  We have systematically studied the nematic fluctuations in the electron-doped
iron-based superconductor BaFe$_{2-x}$Ni$_x$As$_2$ by measuring the in-plane
resistance change under uniaxial pressure. While the nematic quantum critical
point can be identified through the measurements along the (110) direction as
studied previously, quantum and thermal critical fluctuations cannot be
distinguished due to similar Curie-Weiss-like behaviors. Here we find that a
sizable pressure-dependent resistivity along the (100) direction is present in
all doping levels, which is against the simple picture of an Ising-type nematic
model. The signal along the (100) direction becomes maximum at optimal doping,
suggesting that it is associated with nematic quantum critical fluctuations.
Our results indicate that thermal fluctuations from striped antiferromagnetic
order dominate the underdoped regime along the (110) direction. We argue that
either there is a strong coupling between the quantum critical fluctuations and
the fermions, or more exotically, a higher symmetry may be present around
optimal doping.
