Magnetostructural features of phase transitions in the Mn$_{1-x}$Co$_{x}$NiGe system. Part 1. Experimental results

Experimental studies of the magnetic and structural properties of solid solutions of the Mn$_{1-x}$Co$_{x}$NiGe system in a wide range of Co concentrations (0.05 $\le x\le$ 0.8), temperatures (5 K $\le x\le$ 600 K) and magnetic fields (0.016 T $\le x\le$ 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 $\le x\le$ 0.15) to isostructural transitions of the 2nd order paramagnetism-ferromagnetism (0.15 $\le x\le$ 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.