  In this paper, a comprehensive study of the effects of Ni-doping on
structural, electrical, thermal and magnetic properties of the NbB2 is
presented. Low amounts (\leq 10 %) of Ni substitution on Nb sites cause
structural distortions and induce drastic changes in the physical properties,
such as the emergence of a bulk superconducting state with anomalous behaviors
in the critical fields (lower and upper) and in the specific heat. Ni-doping at
the 9 at.% level, for instance, is able to increase the critical temperature
(TC) in stoichiometric NbB2 (< 1.3 K) to approximately 6.0 K. Bulk
superconductivity is confirmed by magnetization, electronic transport, and
specific heat measurements. Both Hc1 and Hc2 critical fields exhibit a linear
dependence with reduced temperature (T/TC), and the specific heat deviates
remarkably from the conventional exponential temperature dependence of the
single-band BCS theory. These findings suggest multiband superconductivity in
the composition range from 0.01 \leq x \leq 0.10.
