Within the framework of the model of interacting parameters of magnetic and structural orders, a theoretical analysis of magnetostructural reversible first-order phase transitions is carried out. Reversible phase transitions are characterized by a jump-like appearance of magnetic order with decreasing temperature (as in a first-order phase transition), and with a reverse increase in temperature, the magnetic order gradually disappears (as in a second-order phase transition). Such transitions are observed in some alloys of the Mn_{1-x}Cr_{x}NiGe magnetocaloric system under pressure (x = 0.11) and without (x = 0.18) and are accompanied by specific magnetic and magnetocaloric features. A phenomenological description of these features is carried out within the concept of a soft mode for the structural subsystem undergoing first-order structural phase transition (P6_{3}/mmc-P_{nma}) and the Heisenberg model for the spin subsystem. For systems with magnetostructural instability within the molecular field approximation for the spin subsystem and the shifted harmonic oscillator approximation for the lattice subsystem, it is shown that the reversible phase transitions arise when the temperature of magnetic disordering is in the temperature hysteresis region of the 1st order structural phase transition P6_{3}/mmc-P_{nma}. It is also shown that the two-peak form of the isothermal entropy, which is characteristic of reversible transitions, is due to the separation of the structural and magnetic entropy contributions.

Using pnictides MnAs and germaindes Mn0.89Cr0.11NiGe as an example, the transformation of thermobaric features of their magnetic characteristics at high pressures is considered. A unified approach is used to describe paramagnetic (PM) structural transitions of the displacement type with a change in the symmetry PM(P63/mmc)-PM(Pnma) from hexagonal to orthorhombic. It is shown that the competition between the parameters of the structural and magnetic orders in both systems manifests itself differently in the stabilization and alternation of the so-called high-spin and low-spin magnetically ordered states initiated by pressure. As a consequence, the structural contribution in these systems weakens (MnAs) or enhances (Mn0.89Cr0.11NiGe ) the giant magnetocaloric effect in the temperature-baric region of the first-order magnetostructural phase transitions.