The paper shows the effect of homogenization annealing on the ferromagnet-paramagnet phase transition in the GdNi2 alloy. In the cast sample, at the Curie point, a temperature hysteresis is observed, which in an external magnetic field of 13.5 T can be at least 3 degrees. The data of scanning electron microscopy of the electropolished samples showed a decrease in the impurity phase of GdNi and practically no temperature hysteresis of the phase transition in the annealed GdNi2 sample. The value of the isothermal change entropy Delta Sm=-18.931 J/(kg·K) for an annealed sample of the GdNi2 alloy at the Curie point of 75 K in a magnetic field of 13.5 T. Keywords: magnetocaloric effect, GdNi2, Laves phases.

Experimental studies of the magnetic and structural properties of solid solutions of the Mn1-xCoxNiGe system in a wide range of Co concentrations (0.05≤ x≤ 0.8), temperatures (5 K≤ x≤600 K) and magnetic fields (0.016 T≤ x≤ 13.5 T) have revealed a number of nontrivial magnetic and magnetocaloric features of this system. The latter include: 1) a change in the nature of magnetic phase transitions from magnetostructural transitions of the 1st order paramagnetism-antiferromagnetism (0.05≤ x≤ 0.15) to isostructural transitions of the 2nd order paramagnetism-ferromagnetism (0.15≤ x≤0.8) with a change in the concentration of Co ; 2) anomalous behavior of low-temperature regions of magnetization in weak magnetic fields; 3) a change in the saturation magnetization and the appearance of irreversible magnetic field-induced transitions at helium temperatures in strong magnetic fields. Keywords: irreversible magnetostructural first-order phase transition, helimagnetism, direct and inverse magnetocaloric effects.

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 Mn1-xCoxNiGe 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. Keywords: irreversible magnetostructural first-order phase transition, helimagnetism, direct and inverse magnetocaloric effects.