Jumplike magnetic disordering processes stimulated by a magnetic field in systems with structural instability

A theoretical analysis of the features of first-order structural and magnetostructural transitions in magnetocaloric helimagnetic alloys of the Mn$_{1-x}$Cr$_{x}$NiGe system is performed. The observed structural transitions of the displacement type hex($P6_{3}/mmc)\leftrightarrow$ orth$(Pnma)$ are described using the local soft mode model in the approximation of a displaced harmonic oscillator. In the absence of a magnetic field, the emergence of a helimagnetic order as a structurally induced second-order transition is described within the Heisenberg model with allowance for the dependence of the exchange integrals on the structural order parameters and elastic strains. In the presence of a magnetic field, it was found that the mutual approach of the characteristic temperatures for the helimagnetic state (HM$(Pnma)$) and the lability temperatures of the hexagonal paramagnetic state (PM($P6_{3}/mmc$)), due to the action of the magnetic field, leads to the appearance of previously unexplored peripheral first-order magnetostructural transitions with insignificant magnetization jumps, increasing with increasing magnetic induction field. In this case, as the pressure increases to 4 kbar with a constant magnetic induction field, the peripheral transitions are transformed into reversible first-order magnetostructural transitions and, at even higher pressures (10–14 kbar), into full-scale first-order magnetostructural transitions with magnetization jumps comparable with the maximum magnetization. Experimental baric studies of the temperature dependences of the magnetization in static magnetic fields with an induction up to 1 T and pressure up to 14 kbar confirm the theoretical results.