  Andreev reflection spectroscopy with unpolarized and highly spin-polarized
currents has been utilized to study an intermetallic single-crystal
superconductor NiBi3. Magnetoresistance at zero bias voltage of point contacts
shows the occurrence and suppression of Andreev reflection by unpolarized and
polarized current, respectively. The gap value, its symmetry and temperature
dependence have been determined using an unpolarized current. The spin state in
the NiBi3 sample is determined to be antiparallel using a highly spin-polarized
current. The gap value 2Delta/kBT, gap symmetry and its temperature dependence,
combined with the antiparallel spin state show that the bulk NiBi3 is a singlet
s-wave superconductor.
