Organic-inorganic hybrid spintronics
Zusammenfassung der Projektergebnisse
B.3 RESUME CONSOLIDE PUBLIC EN ANGLAIS Suivre impérativement les instructions ci-dessus. Controlling ferroelectric and spintronic properties of hybrid organic-inorganic systems Towards ferroelectronics-spintronic organic inorganic devices Organic materials may provide added advantages due to their flexibility, chemical functionality, nontoxicity, low cost, easy processing and low power consumption. For spintronic devices, aside the ferroelectric properties of bulk materials, the hybrid ferromagnetic metal/molecular interfaces play an essential role in device performance. 1-Ferroelectric characterizations of organic ferroelectric Croconic Acid films We studied the microscopic ferroelectric properties of CA thin films deposited on gold (Au) and comparatively more reactive cobalt (Co) surfaces which are relevant to fabricate spintronic devices. Piezoresponse Force Microscopy (PFM) results demonstrated that the ability to switch the polarization state of a domain within the CA film depended on which underlying substrate was used. We found that robust and reproducible switching can occur for a Co substrate, but not for an Au substrate which is normally less reactive than the Co surface. We were able to successfully measure the tiny switching currents originating from the polarization reversal within very few grains of CA. We obtained remnant polarization charge densities of around 12 µC/cm2 which is in the order of reported Polarization charge densities for bulk CA crystals. STM investigations of switching of CA films were unsuccessful. The films that were thin enough to allow a tunnelling current to flow showed no switching due to an in-plane orientation of the dipole moment. Thicker films were insulating. Instead, we studied molecules with a rotatable dipole moment that is lifted from the metallic surface by a ligand. We managed to realize a molecular rotor and a lattice of rotatable dipoles, studied their switching behaviour and their dipole-dipole interaction. 2-Manipulate magnetic state of spin crossover organics complexes on interaction with ferromagnetics surfaces The basic idea of this part of the project was to exchange couple spin crossover molecules to a ferromagnetic substrate, maintain spin crossover and by this modify the magnetic properties of also the ferromagnetic substrate by the crossover. The initial system we investigated was Fe-phen molecules adsorbed on the ferromagnetic Fe2N layers on Cu(100). First results indicated that the molecules can be switched between two states using the electric field of the STM tip but critical investigations of the second Phd student revealed that it was a mere rotation of the molecule. Fe-phen molecules deposited onto LSMO showed a disordered growth and a frozen spin state and no spin crossover. Fe-pryz molecules on LSMO substrates showed a better growth but unfortunately no exchange or spin crossover. Finally, Fe-pyrz on Au passivated Co films worked out. We clearly observed spin crossover using STM and XMCD. There is a large indirect exchange interaction between Co and Fe-pyrz. XMCD measurements on the influence of the spin crossover on the Co hysteresis loop are schedules for 28th of May 2021. Résultats majeurs du projet We succeeded to fabricate switchable and robust ferroelectric Croconic Acid thin films on Co substrate. Moreover, our work also hints at possible rich interface phenomena that may be occurring at a ferromagnetic/organic ferroelectric interface and its role in the stabilization of polarization reversal. Référence du formulaire : ANR-FORM-090601-01-01 6/17 To explore the possibility of measuring the switching currents from nanoscopic ferroelectric regions, we obtained some very promising results in the direction of directly detecting switching currents from the organic ferroelectric, Croconic Acid, at the nanoscale. Further, we were able to demonstrate that spin crossover of molecules between high- and low-spin state may indeed be preserved when adsorbed on a ferromagnetic structure. The molecules showed at the same time an exchange interaction of the high-spin species with the ferromagnetic substrate. This is thus the first demonstration of a magnetic hybrid