Strain-induced altermagnetism to ferromagnetism transition in monolayer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>Cr</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi>Si</mml:mi> <mml:

Magnetic phase transition, especially that involving altermagnets, can provide diverse mechanisms and ideas for the development of multifunctional materials. Here, we employ first-principles calculations to predict the altermagnetic monolayerCr2Si2S3Se3with excellent stability. TheCr2Si2S3Se3is an indirect bandgap semiconductor, which features<i>i</i>-wave spin splitting symmetry due to the spin layer group of13¯2m. Tensile strain can induce a phase transition from altermagnetism to ferromagnetism, with the critical strain ratioa/a0being 1.02. This phase transition causes the spin splitting to evolve from momentum-dependent one to global one (<i>i</i>-wave to<i>s</i>-wave). When spin-orbital coupling is taken into account, the magnetic phase transition is also accompanied by the disappearance of the valley polarization between the -<i>K</i>and<i>K</i>valleys. Our works broaden the scope of magnetic phase transitions and highlight the connection with valley polarization, which is beneficial for the integration of spintronics and valleytronics.