Structural and magnetic properties of Ni nanofilms on Ge(001) by molecular beam epitaxy

Ni films of 20 nm nominal thickness were grown on Ge(001) substrates by molecular beam epitaxy at several different temperatures from room temperature up to 400 °C. X-ray diffraction and X-ray photoelectron spectroscopy reveal the nucleation of Ni-Ge compounds (NiGe, Ni2Ge, Ni5Ge2) as well as a departure from the fcc Ni structure exhibited by the films at and beyond a temperature of 100 °C. The binding energy of the Ni 2p peak increases from the RT value (852.7 eV) by 0.5-1.1 eV for the Ni/Ge(001) samples, while the Ge 2p binding energy changes by 0.6-0.7 eV after Ni growth compared to a clean Ge(001) substrate (there is only a ±0.15 eV shift among the samples grown on substrates at higher temperatures). By increasing substrate temperature, we obtained higher intermixing of Ni and Ge, but rather than both Ni and Ge interdiffusing, we find that Ni diffuses further into the germanium with higher substrate temperature, forming increasingly Ni-rich Ni-Ge compounds diluted into the Ge matrix. Based on Magneto-optic Kerr Effect measurements, Ni/Ge(001) grown on substrates at 100 and 200 °C does not exhibit a hysteresis loop, while the samples on 300 and 400 °C substrates show magnetic behavior, which we attribute to the magnetic character of hexagonal Ni5Ge2 (which is determined here for the first time to be a ferromagnetic phase).