DFT study of itinerant ferromagnetism in p-doped monolayers of MoS2

We use density functional theory to explore the possibility of making the semiconducting transition-metal dichalcogenide MoS2 ferromagnetic by introducing holes into the narrow Mo d band that forms the top of the valence band. In the single impurity limit, the repulsive Coulomb potential of an acceptor atom and intervalley scattering lead to a twofold orbitally degenerate effective-mass-like e' state being formed from Mo d(x2-y2) and d(xy) states, bound to the K and K' valence band maxima. It also leads to a singly degenerate a'(1) state with Mo d(3z2-r2) character bound to the slightly lower lying valence band maximum at Gamma. Within the accuracy of our calculations, these e' and a'(1) states are degenerate for MoS2 and accommodate the hole that polarizes fully in the local spin density approximation in the impurity limit. With spin-orbit coupling included, we find a single ion magnetic anisotropy of similar to 5 meV favoring out-of-plane orientation of the magnetic moment. Pairs of such hole states introduced by V, Nb, or Ta doping are found to couple ferromagnetically unless the dopant atoms are too close in which case the magnetic moments are quenched by the formation of spin singlets. Combining these exchange interactions with Monte Carlo calculations allows us to estimate ordering temperatures as a function of the dopant concentration x. For x similar to 9%, Curie temperatures as high as 100 K for Nb and Ta and in excess of 160 K for V doping are predicted. Factors limiting the ordering temperature are identified and suggestions made to circumvent these limitations.