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Multi layer systems

multi-layer systems

Multi-layer systems with combinations of insulators, semiconductors and ferromagnets are of great importance in terms of several electric components like spintronic devices.

Especially the characteristics of the surfaces and interfaces between separating layers have a great impact on physical effects, for example the Tunnel Magneto Resistance (TMR) [4] or the Giant-Magneto Resistance (GMR) [5,6]. So-called magnetic tunnel junctions (MTJ) mean devices using the TMR, where electrons pass between two ferromagnetic electrodes through a thin insulating film, depending on its magnetization. Here the thickness of the insulating barrier, as well as the structure of the layers and interfaces highly influences the TMR effect. Not only the structural, but also chemical and magnetic properties are of present interest in multiple research. (Quellen)

Favorable materials for MTJ devices are Magnesium Oxide (MgO) as insulator with a great bandgap, Iron (Fe) or Cobalt (Co) as ferromagnetic component and Gallium-Arsenide (GaAs) as semiconductor due to the possibility of bandgap-variation and its high electron transit frequency. [2, 3]

An extensive research of both systems in terms of structure determination, as well as an investigation of electronic, chemical and magnetic properties was made.


The tunnel magnetoresistance effect in magnetic tunnel junctions.

The resistance between two electrodes depends on its magnetization. The number of transmitted electrons can be tuned by choosing a parallel (left) or an anti-parallel (right) magnetization of the ferromagnets.   

S. Yuasa and D. D. Djayaprawira, J. Phys. D: Appl. Phys. 40, R337 (2007).

MgO/Fe/GaAs (001)

The growth of Fe onto an non-reconstructed and a (4x2)-reconstructed GaAs surface were obtained.
A combinated study of XPS, XPD revealed a strong inter-diffusion with an amorphous phase of Iron on the GaAs surface. The (4x2)-reconstructed GaAs surface allowed a formation of bcc structure within the Fe film.
A capping layer of MgO was deposited on top of the Fe/GaAs(4x2)-system.

Well ordered interfaces and crystalline layers could be obtained.
T-MOKE measurements revealed no ferromagnetic properties of the MgO or the GaAs layer, but strong ferromagnetic signals were found under the MgO capping layer in the Fe-interlayer. This makes the combination of the materials a promising system for spintronic devices. [2,3]


A schematic structure model of the MgO/Fe/GaAs(001) multilayer system.

D. Krull, Ph.D. thesis, Technische Universität Dortmund, (2014).

MgO/Co/GaAs (001)

In this work we performed the interface analysis of another bcc-ferromagnet/MgO hybrid system by means of XPS and XPD. Therefore the Ga-rich c(8x2) surface reconstruction of GaAs(001) was used to grow 12 ML of Co epitaxially on it. Recording XPS spectra and XPD patterns of the interface reveal the formation of a crystalline Co3Ga alloy in the rare D03-structure. Finally the deposition of the insulator material MgO on the Co layer was obtained. It was found that MgO layers of 4 ML or less grow in an amorphous phase while 5 ML and more of MgO form a distorted halite structure. With these results a minimal thickness for crystalline MgO-films could be defined. Moreover the investigation of the Co/MgO interface revealed no comound formation, such as the oxidation of Co. [7, 8]


Revealing the interfaces of the hybrid system MgO/Co/GaAs(0 0 1): a structural and chemical investigation with XPS and XPD

Structural determination of the multi-system MgO/Co/GaAs(001) by the use of photoelectron diffraction pattern. R-factors of < 0.1 proof the almost perfect match between the simulated and the measured structure. 

left: K. Shamout, Ph.D. thesis, Technische Universität Dortmund, (2018).
right: K. Shamout et al., J. Phys.: Condens. Matter 30, 075003 (2018).

[2] D. Krull, Appl. Surf. Sci. 367 (2016) 391-400.

[3] D. Krull, Ph.D. thesis, Technische Universität Dortmund, (2014).

[4] M. Jullière, Phys. Lett. A 54 (1975) 225.

[5] G. Binasch, P. Grünberg, F. Saurenbach, W. Zinn, Phys. Rev. B 39 (1989) 4828.

[6] M.N. Baibich, J.M. Broto, A. Fert, F.N. Van Dau, F. Petroff, P. Etienne, G. Creuzet,

A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472.

[7] K. Shamout et al., J. Phys.: Condens. Matter 30, 075003 (2018).

[8] K. Shamout, Ph.D. thesis, Technische Universität Dortmund, (2018).

cobalt intercalated graphene on silicon carbide

Using silicon carbide as a substrate for the preparation of graphene does not directly lead to free standing graphene. Instead, the first carbon layer is partly bonded to the substrate, which impairs its properties compared to free standing graphene. By intercalating an additional layer between the substrate and the carbon layer the unwanted bondings can be released and convert the first carbon layer to free standing graphene. Furthermore, the interaction between the added layer and the graphene can lead to special electronic and magnetic configurations. One example for such an interaction is cobalt. In this system the graphene induces an enhancement of the perpendicular magnetic anisotropy (PMA) in the cobalt layer caused by a Dzyaloshinskii−Moriya interaction.

In this case, PMA means the alignment of the spins in the cobalt layer perpendicular to the sample surface, which directly leads to the topic of spintronic.