Altermagnetism, as a new category of collinear magnetism distinct from traditional ferromagnetism and antiferromagnetism, exhibits the spin splitting without net magnetization. Currently, researchers are focused on searching three-dimensional altermagnetism and exploring its novel physical properties. However, there is a lack of understanding of the physical origin and valley splitting of two-dimensional altermagnetic emergent behavior. Here, we propose an approach to realize the transition from Néel antiferromagnetism to altermagnetism in a two-dimensional system using an electric field, Janus structure, and ferroelectric substrate. In monolayer ${\mathrm{VPSe}}_3$, we demonstrate that multiple physical fields cause the upper and lower Se atoms unequally to break $PT$ symmetry, resulting in altermagnetic spin splitting. We note that monolayer ${\mathrm{VPSe}}_3$ produces a spontaneous valley splitting of 2.91 meV at the conduction band minimum. The electric field can tune the valley splitting magnitude, while the Janus structure not only changes the valley splitting magnitude, but also alters the direction. More interestingly, when the ferroelectric polarization of ${\mathrm{Al}}_2{\mathrm{S}}_3$ is $\mathrm{P}\uparrow$, the direction of valley polarization is switched and the magnitude is almost unchanged. However, the valley splitting significantly increases under the $\mathrm{P}\downarrow$. It is worth noting that the ferroelectric polarization can switch altermagnetic effect and realize anomalous valley Hall effect. Besides, we reveal the microscopic mechanism of valley splitting by an effective Hamiltonian. Our findings not only provide a method to designing the altermagnet, but also enrich the valley physics.

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• Electronic structure
• Ferroelectricity
• First-principles calculations
• Magnetic phase transitions
• Valley degrees of freedom
• Valleytronics