Growth, structure, electronic, and magnetic properties of MgO/Fe(001) bilayers and Fe/MgO/Fe(001) trilayers

Abstract

Single-crystal epitaxial MgO thin films were grown directly onto high-quality Fe single crystal and Fe whisker substrates and covered with Fe/Au layers. Reflection high-energy electron diffraction and low-energy electron diffraction patterns and scanning tunneling microscopy images showed that the growth of MgO proceeded pseudomorphically in a nearly layer-by-layer mode up to six monolayers. A misfit dislocation network is formed for MgO layers thicker than six monolayers. The thin MgO films were characterized electrically by scanning tunneling spectroscopy. The tunneling barrier in MgO was found to depend on the MgO layer thickness, starting from 2.5 eV at two monolayer thickness to the expected full barrier of MgO of 3.6 eV at six monolayers. A small fraction of the scanned area showed randomly placed spikes in the tunneling conductance. Tunneling I-$V$ curves at the defects showed a lower tunneling barrier than that in the majority of the MgO film. The total tunneling current integrated over areas of $100\ifmmode\times\else\texttimes\fi{}100 {\mathrm{nm}}^{2},$ however, was not dominated by spikes of higher conductance. These local defects in the MgO barrier were neither related to atomic steps on the Fe substrates nor to individual misfit dislocations. Magnetic anisotropies and exchange coupling in Fe/MgO(001) and Fe/MgO/Fe(001) structures were studied using ferromagnetic resonance and Brillouin light scattering.