  We investigate spin pumping and the effective spin mixing conductance in
heterostructures based on magnetic oxide trilayers composed of
La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO), LaNiO$_3$ (LNO), and SrRuO$_3$ (SRO). The
heterostructures serve as a model system for an estimation of the effective
spin mixing conductance at the different interfaces. Our results show that by
introducing a LNO interlayer between LSMO and SRO, the total effective spin
mixing conductance increases due to the much more favourable interface of
LSMO/LNO with respect to the LSMO/SRO interface. Neverheless, the spin current
into the SRO does not decrease because of the spin diffusion length of
$\lambda_\text{LNO}\approx$3.3 nm in the LNO. This value is two times higher
than that of SRO. Our results show the potential of using oxide interfaces to
tune the effective spin mixing conductance in heterostructures and to bring
novel functionalities into spintronics by implementing complex oxides.
