The paramagnetic defects in the magnesium-oxide (MgO) thin-film grown on a silicon (Si) substrate (MgO|Si) by a molecular beam epitaxy method are reported. These thin films have been used as an MgO tunnel barrier material for the spin-injection to Si from Fe. According to the crystallinity evaluation, the MgO film possessed a (001) oriented structure. We investigated the paramagnetic defects and impurities of several MgO|Si samples by electron paramagnetic resonance (EPR). The EPR spectra showed a strong isotropic signal with a g-value of 2.002 and six weak satellite peaks which were assigned to oxygen vacancies of highly oriented MgO(001) (F+ center) coupled with 25Mg nuclei (I = 5/2). The spin area density of MgO (thickness = 1.6 nm), which can be used as a tunnel barrier for spin injection, was determined to be 7 × 1013 cm−2. This value corresponds to 1% of O atoms replaced with vacancies. The identification of the paramagnetic centers and the quantitative estimation of their concentration in MgO films is important and may pave the way for highly efficient spin injection from Fe to Si via an MgO tunnel barrier.

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