Specific features of magnetic properties upon martensitic transition $M\leftrightarrow L2_1$ in Ni$_{2+x}$Mn$_{1-x}$Ga alloys

The paramagnetic susceptibility of Ni$_{2+x}$Mn$_{1-x}$Ga alloys with $x$ = 0.12, 0.16, 0.19, 0.21, 0.23, 0.24, 0.27, 0.3, 0.33, 0.36, and 0.39 was measured at $H$ = 5 kOe in the temperature range of 400–800 K. It was shown that the degree of tetragonal distortion $c/a$ and the parameters characterizing the electronic $(\chi_0)$ and magnetic ($\theta_P$ and $\mu^2_{\mathrm{eff}}$) subsystems of the alloys change anomalously sharply in the range 0.27 $< x <$ 0.3 when the number of valence electrons per atom is $e/a\sim$ 7.7.