Interface-driven static and dynamic magnetic properties of ultrathin Fe/Ge multilayers

Abstract

Ferromagnet-semiconductor (FM-SC) alloy phase with room temperature ferromagnetic properties at the interfaces of FM/SC heterostructures are believed to improve the electron spin injection from the FM to the SC for the realization of spintronic devices. Here we report the formation of a crystalline alloy phase (Fe3Ge) at the interfaces on growing ultrathin Fe/Ge multilayers at an elevated substrate temperature of ∼ 523 K as well as on annealing the multilayer at a temperature > 523 K. The alloy phase formation at interfaces is achieved at a much lower temperature than the Fe3Ge phase formation in bulk material (∼973 K), mainly due to rapid interdiffusion of Fe and Ge at the interfaces which is about 4 (∼2) order higher in magnitude than the diffusion of Ge (Fe) in the bulk crystalline phase. We find a large magnetic anisotropy constant (K ∼ 3 × 106 to 7 × 107 erg/cm3) on the formation of the nanocrystalline alloy phase at interfaces. Ultrathin Fe/Ge multilayers were characterized using x-ray scattering, macroscopic magnetization measurement, and polarized neutron reflectivity (PNR). The temperature-induced diffusion process indicates that the alloy phase is structurally stable up to higher temperature. PNR provided a detailed depth profile of the magnetization in the system and suggested that the alloy phase is ferromagnetic at room temperature. We have also studied the dynamic magnetic properties of these systems by employing ferromagnetic resonance measurements.