Strain-tunable magnetic phase transition and magnetocaloric effect in the CrS2 monolayer with bipolar magnetic semiconducting characteristics

Two-dimensional (2D) ferromagnetic materials with high-temperature intrinsic magnetic ordering, electrically tunable spin polarity, and significant magnetocaloric effect (MCE) are highly desirable for designing nanoscale spintronic devices and developing multifunctional materials. In this work, we employ first-principles calculations in conjunction with atomic spin dynamics (ASD) simulations to identify a 2D monolayer phase of ${\mathrm{CrS}}_{2}$ with space group $P\overline{4}{2}_{1}m$, exhibiting high-Curie-temperature (${T}_{\mathrm{C}}$) Ising ferromagnetism and bipolar magnetic semiconducting characteristics. The intrinsic ferromagnetism originates from the strong superexchange interactions between Cr cations mediated by S anions. Moreover, we explore its layer-dependent electronic structure and find that even-layered ${\mathrm{CrS}}_{2}$ systems exhibit altermagnetism with the momentum-dependent spin splitting. ASD simulations demonstrate that the ${\mathrm{CrS}}_{2}$ monolayer possesses an excellent MCE with the maximum magnetic entropy change $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{M}}^{max}$ of 8.03 $\textmu{}\mathrm{J}/{\mathrm{m}}^{2}$/K, the maximum adiabatic temperature change $\mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{max}$ of 2.17 K, and a relative cooling power (RCP) of 722.70 $\textmu{}\mathrm{J}/{\mathrm{m}}^{2}$ under an out-of-plane magnetic field of 5 T. Additionally, we systematically investigate the effects of biaxial strain on its magnetic phase and MCE. Our results indicate that a moderate compressive strain induces a magnetic phase transition from ferromagnetism to antiferromagnetism and significantly enhances the MCE by increasing $\ensuremath{-}\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{M}}^{max}, \mathrm{\ensuremath{\Delta}}{T}_{\mathrm{ad}}^{max}$, and RCP. Furthermore, utilizing the carrier-doping controllable spin polarity of the bipolar magnetic semiconductor, we propose a conceptual model for a dual-gated magnetic tunnel junction, where the ``on'' and ``off'' states are achieved by switching the voltage signs of the dual gates. These findings not only deepen our understanding of the electrically tunable spin polarity, but also emphasize the profound impact of strain effects on magnetic ground states and MCE enhancement, thereby paving the way for future innovations in spintronic devices and magnetic refrigeration materials.