Field control of bound magnetic polarons in magnetic oxides (ConBMP)

• Applicant: Professorin Dr. Heidemarie Schmidt
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 270442279

The control of the magnetic, optical, and transport properties of magnetic transition metal oxides (TMO) by external fields is important for future transparent spintronics. We have shown that the magnetotransport properties of magnetic zinc oxide films can be controlled in a magnetic and in an electric field (Phys. Rev. Lett. 101 (2008); Appl. Phys. Lett. Materials 2 (2014)). In the intrinsically n-type conducting TMO materials oxygen vacancies are used to make the TMO highly conductive. Nowadays advances in the fabrication of TMO materials make it possible to fabricate thin TMO films with a well controlled concentration of oxygen vacancies and with up to 10-20 at. % diluted magnetic ions. The oxygen vacancies can be created by sputtering or by oxygen poor atmosphere. Often, very small changes in the oxygen vacancy chemistry cause profound changes in TMO function, for example, inducing magnetization from a nonmagnetic state. Since the prediction of room temperature ferromagnetism in magnetic TMOs and despite huge efforts to clarify the role of dopants and defects, the observed room temperature ferromagnetism in magnetic TMOs is still a matter of controversy. Also in most reports the increase of the static dielectric constant with increasing concentration of isovalent magnetic ions in magnetic TMO has been neglected and as a consequence the radius of bound magnetic polarons (BMP) is underestimated.Earlier magnetization measurements on magnetic TMO thin films demonstrated that the ferromagnetism may be mediated by neutral oxygen vacancies (VO), which form F-centers with two trapped electrons. However, it can be stated that mainly in magnetic TMO nanoparticles with a large surface to volume ratio room temperature ferromagnetism has been observed. Using electron paramagnetic resonance and X-ray emission spectroscopy measurements singly ionized oxygen vacancies (VO+) located near the surface of the magnetic TMO nanoparticles were suggested by Eichel et al. [Functional Materials Letters 6 (2013)] as the main defects that contribute to the observed room temperature ferromagnetic ordering in magnetic TMO nanoparticles. Within the project we will investigate ferromagnetism in depleted magnetic TMO thin films with singly ionized oxygen vacancies (VO+). We will 1.) determine the static dielectric constant in dependence on the concentration of magnetic ions, 2.) adapt the existing hydrogenic BMP model to VO+ oxygen vacancies which form F+ centers with one trapped electron, 3) experimentally realize depleted magnetic ZnO and depleted magnetic MgO films with stable F+ centers, 4.) present the fingerprints of F+ center BMPs in magnetization and magnetotransport properties, and 5.) devise new approaches for exploiting the huge internal magnetic fields felt by charge carriers in magnetic TMO films with F+ center BMPs.

• DFG Programme: Research Grants