Within the framework of the model of interacting parameters of the magnetic and structural orders, taking into account the internal periodic magnetic field orthogonal to the exchange field, we analyzed the features of magnetostructural transitions in the Mn_{1-x}Co_{x}NiGe system. A qualitative description of changes in the nature of magnetic phase transitions from magnetostructural transitions of the 1st order paramagnetism-antiferromagnetism (x = 0.05-0.1) to isostructural transitions of the 2nd order paramagnetism-ferromagnetism (x = 0.15-0.8) with a change in the concentration of Co is presented. An explanation is given for the onset of irreversible magnetic-field-induced transitions at temperatures on the order of 5 K in strong magnetic fields, accompanied by a change in the saturation magnetization for samples x = 0.15-0.8. The low-temperature inverse magnetocaloric effect at liquid helium temperatures is predicted for these samples.

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Substitutional solid solutions Mn1-xFexNiGe (0.05≤x≤1.00) were synthesized. The crystal structure in the temperature range of 20 - 300 K has been studied by X-ray and neutron diffraction. At T = 200 K, a phase transformation from a hexagonal structure of the Ni2In type to an orthorhombic structure (S.G. Pnma) has been observed. The magnetic properties of these compounds were also investigated in the temperature range of 5 - 300 K and external magnetic fields up to 10 T. The temperature of the "magnetic order - magnetic disorder" phase transformation was found to decrease from 272 to 132 K with an increase in concentration x from 5 to 30 mol%.

The Stockbargard – Bridgman method yielded single crystals Mn0.99Fe0.01As. The effect of an external magnetic field with an intensity of up to 10 T on phase transitions in the single crystal Mn0.99Fe0.01As is studied. It is established that the magnetostructural phase transition in Mn0.99Fe0.01As is accompanied by a change in the entropy ΔSm, which is due to the transformation of the crystal structure. At temperatures above the temperature of the magnetostructural transition Tu = 290 K, the existence of an unstable magnetic structure is obtained. The magnetocaloric characteristics of the material under study are determined by an indirect calculation method based on the Maxwell thermodynamic relations and the Clapeyron – Clausius equation.

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