On Order-Disorder ($L2_1 \to B2^\prime$) Phase Transition in   Ni$_{2+x}Mn_{1-x}$Ga Heusler Alloys
  Order-disorder phase transition in Ni$_{2+x}Mn_{1-x}$Ga Heusler alloys has been studied. It was found that the $L2_1 \to B2^{\prime}$ phase transition in Ni2+xMn1-xGa (x = 0.16 - 0.20) Heusler alloys is of second order and the temperature of this transition decreases with Ni excess. 
V.V. Khovailo, T. Takagi, A.N. Vasil'ev, H. Miki, M. Matsumoto, and R.   Kainuma