  We perform Feshbach spectroscopy in an ultracold mixture of $^{23}$Na and
$^{40}$K with different spin-state combinations. We have observed 24 new
interspecies Feshbach resonances at magnetic field up to 350 G. A full
coupled-channel calculation is performed to assign these resonances. Among
them, 12 resonances are identified as d-wave Feshbach resonances. These d-wave
Feshbach resonances are about 5 G systematically smaller than the predications
based on previous model potential. Taking into account these new experimental
results, we improve the Born-Oppenheimer potentials between Na and K, and
achieve good agreement between the theory and experiment for all the observed
Feshbach resonances.
