  Strong interplay of spin and charge/orbital degrees of freedom is the
fundamental characteristic of the iron-based superconductors (FeSCs), which
leads to the emergence of a nematic state as a rule in the vicinity of the
antiferromagnetic state. Despite intense debate for many years, however,
whether nematicity is driven by spin or orbital fluctuations remains unsettled.
Here, by use of transport, magnetization, and $^{75}$As nuclear magnetic
resonance (NMR) measurements, we show a striking transformation of the
relationship between nematicity and spin fluctuations (SFs) in
Na$_{1-x}$Li$_x$FeAs; For $x\leq 0.02$, the nematic transition promotes SFs. In
contrast, for $x\geq 0.03$, the system undergoes a non-magnetic phase
transition at a temperature $T_0$ into a distinct nematic state that suppresses
SFs. Such a drastic change of the spin fluctuation spectrum associated with
nematicity by small doping is highly unusual, and provides insights into the
origin and nature of nematicity in FeSCs.
