  Our measurements of $^{125}$Te NMR relaxations reveal an enhancement of
electronic spin fluctuations above $\mu_0H^*\sim15$ T, leading to their
divergence in the vicinity of the metamagnetic transition at
$\mu_0H_m\approx35$ T, below which field-reinforced superconductivity appears
when a magnetic field ($H$) is applied along the crystallographic $b$ axis. The
NMR data evidence that these fluctuations are dominantly longitudinal,
providing a key to understanding the peculiar superconducting phase diagram in
$H\|b$, where such fluctuations enhance the pairing interactions.
