  The magnetic properties and transition from an antiferromagnetic (AFM) to a
ferromagnetic (FM) state in semi Heusler alloys Cu1-xNixMnSb, with x < 0.3 have
been investigated in details by dc magnetization, neutron diffraction, and
neutron depolarization. We observe that for x < 0.05, the system Cu1-xNixMnSb
is mainly in the AFM state. In the region 0.05 \leq x \leq 0.2, with decrease
in temperature, there is a transition from a paramagnetic to a FM state and
below ~50 K both AFM and FM phases coexist. With an increase in Ni
substitution, the FM phase grows at the expense of the AFM phase and for x >
0.2, the system fully transforms to the FM phase. Based on the results
obtained, we have performed a quantitative analysis of both magnetic phases and
propose a magnetic phase diagram for the Cu1-xNixMnSb series in the region x <
0.3. Our study gives a microscopic understanding of the observed crossover from
the AFM to FM ordering in the studied semi Heusler alloys Cu1-xNixMnSb.
