  The thermopower and Nernst effect were investigated for undoped parent
compounds LaFeAsO and LaNiAsO. Both thermopower and Nernst signal in iron-based
LaFeAsO are significantly larger than those in nickel-based LaNiAsO.
Furthermore, abrupt changes in both thermopower and Nernst effect are observed
below the structural phase transition temperature and spin-density wave (SDW)
type antiferromagnetic (AFM) order temperature in Fe-based LaFeAsO. On the
other hand, Nernst effect is very small in the Ni-based LaNiAsO and it is
weakly temperature-dependent, reminiscent of the case in normal metals. We
suggest that the effect of SDW order on the spin scattering rate should play an
important role in the anomalous temperature dependence of Hall effect and
Nernst effect in LaFeAsO. The contrast behavior between the LaFeAsO and LaNiAsO
systems implies that the LaFeAsO system is fundamentally different from the
LaNiAsO system and this may provide clues to the mechanism of high $T_c$
superconductivity in the Fe-based systems.
