, Si substrate/SiO₂/CoFeB (5 nm)/R (t_Ru)/CoFeB (5 nm)/Pt (5 nm) trilayer films were grown with a Ru spacer layer thickness varying between t_Ru = 0.25 to 3.0 nm, in increments of 0.125 nm steps. A single layer of CoFeB with thickness 10 nm (without a Ru spacer in the middle) was also deposited and used to calibrate the total magnetic moment expected from the SAF multilayer films. Upon inspection, the second antiferromagnetic RKKY peak appeared to correspond to the SAF trilayer with a Ru thickness, t_Ru = 1.875 nm, which was thicker than the 1.1 nm Ru thickness observed by Waring, et al. Nevertheless, by analyzing magnetization hysteresis behavior for each of the grown films, we demonstrate the gradual transitions between interlayer coupling configurations and strengths.<br/><br/>At the optimal antiferromagnetic interlayer coupling, t_Ru = 1.875 nm, the thickness of the ferromagnetic layers was also varied. For thinner ferromagnetic films, the exchange coupling across the Ru thickness is expected to have a strong effect throughout the entire magnetic thickness. However, as the ferromagnetic layer thickness increases, the antiferromagnetic interlayer exchange coupling becomes less pronounced. This behavior is seen experimentally as the thickness of the ferromagnetic layer in the stack substrate/SiO₂/CoFeB (t_FM)/Ru (1.875 nm)/CoFeB (t_FM)/Pt (5 nm) is varied between t_FM = 2.5 nm to 25 nm in steps of 2.5 nm. As expected the antiferromagnetic exchange decreases as the thickness of the ferromagnetic layer it has to act upon increases, corresponding to an overall reduction in the anisotropy. Thus, this research illustrates that optimization of both the non-magnetic metal spacer and the ferromagnetic layer thicknesses can be used to achieve strongly-coupled and highly-efficient synthetic antiferromagnetic films for future low-power integrated systems.<br/><br/>[1] H. J. Waring, N. A. B. Johansson, I. J. Vera-Marun, and T. Thomson, “Zero-field Optic Mode beyond 20 GHz in a Synthetic Antiferromagnet,” <i>Phys. Rev. Appl.</i>, vol. 13, no. 3, p. 1, 2020.