Elsevier

Thin Solid Films

Volume 680, 30 June 2019, Pages 67-74
Thin Solid Films

Epitaxial and contamination-free Co(0001) electrodes on insulating substrates for molecular spintronic devices

https://doi.org/10.1016/j.tsf.2019.04.021Get rights and content

Highlights

  • In-situ UHV preparation of Cobalt electrodes on insulating substrate

  • Implementation of organic molecules as insulating barrier

  • Test devices show high resistance in transport measurements.

  • Resistance is temperature dependent.

Abstract

The growing field of molecular spintronics is an auspicious route to future concepts of data storage and processing. It has been reported that the hybridization of the electronic structures of non-magnetic organic molecules and ferromagnetic transition-metal (FM) surfaces can form new magnetic units, so-called hybrid molecular magnets, with distinct magnetic properties, which promise molecular spintronic devices with extremely high information density and low energy consumption. The investigation and profound understanding of these device concepts require the formation of clean and epitaxial interfaces between the surface of a FM bottom electrode and molecular thin films. This can only be realized under ultra-high vacuum conditions. In addition, the FM electrodes must be grown on an insulating substrate to electrically separate neighboring devices. Here, we report on procedures to realize an entirely in-situ preparation of mesoscopic test devices featuring structurally and chemically well-defined interfaces. Au(111)-buffered Co(0001) electrodes are deposited by molecular-beam epitaxy onto sapphire or mica substrates using a shadow-mask to define the geometry. The surface quality is subsequently characterized by scanning tunneling microscopy (STM) and other surface science analysis tools. 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI), which serves as an exemplary molecule, is sublimed through another shadow-mask, and the interface formation in the monolayer regime is also studied by STM. Finally, we deposit a Cu top electrode through yet another shadow-mask to complete a mesoscopic (200 × 200 μm2) test device, which reveals in ex-situ transport measurements for the Co/BNTCDI/Cu junction non-metallic behavior and a resistance-area product of 24 MΩ·μm2 at 10 K.

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.

References (62)

  • J. DeRose et al.

    Gold grown epitaxially on mica: conditions for large area flat faces

    Surf. Sci.

    (1991)
  • U. Höpfner et al.

    Preparation of ordered thin gold films

    Appl. Surf. Sci.

    (1999)
  • B. Lüssem et al.

    The origin of faceting of ultra at gold films epitaxially grown on mica

    Appl. Surf. Sci.

    (2005)
  • J.H. Scofield

    Hartree-slater subshell photoionization cross-sections at 1254 eV and 1487 eV

    J. Electron Spectrosc. Relat. Phenom.

    (1976)
  • D. Zhang et al.

    Characterization of critically cleaned sapphire single-crystal substrates by atomic force microscopy, XPS and contact angle measurements

    Appl. Surf. Sci.

    (2013)
  • F. Tautz

    Structure and bonding of large aromatic molecules on noble metal surfaces: the example of PTCDA

    Prog. Surf. Sci.

    (2007)
  • C. Joachim et al.

    Electronics using hybrid-molecular and mono- molecular devices

    Nature

    (2000)
  • A.R. Rocha et al.

    Towards molecular spintronics

    Nat. Mater.

    (2005)
  • S. Sanvito

    The rise of spinterface science

    Nat. Phys.

    (2010)
  • D. Gatteschi et al.

    Molecular Nanomagnets

  • L. Bogani et al.

    Molecular spintronics using single-molecule magnets

    Nat. Mater.

    (2008)
  • M. Mannini et al.

    Magnetic memory of a single-molecule quantum magnet wired to a gold surface

    Nat. Mater.

    (2009)
  • K. Bairagi et al.

    Tuning the magnetic anisotropy at a molecule-metal Interface

    Phys. Rev. Lett.

    (2015)
  • F.A. Ma'Mari et al.

    Beating the Stoner criterion using molecular interfaces

    Nature

    (2015)
  • M. Callsen et al.

    Magnetic hardening induced by nonmagnetic organic molecules

    Phys. Rev. Lett.

    (2013)
  • K. Raman

    Interface-assisted molecular spintronics

    Appl. Phys. Rev.

    (2014)
  • M. Cinchetti et al.

    Activating the molecular spinterface

    Nat. Mater.

    (2017)
  • S. Schmaus et al.

    Giant magnetoresistance through a single molecule

    Nat. Nanotechnol.

    (2011)
  • K.V. Raman et al.

    Interface-engineered templates for molecular spin memory devices

    Nature

    (2013)
  • M. Galbiati et al.

    Spinterface: crafting spintronics at the molecular scale

    MRS Bull.

    (2014)
  • T. Esat et al.

    Quantum interference effects in molecular spin hybrids

    Phys. Rev. B

    (2017)
  • Cited by (1)

    • Determination of magnetic properties of Co(0 0 0 1) using MCDAD

      2020, Journal of Magnetism and Magnetic Materials
    View full text