Metastable defect ferromagnetic phases by low temperature interface reactions between transition metal oxides

Recently we showed [Nature Materials 3, 709 (2004)] that the room temperature ferromagnetism observed in low temperature (500 $^{\circ}$C) processed mixtures of 2 at{\%} MnO$_{2}$ with ZnO [Nature Materials 2, 673 (2003)] is caused by an interface phase suggested to be of the form Mn$_{2-x}$Zn$_{x}$O$_{3-\delta , }$wherein Zn is incorporated into Mn$_{2}$O$_{3}$. In order to establish the anticipated generic nature of the process, experiments were performed on low temperature sintering of 2 at{\%} MnO$_{2}$ with other transition metal oxides such as NiO, TiO$_{2}$ or CuO. Room temperature ferromagnetism was observed in all the three cases. Our results suggest that ferromagnetism in these new cases also resides at the interface. The same mixtures when sintered at 800 $^{\circ}$C resulted in compound phases (e.g. NiMn$_{2}$O$_{4}$ in the Ni-Mn-O case) that are nonmagnetic at room temperature. We suggest a new mechanism of ferromagnetism based on the valence and spin controlled defect state in the interface phases.