Ferroelectric control of organic/ferromagnetic spinterface – FEOrgSpin

1. Structure syntheses and characterizations of FE-Org thin film
We synthesize FE-Org thin film with spin-coating or LB methods. Good FE and tunneling properties of FE-Org will be firstly optimized by PFM and Conductive Atomic Force Microscopy (C-AFM) characterizations. TEM will be employed to characterize the metal/organic interface diffusion. ARXPS can be employed to study the electronic structure at FE-Org/Co interface.
2. Magneto-transport characterization
The device with junction structure of FM/FE-Org/FM will be fabricated on different substrates including flexible substrate. Either we can use in-situ mask with large junction size, or we can elaborate nano-size tunnel junctions (10^4 nm^2) with controlled organic barrier thickness by Conductive Tip-AFM (CT-AFM). Magneto-transport measurements will allow us to characterize the tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) of our devices.
3. Ab-initio calculation
Ab initio calculations will be necessary to evaluate the spin-polarization at the interface and to elucidate the observed TMR effect mechanisms and propose novel organic structure for efficient FE modulation of spin-polarization.

1. We have reported the evidence of ferroelectric “ailing channel” in the organic barrier, which can effectively pin the ferroelectric domain, resulting in non-switchable spin polarization at the FM/FE-Org interface. In particular, OMFTJs based on La0.6Sr0.4MnO3/P(VDF-TrFE) (t)/Co/Au structures with different P(VDF-TrFE) thickness (t) were fabricated. Pot-hole structures at the boundary between the P(VDFTrFE) needle-like grains are evidenced to induce “ailing-channels” that hinder efficient ferroelectric polarization of the organic barrier and result in the quenching of the spin polarization switching at Co/P(VDF-TrFE) interface. Furthermore, the spin diffusion length in the negatively polarized P(VDF-TrFE) is measured to be about 7.2 nm at 20K. The evidence of the mechanism of ferroelectric “ailing-channels” is of essential importance to improve the performance of OMFTJ and master the key condition for an efficient ferroelectric control of the spin polarization of “spinterface”. This work has been published in [ACS Appl. Mater. Interfaces 10, 30614 (2018)].
2. We have reported on the fabrication of an organic multiferroic tunnel junction (OMFTJ) based on an organic barrier of the Poly(vinylidene fluoride) (PVDF):Fe3O4 nanocomposite. By adding Fe3O4 nanoparticles into the PVDF barrier, we found that the ferroelectric properties of the OMFTJ are considerably improved compared to those with a pure PVDF barrier. This can lead to tunneling electroresistances of about 450% at 10 K and 100% at room temperature (RT), which are much higher than those of the pure PVDF based device (70% at 10K and 7% at RT). OMFTJs based on the PVDF:Fe3O4 nanocomposite could open new functionalities in smart multiferroic devices via the interplay of the magnetism of nanoparticles with the ferroelectricity of the organic barrier. This work has been published in [Appl. Phys. Lett. 116, 152905 (2020)].

This project will open new functionality in controlling the injection of spin polarization into organic materials via the ferroelectric polarization of the barrier. The product of this project will lead to the new functionalities of spintronic device based on ferroelectric organic materials, which certainly gives a significant impact on the actual science and technology of information. For example, the room temperature operated organic multiferroic tunnel junctions will be integrated on the flexible substrate to be used as magnetic sensor in robot skin and wearable device applications (market of $20 billion in 2020). If we can integrate FE-Org on non-magnetic semiconductor (GaAs, Si…) as a tunneling barrier, by ferroelectrically switching FE-Org polarization, we could electrically control the spin injection direction without any magnetic field, which could significantly promote the practical application of semiconductor spintronics (eg. spin light emitting diode and spin laser).

Publications:
1. Shiheng Liang, et al., “Quenching of Spin Polarization Switching in Organic Multiferroic Tunnel Junctions by Ferroelectric ‘Ailing-Channel’ in Organic Barrier”, ACS Applied Materials & Interfaces, 10, 30614 (2018).
2. Xue Gao, et al., “Enhancement of ferroelectric performance in PVDF:Fe3O4 nanocomposite based organic multiferroic tunnel junctions”, Applied Physics Letters 116, 152905 (2020).
3. Can Xiao, et al., “Temperature dependence of transport mechanisms in organic multiferroic tunnel junctions”, Journal of Physics D: Applied Physics 53, 325301 (2020).
4. F. Ibrahim, et al., «Unveiling multiferroic proximity effect in graphene«, 2D Materials 7, 015020 (2020)

Communications:
1. Shiheng Liang, et al., «Ferroelectric Control of Organic/Ferromagnetic Spinterface«, 7th International Meeting on Spin in Organic Semiconductors (SpinOS 2018), August 13-16, 2018, Halle (Saale) Germany.(poster)
2. Shiheng Liang, et al., «Quenching of Spin Polarization Switching in Organic Multiferroic Tunnel Junctions by Ferroelectric “Ailing-Channel” in Organic Barrier», Forum des microscopies à sonde locale – 19-22 mars 2019 – Carry-le-Rouet (poster)
3. M. Chshiev, “Spin-Orbit and Magnetic Proximity-Induced Phenomena in Nanostructures Comprising Transition Metals, Insulators and 2D Materials “, Online Spintronics Seminar Series, June 26, 2020 (invited talk)

Hybrid ferromagnetic metal/organic interface, as known as “spinterface”, can exhibit highly efficient spin-filtering properties and presents a promising class of materials for future spintronic devices. However, the spin-polarization of spinterface at Fermi level can be different or even opposite in sign to that of adjacent ferromagnetic electrode. Our recent achievement demonstrates that the spin-polarization at poly(vinylidene fluoride) (PVDF)/Co spinterface can be actively
modulated (even change the sign) by switching the ferroelectric (FE) polarization of PVDF. In this project, we aim to expand our knowledge on ferroelectric control of spin polarization at different FE-organic/ferromagnetic spinterface and develop novel functionalities of spintronic device based on ferroelectric organics.