We present the results of broadband ferromagnetic resonance studies on bilayers comprised of Finemet thin films covered by Pt and Ta wedge layers with the aim to observe spin pumping effects and to evaluate both the real and imaginary parts of the spin mixing conductance. The obtained experimental results are analyzed in the framework of a recent microscopic theory of spin pumping effects and confirm the important role of spin-orbit interactions at interfaces between Finemet and nonmagnetic heavy metal such as Pt and Ta. In particular, we show that the imaginary part of spin mixing conductance has comparable value to the real part and discuss its influence on magnetization dynamics.