  We studied metastable $\alpha$-FeGe$_2$, a novel layered tetragonal material,
embedded as a spacer layer in spin valve structures with ferromagnetic Fe$_3$Si
and Co$_2$FeSi electrodes. For both types of electrodes, spin valve operation
is demonstrated with a metallic transport behavior of the $\alpha$-FeGe$_2$
spacer layer. The spin valve signals are found to increase both with
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.
