Unravelling the role of the interface for spin injection into organic semiconductors

Abstract: Whereas spintronics brings the spin degree of freedom to electronic devices, molecular/organic electronics adds the opportunity to play with the chemical versatility. Here we show how, as a contender to commonly used inorganic materials, organic/molecular based spintronics devices can exhibit very large magnetoresistance and lead to tailored spin polarizations. We report on giant tunnel magnetoresistance of up to 300% in a (La,Sr)MnO3/Alq3/Co nanometer size magnetic tunnel junction. Moreover, we propose a spin dependent transport model giving a new understanding of spin injection into organic materials/molecules. Our findings bring a new insight on how one could tune spin injection by molecular engineering and paves the way to chemical tailoring of the properties of spintronics devices.

• The paper presents a model for spin-dependent transport, revealing enhanced spin lifetime in OSCs due to weak spin-orbit coupling.
• It employs CT-AFM nanoindentation to fabricate controlled LSMO/Alq/Co MTJs, achieving reproducible 300% tunnel magnetoresistance.
• The research demonstrates potential for chemical tuning at the interface to optimize spin injection and improve device performance.

The Role of Interfaces in Spin Injection into Organic Semiconductors

This paper presents an in-depth exploration of the interface dynamics that influence spin injection in organic semiconductors (OSCs), elucidating how molecular spintronics can emerge as a viable alternative to traditional inorganic materials. It highlights the emergence of a significant tunnel magnetoresistance (TMR) effect, achieving values up to 300% within (La,Sr)MnO/Alq/Co nanometer-sized magnetic tunnel junctions (MTJs). This impressive figure introduces considerable potential for tailoring spin polarizations through molecular engineering.

Contributions and Findings

The authors provide a detailed account of fabricating LSMO/Alq/Co magnetic tunnel junctions by leveraging conductive tip atomic force microscopy (CT-AFM). Through precise nanoindentation techniques, they address common issues like inhomogeneity and metal diffusion in wide area tunnel junctions, resulting in a well-controlled organic barrier thickness.

This research proposes a model for spin-dependent transport, enhancing the understanding of spin injection into OSCs. Two key contributions arise from this study:

1. Enhanced Spin Lifetime: The paper discusses the substantial enhancement in spin relaxation times that can be achieved in organic semiconductors, attributed to weak spin-orbit coupling and delocalized orbital electron transport. This provides organic materials a distinct advantage over inorganic counterparts.
2. Interface Dynamics: At the LSMO/Alq/Co tri-layer, the authors analyze the magnetoresistance (MR) characteristics. With reproducible positive MR effects confirmed through various tests, the study emphasizes the importance of potential chemical tuning at the interface to optimize spin injection properties.

Theoretical Implications

By introducing a model based on spin transport via donor-acceptor mediated mechanisms, the authors explore the rectification of traditional inconsistencies observed in MR sign variations. Utilizing a modified Bardeen approach, they postulate that localized states formed at the metal/molecule interface can significantly enhance spin polarization or even reverse it. The analysis distinguishes between weak and strong couplings, demonstrating the resultant variations in spin-dependent density of states (DOS).

Practical Implications

The reported TMR values suggest that OSC-based devices exhibit promising performance metrics and pave the way for further chemical tailoring. This can be particularly impactful for the electronics industry, offering a cost-effective, chemically versatile alternative to inorganic MTJs. The potential applications extend to developing highly efficient organic spin-valves and other spintronic devices, incorporating tailored molecular interfaces to optimize electronic and magnetic performance.

Future Directions

Going forward, the refinement in understanding spin injection mechanisms offers avenues for additional experimentation with self-assembled monolayers and other organic interfaces. The prospective ability to control device properties through manipulation of molecular composition and interface coupling presents a promising research trajectory. Future efforts could aim to integrate diverse organic materials and explore their compatibility with various magnetic electrodes, expanding the functional scope of organic spintronics.

In conclusion, this paper signifies a step towards unlocking the potential of molecular electronics in the field of spintronics by meticulously unraveling interface dynamics and exploiting organic compounds' inherent capacities to specialize spin transport phenomena. As the exploration of this emergent intersection progresses, it is poised to incite further innovations in the domain of next-generation electronic devices.