Electronic and magnetic properties of α−FeGe2 films embedded in vertical spin valve devices

We studied metastable $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Ge}}_{2}$, a layered tetragonal material, embedded as a spacer layer in spin valve structures with ferromagnetic ${\mathrm{Fe}}_{3}\mathrm{Si}$ and ${\mathrm{Co}}_{2}\mathrm{Fe}\mathrm{Si}$ electrodes. For both types of electrodes, spin valve operation is demonstrated with A metallic transport behavior of the $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}{\mathrm{Ge}}_{2}$ spacer layer. The spin valve signals are found to increase with both temperature and spacer thickness, which is discussed in terms of a decreasing magnetic coupling strength between the ferromagnetic bottom and top electrodes. The temperature-dependent resistances of the spin valve structures exhibit characteristic features, which are explained by ferromagnetic phase transitions between 55 and 110 K. The metallic transport characteristics as well as the low-temperature ferromagnetism are found to be consistent with the results of first-principles calculations.