Exploiting the valley degree of freedom introduces a paradigm for advancing quantum information technology. Currently, the investigation on spontaneous valley polarization mainly focuses on two major types of systems. One type of magnetic systems is by breaking the time-reversal symmetry, the other is ferroelectric materials through breaking the inversion symmetry. Might there be additional scenarios? Here, we propose to realize spontaneous valley polarization by breaking the mirror symmetry in the altermagnets, named type III valley polarization. Through symmetry analysis and first-principles calculations, we confirm that this mechanism is feasible in non-Janus ${\mathrm{Fe}}_2{\mathrm{WS}}_2{\mathrm{Se}}_2$. Monolayer non-Janus and Janus ${\mathrm{Fe}}_2{\mathrm{WS}}_2{\mathrm{Se}}_2$ are stable Néel-type antiferromagnetic state with the direct band gap semiconductor. More interestingly, their magnetic anisotropy energy exhibits the rare biaxial anisotropy and a four-leaf clover shape in the xy plane, while the xz and yz planes show the common uniaxial anisotropy. This originated from the fourth-order single ion interactions. More importantly, the valley splitting is spontaneously generated in the non-Janus ${\mathrm{Fe}}_2{\mathrm{WS}}_2{\mathrm{Se}}_2$ due to the ${\mathrm{M}}_{xy}$ symmetry breaking, without requiring the SOC effect. Both the non-Janus and Janus ${\mathrm{Fe}}_2{\mathrm{WS}}_2{\mathrm{Se}}_2$ exhibit diverse valley polarization and anomalous valley Hall effect properties. In addition, the magnitude and direction of valley polarization can be effectively tuned by the biaxial strain and magnetic field. Our findings not only expand the realization system of spontaneous valley polarization, but also provide a theoretical basis for the high-density storage of valley degrees of freedom.

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• Density of states
• Electronic structure
• First-principles calculations
• Magnetic anisotropy
• Magnetism
• Spin-orbit coupling
• Valley degrees of freedom
• Valleytronics