The phase diagrams and magnetic behavior of the \(\hbox {Ni}_2\) MnGa full-Heusler alloy, modeled as a mixed spin-1 and spin-5/2 Ising ferromagnetic system, are investigated using the mean-field approximation, with Bogoliubov’s inequality for the Gibbs free energy as the theoretical framework. Although the magnetic properties of \(\hbox {Ni}_2\) MnGa have been studied, there remains a gap in the understanding of its phase transitions under different crystal fields and reduced temperatures. This work aims to address this gap by providing detailed ground-state phase diagrams and phase diagrams in different parameter planes, offering new insights into the interplay between temperature, magnetic ordering, and crystal field effects. Our model highlights first- and second-order phase transitions, as well as the emergence of a tricritical point and reentrant phenomena, that have not been sufficiently explored in previous studies. The results found not only extend the understanding of phase transitions in \(\hbox {Ni}_2\) MnGa but also provide a reference for future theoretical and experimental work.
