  We study the occurrence and the origin of ferromagnetic fluctuations in the
longitudinal spin susceptibility of the $t$-$t'$-Rashba-Hubbard model on the
square lattice. The combined effect of the second-neighbor hopping $t'$ and the
spin-orbit coupling leads to ferromagnetic fluctuations in a broad filling
region. The spin-orbit coupling splits the energy bands, leading to two van
Hove fillings, where the sheets of the Fermi surface change their topology.
Between these two van Hove fillings the model shows ferromagnetic fluctuations.
We find that these ferromagnetic fluctuations originate from interband
contributions to the spin susceptibility. These interband contributions only
arise if there is one holelike and one electronlike Fermi surface, which is the
case for fillings in between the two van Hove fillings. We discuss implications
for experimental systems and propose a test on how to identify these types of
ferromagnetic fluctuations in experiments.
