Stepping stone mechanism: carrier-free long range magnetism mediated by magnetized cation states*

The long range magnetism observed in group-V tellurides is the only working example of carrier-free dilute magnetic semiconductors (DMS), but the physical mechanism is unclear. Based on DFT calculations, we discovered a new ferromagnetic order in Cr-doped Bi₂Te₃ and Sb₂Te₃, with the dopant separation more than 9 Å. This configuration is the global energy minimum among all configurations. Different from the conventional super exchange theory, the magnetism is facilitated by the lone pair derived anti-bonding states near the cations. Such anti-bonding states work as stepping stones merged in the electron sea and conduct magnetism. Further, spin orbit coupling induced band inversion was found insignificant in the magnetism. Our findings directly dismiss the common misbelief that band topology is the only factor that enhances the magnetism. We further demonstrated that removal of the lone pair derived states destroys the long range magnetism. This novel mechanism sheds light on the fundamental understanding of long range magnetism and may lead to discoveries of new classes of DMS.