Magnetic and structural phase transitions and the tetragonality of thermoelastic martensite in quasi-binary Heusler alloys Ni$_{2+x}$Mn$_{1-x}$Ga

This paper reports on the results of investigations into the structure of martensite and the electronic and magnetic properties of Ni$_{2+x}$Mn$_{1-x}$Ga alloys in the concentration range 0.12 $\le x\le$ 0.39. The concentration dependences of the critical temperatures and the anomalous behavior of the degree of tetragonal distortion of the martensite have been discussed, as well as the changes in the parameters characterizing the electronic and magnetic subsystems in the thermoelastic martensitic transition $L2_1\leftrightarrow M$, which occurs in atomically ordered $L2_1$-type alloys upon doping in the range 0.12 $\le x\le$ 0.39 at different temperatures. The temperature and concentration dependences of the magnetic properties have been determined for the state of the single-phase $L2_1$ austenite (at $T>T_C$ and $A_f$) and martensite (at $T<T_C$ and $M_f$), which undergoes the transition from a modulated $10M$- or $14M$-type structure to a nonmodulated (NM) tetragonal structure.