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

Abstract

Klaua, M.; Ullmann, D.; Barthel, J.; Wulfhekel, W.; Kirschner, J.; Urban, R.; Monchesky, Theodore L.; Enders, Axel; Cochran, John F.; and Heinrich, Brett, "Growth, structure, electronic, and magnetic properties of MgO/Fe(001) 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 100ϫ100 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/ MgO͑001͒ and Fe/MgO/Fe͑001͒ structures were studied using ferromagnetic resonance and Brillouin light scattering.

Electron trapping in misfit dislocations of MgO thin films

“…In such systems, dense dislocation networks develop spontaneously in order to compensate the lattice mismatch with the substrate. [17][18][19] A systematic characterization of such line defects and the elucidation of their role as electron traps is however missing, mostly because of experimental difficulties to investigate electrically insulating oxide materials. In this STM and EPR study, we demonstrate the trapping ability of misfit dislocations formed in MgO/ Mo͑001͒ thin films.…”

Growth of a crystalline and ultrathin MgO film on Fe(001)

“…A relatively small band off set of 3 eV is thought to be due to defect states within the band gap of MgO. 6 Figure 1(b) is a line profile following the black arrow in Fig. 1(a).…”

“…1,5 Up to now, MgO layers on the Fe(001) surface have been prepared mostly by electron bombardment on MgO sources, but any atomic resolution STM image of MgO layers has not been reported. 2,5,6 On the other hand, on the Mo(001) and Ag(001) surfaces, thin MgO films have been prepared by Mg deposition on the substrate held at 470-560 K in an O 2 environment (i.e., reactive oxidation). 7,8 Atomic resolution of the MgO layer has also been obtained on the Ag(001) surface.…”

Gundlach oscillations and Coulomb blockade of Co nanoislands on MgO/Mo(100) investigated by scanning tunneling spectroscopy at 300 K

“…Both fields rely on the good epitaxial quality of MgO films, while the former was additionally boosted by the theoretical insight of a symmetry induced strong spin selectivity of MgO(001) [22].MgO might also be an excellent template for nanoelectronic studies by scanning probe microscopy similar to the ones that have been performed recently on NaCl with respect to charge manipulation and bond formation [23][24][25] or on Al 2 O 3 and CuN with respect to the determination of magnetic properties of individual atoms on a substrate [26][27][28]. Thin films of MgO, exhibiting a wide band gap and a simple rock salt structure, grow epitaxially on different metal substrates as Ag(001) [10][11][12][13][14][15][16], Fe(001) [5][6][7][8][9] and Mo(001) [17][18][19][20][21]. Here, we choose Mo, since it allows high annealing temperatures exceeding 1000 K, which might foster an improved MgO film quality.…”

“…Magnesium oxide (MgO) is a preferred insulator for magnetic tunnel junctions [1][2][3][4][5][6][7][8][9] and is extensively used as a template for the microscopic study of catalytic reactions [10][11][12][13][14][15][16][17][18][19][20][21]. Both fields rely on the good epitaxial quality of MgO films, while the former was additionally boosted by the theoretical insight of a symmetry induced strong spin selectivity of MgO(001) [22].MgO might also be an excellent template for nanoelectronic studies by scanning probe microscopy similar to the ones that have been performed recently on NaCl with respect to charge manipulation and bond formation [23][24][25] or on Al 2 O 3 and CuN with respect to the determination of magnetic properties of individual atoms on a substrate [26][27][28].…”