  Materials with large intrinsic valley splitting and high Curie temperature
are a huge advantage for studying valleytronics and practical applications. In
this work, using first-principles calculations, a new Janus TaNF monolayer is
predicted to exhibit excellent piezoelectric properties and intrinsic valley
splitting, resulting from the spontaneous spin polarization, the spatial
inversion symmetry breaking and strong spin-orbit coupling (SOC). TaNF is also
a potential two-dimensional (2D) magnetic material due to its high Curie
temperature and huge magnetic anisotropy energy. The effective control of the
band gap of TaNF can be achieved by biaxial strain, which can transform TaNF
monolayer from semiconductor to semi-metal. The magnitude of valley splitting
at the CBM can be effectively tuned by biaxial strain due to the changes of
orbital composition at the valleys. The magnetic anisotropy energy (MAE) can be
manipulated by changing the energy and occupation (unoccupation) states of d
orbital compositions through biaxial strain. In addition, Curie temperature
reaches 373 K under only -3% biaxial strain, indicating that Janus TaNF
monolayer can be used at high temperatures for spintronic and valleytronic
devices.
