Ferromagnetism in defect-ridden oxides and related materials

Abstract

The existence of high-temperature ferromagnetism in thin films and nanoparticles of oxides containing small quantities of magnetic dopants remains controversial. Some regard these materials as dilute magnetic semiconductors, while others think they are ferromagnetic only because the magnetic dopants form secondary ferromagnetic impurity phases such as cobalt metal or magnetite. There are also reports in d 0 systems and other defective oxides that contain no magnetic ions. Here, we investigate TiO 2 (rutile) containing 1 -5% of iron cations and find that the roomtemperature ferromagnetism of films prepared by pulsed-laser deposition is not due to magnetic ordering of the iron. The films are neither dilute magnetic semiconductors nor hosts to an iron-based ferromagnetic impurity phase. A new model is developed for defect-related ferromagnetism which involves a spin-split defect band populated by charge transfer from a proximate charge reservoir -in the present case a mixture Fe 2+ and Fe 3+ ions in the oxide lattice. The phase diagram for the model shows how inhomogeneous Stoner ferromagnetism depends on the total number of electrons N tot , the Stoner exchange integral I and the defect bandwidth W; the band occupancy is governed by the d-d Coulomb interaction U. There are regions of ferromagnetic metal, half-metal and insulator as well as nonmagnetic metal and insulator. A characteristic feature of the high-temperature Stoner magnetism is an an anhysteretic magnetization curve which is practically temperature independent below room temperature. This is related to a wandering ferromagnetic axis which is determined by local dipole fields. The magnetization is limited by the defect concentration, not by the 3d doping. Only 1-2 % of the volume of the films is magnetically ordered.

Defect-induced magnetism in SiC: Interplay between ferromagnetism and paramagnetism

“…Therefore, the magnetic moments can not be increased by increasing the fluence of irradiation all the time, while, the amount of paramagnetic centers can be scaled up with neutron fluence and show saturation at very large fluences. The locally accumulation of ferromagnetic defects has also been suggested for defective ferromagnetic oxides [14,19,79].…”

Carrier-mediated nonlocal ferromagnetic coupling between local magnetic polarons in Fe-doped In2O3and Co-doped ZnO

“…16 In the carrier-mediated model, the local moments on the TM dopants are coupled with each other ferromagnetically through polarized mobile carriers. 11 In the BMP model, the local moments on the TM dopants are coupled with each other ferromagnetically through polarized carriers trapped by defects, such as the electrons associated with an oxygen vacancy.…”

“…In contrast, the CTF model predicts that the moments are entirely localized on itinerant carriers confined to small regions possibly grain boundaries. 16 Although each model has its merits in certain aspects, [17][18][19][20][21][22][23][24][25][26] a complete understanding of the ferromagnetic behavior in oxide DMSs still calls for more microscopic insights into the defect structure, transport property, and moment distribution in these systems.…”

Fabrication and magnetic properties of Fe and Co co-doped ZrO2

“…5,6 TiO 2 , SnO 2 , and ZnO-based DMOs have been extensively studied in pursuit of room temperature ferromagnetism, and doner impurity-band exchanged models through the oxygen vacancies and the spinodal decomposition in nanoscaled regions are proposed for room temperature ferromagnetism. [7][8][9][10] Ferromagnetism in oxide-based semiconductors or insulators is strongly related to the existence of oxygen vacancies. 7,8 Fabrication of ZrO 2 -based DMOs is a challenging goal, as this would combine the advantages of ZrO 2 with enhanced magnetic properties.…”

“…[7][8][9][10] Ferromagnetism in oxide-based semiconductors or insulators is strongly related to the existence of oxygen vacancies. 7,8 Fabrication of ZrO 2 -based DMOs is a challenging goal, as this would combine the advantages of ZrO 2 with enhanced magnetic properties. Theoretical calculations predict that Mn-doped ZrO 2 in the cubic structure has the potential to be a high Curie temperature DMO.…”