  Spin transport in thin-film materials can be studied by ferromagnetic
resonantly (FMR) driven spin pumping of a charge-free spin current which
induces an electromotive force through the inverse spin Hall effect (ISHE). For
quantitative ISHE experiments, precise control of the FMR driving field
amplitude $B_1$ is crucial. This study exploits in situ monitoring of $B_1$ by
utilization of electron paramagnetic resonantly (EPR) induced transient
nutation of paramagnetic molecules (a 1:1 complex of
{\alpha},{\gamma}-bisdiphenylene-{\beta}-phenylallyl and benzene, BDPA) placed
as $B_1$ probe in proximity of a NiFe/Pt-based ISHE device. Concurrent to an
ISHE experiment, $B_1$ is obtained from the inductively measured BDPA
Rabi-nutation frequency. Higher reproducibility is achieved by renormalization
of the ISHE voltage to $B_1^2$ with an accuracy that is determined by the
homogeneity of the FMR driving field and thus by the applied microwave
resonator and ISHE device setup.
