Enhanced Itinerant Ferromagnetism in Hole-doped Transition Metal Oxides: Beyond the Canonical Double Exchange Mechanism

Abstract: Here we demonstrate the occurrence of robust itinerant ferromagnetism in Mott-Hubbard systems at both low and high doping concentrations. Specifically, we study the effect of hole doping on the experimentally synthesized LaCrAsO via first-principles calculations and observe that the parent G-type antiferromagnetism vanishes quickly at low doping concentration ($x$ $\sim$ 0.20) and the system becomes ferromagnetic metal due to the canonical double exchange (CDE) mechanism. As $x$ continues to increase, the onsite energy difference between Cr 3$d$ and As 4$p$ orbitals decreases and the system transitions to a ferromagnetic negative charge-transfer energy metal. Therefore, the itinerant ferromagnetism doesn't terminate at intermediate $x$ as CDE mechanism usually predicts. Furthermore, our calculations reveal that both nearest and next-nearest ferromagnetic exchange coupling strengths keep growing with $x$, showing that ferromagnetism caused by negative charge-transfer energy state is "stronger" than that of CDE picture. Our work not only unveils an alternative mechanism of itinerant ferromagnetism, but also has the potential to attract immediate interest among experimentalists.