  When spin-orbit torques are measured using spin-torque ferromagnetic
resonance (ST-FMR), two alternative ways of analyzing the results to extract
the torque efficiencies -- lineshape analysis and analysis of the change in
linewidth versus DC current -- often give inconsistent results. We identify a
source for these inconsistencies. We show that fits of ST-FMR data to the
standard analysis framework leave significant residuals that we identify as due
to (i) current-induced excitations of a small volume of magnetic material with
magnetic damping much larger than the bulk of the magnetic layer, that we
speculate is associated with the heavy-metal/magnet interface and (ii)
oscillations of the sample magnetization at the modulation frequency due to
heating. The dependence of the residual signals on DC current can interfere
with an accurate extraction of spin-torque efficiencies by the linewidth
method. We show that the discrepancies between the two types of analysis can be
largely eliminated by extrapolating the window of magnetic fields used in the
linewidth fits to small values so as to minimize the influence of the residual
signals.