system showing spin crossover. Our observations could work as a guide for the development of multifunctional devices based on ferroelectric Croconic Acid composite as well as a demonstrator to extend e.g. optically induced switching of a spin crossover complex to a hybrid system containing metallic ferromagnets. Illustrations Robust polarization reversibility of CA domain on Co surface. Grey-blue contrast shows in-plane domains of a 2×2μm2 region in the initial state (a), after consecutive poling at +12V (b), -12V (c) and at +12V. Spin crossover of Fe-pyrz on Au/Co/Au(111). The high spin fraction drops from 100% at room temperature to about 33% at low temperatures and can be excited by light to the high spin state. The high spin molecules feel an exchange field to the Co films of about 820 mT. Production scientifique et brevets depuis le début du projet (environ 500 caractères espaces compris) The scientific production from this project has been significant with the publication of 9 papers on top of which 2 or 3 are in the writing stage. The very basic fundamental character of the project did not lead to patentable material. Originality of the treated subject, considering a mix of materials organics, inorganics, ferroelectrics and magnetics, allowed us to keep a large visibility on the international scene in this field. Informations factuelles The project ORINSPIN is a fundamental research international effort coordinated by Victor Da COSTA of IPCMS (Strasbourg) and Wulf Wulfhekel of KIT (Germany). After the decease of Eric Beaurepaire in April 2018, he was the originator of this project together with Wulf Wulfhelkel, the ANR accepted that Victor Da Costa, an Eric’s close collaborator, takes the responsibility to continue the project until its successful end. The project started in April 2017 at the KIT and 1st march 2017 at IPCMS. Considering exceptional factors that occurred last years, the duration of the project was extended to 52 months. It benefitted a grant of 187080€ from the ANR and a grant of 167100€ from the DFG, for a total cost of the order of 354180€ (ANR+DFG).
Projektbezogene Publikationen (Auswahl)
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2017. Molecular Graph Paper. Angewandte Chemie International Edition 56, 8290–8294
Lindner, M., Valášek, M., Mayor, M., Frauhammer, T., Wulfhekel, W., Gerhard, L.
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2018. Abrupt Switching of Crystal Fields during Formation of Molecular Contacts. ACS Nano 12, 3280–3286
Chen, J., Isshiki, H., Baretzky, C., Balashov, T., Wulfhekel, W.
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2018. Linking Electronic Transport through a Spin Crossover Thin Film to the Molecular Spin State Using X-ray Absorption Spectroscopy Operando Techniques. ACS Appl. Mater. Interfaces 10, 31580– 31585
Schleicher, F., Studniarek, M., Kumar, K.S., Urbain, E., Katcko, K., Chen, J., Frauhammer, T., Hervé, M., Halisdemir, U., Kandpal, L.M., Lacour, D., Riminucci, A., Joly, L., Scheurer, F., Gobaut, B., Choueikani, F., Otero, E., Ohresser, P., Arabski, J., Schmerber, G., Wulfhekel, W., Beaurepaire, E., Weber, W., Boukari, S., Ruben, M., Bowen, M.
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2019. Six state molecular revolver mounted on a rigid platform. Nanoscale 11, 9015–9022
Homberg, J., Lindner, M., Gerhard, L., Edelmann, K., Frauhammer, T., Nahas, Y., Valášek, M., Mayor, M., Wulfhekel, W.
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2020. Robust ferroelectric properties of organic croconic acid films grown on spintronically relevant substrates. Mater. Adv.
Mohapatra, S., Da Costa, V., Avedissian, G., Arabski, J., Weber, W., Bowen, M., Boukari, S.
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2021. Addressing a lattice of rotatable molecular dipoles with the electric field of an STM tip. Phys. Chem. Chem. Phys. 23, 4874–4881
Frauhammer, T., Gerhard, L., Edelmann, K., Lindner, M., Valášek, M., Mayor, M., Wulfhekel, W.
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2021. Interplay between point symmetry, oxidation state, and the Kondo effect in 3d transition metal acetylacetonate molecules on Cu(111). Phys. Rev. B 103, 085423
Chen, H., Frauhammer, T., Sasaki, S., Yamada, T.K., Wulfhekel, W.
