  The half-Heusler alloy RuMnGa having valence electron count (VEC) 18 has
recently been theoretically proposed to exhibit compensated ferrimagnetic
(CFiM) character instead of the expected nonmagnetic ground state. On the other
hand, a preliminary experimental study proposed ferromagnetic (FM) ordering. As
no half-Heusler system with VEC 18 is known to exhibit magnetic ordering, we
have investigated the details of crystal structure and magnetic properties of
RuMnGa using a combination of experimental tools, viz., x-ray and neutron
diffraction techniques, dc and ac susceptibility, isothermal magnetisation,
heat capacity, resistivity and neutron depolarisation measurements. Rietveld
refinements of x-ray and neutron diffraction data suggest single phase nature
of the compound with elemental composition RuMn$_{0.86}$Ga$_{1.14}$. We have
shown that the system exhibits FM-type ordering owing to the inherent presence
of this minor off-stoichiometry, showing very low magnetic moment. The system
also exhibits reentrant canonical spin-glass behaviour, which is rarely
observed in half-Heusler alloys. The temperature coefficient of resistivity
changes its sign from negative to positive and further to negative as the
temperature decreases.
