Fe layer thickness driven evolution of magnetic states in Gd/Fe multilayers

This study explores the structural and magnetic properties of as-deposited ${[\mathrm{Gd}(4\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Fe}(t\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{\ifmmode\times\else\texttimes\fi{}10}$ multilayers, with $t=2.0,\phantom{\rule{4pt}{0ex}}2.5$, and $3.0\phantom{\rule{4pt}{0ex}}\mathrm{nm}$. Structural characterization was carried out using both laboratory-based and synchrotron-based x-ray techniques. X-ray reflectivity (XRR) measurements indicate the formation of a GdFe alloy at the interfaces for elevated Fe thickness, suggesting significant interfacial intermixing during deposition. Magnetic properties were investigated using the magneto-optical Kerr effect (MOKE), bulk magnetization measurements, and simulations based on a modified Stoner-Wohlfarth model for a one-dimensional spin chain. The structural and magnetic properties suggest the formation of superparamagnetic Fe clustering at room temperature for the lower Fe thickness sample. Magnetic measurements reveal a distinct evolution from a Gd dominated phase to Fe dominated phase with increasing Fe layer thickness, with the intermediate sample showing both Gd and Fe aligned phase across temperature. Simulations, incorporating inputs from structural measurements, successfully reproduce the evolution of hysteresis loops with temperature and Fe thickness. The simulations provide layer-resolved insights into spin configurations, elucidating the formation of twisted/fan-like magnetic states due to the competition between antiferromagnetic exchange coupling and the external field across varying temperatures. Our results also reinforce the fact that the interfacial alloying significantly decreases the compensation temperature of the system.