Co layer Thickness Driven Structural Evolution and Magnetic Behaviour of Co/Ir/Co Trilayers

Abstract

Ferromagnetic heavy-metal (FM/HM) trilayer heterostructures provide a versatile platform for investigating the interplay between structural and magnetic properties. In this study, we investigate the effect of Co-layer thickness on the structural and magnetic properties of sputter-deposited $\mathrm{Co}(t_{\mathrm{Co}})/\mathrm{Ir}(5\mathrm{nm})/\mathrm{Co}(t_{\mathrm{Co}})$ varying from 2 to 10 nm. Structural characterization using grazing-incidence X-ray diffraction (GIXRD) and high-resolution transmission electron microscopy (HRTEM) reveals a nanocrystalline structure, with the crystallite size increasing with Co-layer thickness. Furthermore, X-ray reflectivity (XRR) reveals a thickness-dependent variation in interface morphology, with the interface roughness ratio reaching a maximum at $t_{\mathrm{Co}}=4$ nm. Temperature-dependent zero-field-cooled and field-cooled (ZFC-FC) magnetization measurements reveal thickness-dependent characteristic temperatures and multiple relaxation regimes in selected samples, indicating complex thermally activated magnetic behaviour. In contrast, field-dependent hysteresis measurements reveal that the saturation magnetization increases with Co-layer thickness, whereas the coercivity exhibits a pronounced non-monotonic variation, with enhanced values at intermediate thicknesses. Moreover, low-temperature hysteresis loops display thickness-dependent exchange-bias-like shifts, particularly for $t_{\mathrm{Co}}=2$-$4$ nm. These findings highlight the strong influence of Co-layer thickness on the structural and magnetic properties of Co/Ir/Co trilayers.