Epitaxial and contamination-free Co(0001) electrodes on insulating substrates for molecular spintronic devices
Introduction
The growing field of molecular spintronics [[1], [2], [3]] is a promising approach for future data storage and processing applications. A key feature of these concepts is to increase the density of information per unit area and to decrease energy consumption in operation. Molecular spintronics is a promising route towards using the electron spin as information carrier mainly because of the weak spin-orbit and hyperfine interactions in organic molecules, which promise the preserving of the spin-coherence for much longer times and over wider distances than in metals or conventional semiconductors [2]. Besides the approach of using intrinsically magnetic molecules, so-called single molecular magnets [[4], [5], [6], [7]], there is an alternative route that employs the hybridization of small non-magnetic aromatic molecules with a ferromagnetic [8] or even non-ferromagnetic [9] surfaces. This direct interaction leads to the formation of new magnetic units at the interface, the so-called hybrid molecular magnets, that consist of the adsorbed molecule and the few substrate atoms it is bound to [[8], [9], [10], [11], [12]]. Anticipated applications of such hybrid molecular magnets and their technical realization motivate our work towards a deeper understanding of the fundamental processes and their technical feasibility likewise.
It has been shown theoretically and experimentally [8,[13], [14], [15], [16]] that hybrid molecular magnets can form upon chemisorption on Co(0001) surfaces, where the π-orbitals of the aromatic molecules hybridize with the spin-split Co 3d-states. Therefore, well-defined and clean surfaces for the molecules to interact with are crucial. The adsorption and initial growth behavior leading to the formation of an interface with specific magnetic properties, the so-called spinterface [3], can be studied on Co(0001) single crystals. However, actual mesoscopic devices that enable (magneto-)transport measurements and operate on the basis of organic molecular layers showing spinterface effects demand an approach, where the Co(0001) surface is part of the bottom electrode of the device. Since electrodes of several devices fabricated on the same substrate must be electrically separated from each other to enable individual addressability, epitaxial and laterally structured Co(0001) electrodes deposited on an insulating substrate are needed. Furthermore, the high reactivity of Co at ambient conditions, in particular the formation of carbides, oxides, and hydroxides [17,18], makes an entirely in-situ preparation under ultra-high vacuum (UHV) conditions for the Co bottom electrode as well as for the subsequently deposited organic layer and top electrode mandatory. Here, we present an entirely UHV-based preparation procedure for mesoscopic test devices comprising crossed bottom and top electrodes and an organic molecular interlayer with structurally and chemically well-defined interfaces.
Section snippets
Experimental details
The sample fabrication and most of the characterizations and measurements are performed in a multi-chamber UHV system featuring a preparation chamber with various evaporators and surface analysis tools, a dedicated molecule deposition chamber, a STM chamber housing a low-temperature scanning tunneling microscope (LT-STM from Omicron), and a SEM chamber featuring an in-situ scanning electron microscope (UHV Gemini column from ZEISS). (Magneto-)transport measurements are performed ex-situ after
Growth and morphology of Co(0001) electrodes
We explore two approaches to achieve an epitaxial Co(0001) surface on an insulating substrate. The first system consists of a mica sheet as insulating substrate and a Au(111) buffer layer on top [[20], [21], [22]]. The second system consists of a c-cut sapphire crystal as an insulating substrate and also a Au(111) buffer layer [23]. In this case, however, we use a very thin Co(0001) seed layer to improve the surface morphology of the Au buffer layer [24]. Both systems yield substrates, on which
Organic layer deposition
Fig. 5(a) shows the structure formula of BNTCDI that we use as an exemplary organic molecule to form an organic barrier layer on the Co(0001) electrode described in the previous Section.
The BNTCDI molecules are sublimed at temperatures between 538 and 548 K for about 25 min, while the substrate is kept at RT. The pressure during deposition did not exceed 1.3∙10−8 Pa. A high-resolution STM image of an isolated BNTCDI molecule on Co(0001) is shown in Fig. 5(b). The apparent shape has C2v symmetry
Test device fabrication and transport measurements
In order to provide proof-of-principle and to demonstrate the feasibility for the presented method of preparing epitaxial Co(0001) bottom electrodes (Section 3.2) and the in-situ deposition of an organic layer of BNTCDI molecules (Section 4), we fabricated test devices with the layer sequence sapphire/2 nm Co(111)/50 nm Au(111)/5 nm Co(0001)/≈5 nm BNTCDI/30 nm Cu/2 nm MgO, see Fig. 7(a).
The Co/Au/Co bottom electrode has been deposited according to the recipe in Section 3.2.
The BNTCDI organic
Conclusions
An entirely in-situ preparation cycle enabling the fabrication of organic spintronic devices involving molecular layers has been presented. Emphasis is put on the preparation of epitaxial and contamination-free Co(0001) electrodes on insulating substrates, which are key for the fabrication of junctions, where the electrodes are not only passive supports, but active functionality-enabling parts of the device, e.g. due to chemical bonding and electronic hybridization between the electrode
Acknowledgements
Financial support from the Volkswagen Foundation through the project “Optically Controlled Spin Logic” is gratefully acknowledged.
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