Ferroelastically tunable altermagnets

Altermagnets have garnered great interest due to their non-relativistic spin splitting and novel physical properties. However, the control of altermagnetic states remains underexplored. Here, we propose a new multiferroic state, i.e. ferroelastic altermagnetic state, in which ferroelastic strain couples directly to the spin-splitting. Through symmetry analysis and first-principles calculations, we identify the ferroelastic $d$-wave altermagnetism of puckered pentagonal CoSe$_2$ monolayer. Interestingly, uniaxial stress can induce a ferroelastic phase transition, accompanied by a $90^\circ$ rotation of the spin-splitting bands. Cooperative rotation of the lattice and Néel vectors preserves the sign of Kerr angle, whereas noncooperative rotation reverses it. Our work provides a general strategy for manipulating altermagnetism in multiferroic systems and opens new avenues for exploring emergent magnetoelastic phenomena.