Strain-tuned room-temperature spin valve effect in the HfCr2N4 monolayer

Traditional van der Waals spin valves, usually comprising of a nonmagnetic monolayer sandwiched between two ferromagnetic monolayers, present a significant challenge for precise synthesis in contemporary experimental techniques. Can spin valves be implemented with a monolayer compound instead of a van der Waals multilayer heterostructure? Here, we propose a room-temperature ${\mathrm{HfCr}}_{2}{\mathrm{N}}_{4}$ monolayer spin valve based on density functional theory and Boltzmann transport theory. The ${\mathrm{HfCr}}_{2}{\mathrm{N}}_{4}$ monolayer is a ferromagnetic half-metal with a high Curie temperature above room temperature. The transition from half-metal to semiconductor can be achieved by manipulating the relative orientation of magnetizations of the Cr atomic layers. Interestingly, the biaxial strain applied in the $xy$ plane is an effective strategy to switch the magnetic configurations. The magnetic ground state of the ${\mathrm{HfCr}}_{2}{\mathrm{N}}_{4}$ monolayer converts to the antiferromagnetic configuration under $\ensuremath{-}4%$ strain. Remarkably, the ${\mathrm{HfCr}}_{2}{\mathrm{N}}_{4}$ monolayer exhibits a substantial room-temperature magnetoresistance ratio of $572%$, attributed to variations in band gaps and carrier mobilities. Our findings pave the way for designing future room-temperature monolayer spin valve devices.