Dynamical Interplay Between Intrinsic Defects and Impurity Ions in Very Dilute Fe-doped MgO Thin Films

The nature of intrinsic defects and impurities in the dielectric layer of a typical magneto tunneling junction is of great interest to understand tunneling of spin-polarized currents. In this connection, we have carried out studies of the electronic and magnetic properties of Mg$_{1-x}$Fe$_{x}$O. In particular, we have compared results from first principles calculations based on Density Functional Theory with highly accurate experiments. The measurements were performed with a Quantum Interference Device and by using soft x-ray absorption spectroscopy. Our study reveals basic defect units composed a Fe impurity coupled to one or two Mg vacancies. The trimer unit (i.e. Fe with two Mg vacancies) produces a magnetic net spin opposed to the dilute magnetism present in the oxide matrix. These findings could pave a way for engineering dielectric layers with high endurance and optimal tunneling properties by controlling the concentration of impurities and defects in the oxide matrix. Work supported by the US DOE.