Investigation of the Microstructural and Magnetic Properties of Ni on GeSn Layers

Thermal processing of nickel (Ni) layers on germanium-tin (GeSn) can strongly modify the Ni film and the Ni/GeSn interface within a restricted thermal budget; however, the influence of GeSn crystalline quality on Ni-Ge-Sn interfacial evolution and the magnetic response of the Ni layer remains incompletely understood. Here, Ni (15~nm)/Ge$_{1-x}$Sn$_x$/Ge/Gd$_2$O$_3$/Si(111) heterostructures ($x = 0.08$ and $0.10$) were investigated following rapid thermal annealing at 573 and 973~K for 10~s using X-ray diffraction, pole-figure analysis, X-ray photoelectron spectroscopy depth profiling, and temperature- and field-dependent magnetization measurements. Annealing at 573~K preserves the crystallographic texture of Ge$_{0.92}$Sn$_{0.08}$, whereas Ge$_{0.90}$Sn$_{0.10}$ exhibits microtwin formation and reduced structural integrity. Annealing at 973~K, employed as a severe upper-bound thermal reference, causes extensive structural degradation in both heterostructures, with substantially greater deterioration in the higher-Sn sample. XPS depth profiling reveals progressive Ni-Ge-Sn intermixing accompanied by pronounced Sn surface segregation with increasing annealing temperature. Magnetization measurements show coexisting ferromagnetic and superparamagnetic contributions in the as-deposited and 573~K samples, whereas annealing at 973~K strongly suppresses the ferromagnetic response, producing highly unsaturated $M(H)$ loops together with reduced saturation magnetization and coercivity. These magnetic changes are consistent with extensive interfacial alloying and dilution of the metallic Ni layer. This study establishes a direct correlation between GeSn crystalline quality, interfacial diffusion, and magnetic behavior.