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Enhanced magnetic hybridization of a spinterface through Insertion of a two-dimensional magnetic oxide layer

Interfaces between organic semiconductors and ferromagnetic metals offer intriguing opportunities in the rapidly developing field of organic spintronics. Understanding and controlling the spin-polarized electronic states at the interface is the key toward a reliable exploitation of this kind of systems. It is indeed important to master and reliably reproduce the chemical reactions responsible of the spin-polarization at the interface.
Here we propose an approach consisting in the insertion of an ultrathin, two-dimensional Cr4O5 magnetic oxide layer at the interface between a C60 fullerene organic semiconductor and a Fe(001) ferromagnetic metal to both maximize the spin polarization and to overcome the reproducibility issues usually present in case of direct interface between metallic layer and organic semiconductor.
C60 fullerene showed a greater surface diffusivity when growing on Cr4O5 compared to the Fe(001) case. From the first stages of surface coverage, C60 tends to form islands rather than isolated molecules, leading to a well-ordered growth at higher thicknesses (Figure 1).


Figure 1STM image 200 x 200 nm2 of the surface of a C60/ Cr4O5/Fe(001) sample with a fullerene coverage of about 0.5 ML. The image was taken at room temperature with ΔV= 1.7 V, I = 400 pA.

We performed XMCD measurements at the C K1 edge at APE-HE beamline. The XAS spectra were acquired in the total electron yield mode, with the photon beam impinging at 45° with respect to the sample surface. The samples were oriented so to have one of the easy axes of magnetization of the Fe(001) surface in the same plane containing both the radiation propagation vector and the normal to the sample surface, in order to maximize the spin-polarized signal. All spectra were acquired at room temperature and in the remnant magnetization condition, after applying a magnetic field pulse of 50 Oe, which is large enough to saturate the substrate magnetization.

Figure 2 shows both XAS spectra at opposite X-ray circular polarizations and the related XMCD spectra for 1 ML C60 grown on Cr4O5 and Fe(001). The XAS spectra are characterized by the typical C60 line shape, where the C 1s → π* absorption peaks are labeled increasingly from LUMO to LUMO+3. The part of the spectra above about 289 eV is instead due to C 1s → σ* transitions. The LUMO+1 and LUMO+2 peaks are merged together in the C60/Fe(001) case, on account of the stronger interaction with the substrate, as already observed in several cases where fullerene was grown on metallic substrates.In the C60/Cr4O5/Fe(001) sample the LUMO+1 and LUMO+2 peaks are better resolved. From the point of view of the magnetic hybridization, the measured XMCD is different from zero at correspondence with different absorption peaks, revealing that a spin-polarization has been induced in the C60 π-conjugated states. Concerning the C60/Fe(001) interface,we observe an oscillatory behavior with XMCD changing sign when passing from the LUMO to the LUMO+1/+2 positions. On the other hand, the XMCD spectrum of C60/Cr4O5/Fe(001) is dominated by a large positive signal close to the LUMO+3 peak. This XMCD peak reaches a value of nearly 12%, which is unprecedented in similar (purely carbon-based) systems.



Figure 2. X-ray absorption spectroscopy at the C K1 edge taken at room temperature with either right (black lines) or left (red lines) circular polarization and corresponding XMCD spectra (blue lines) of (a) 1 ML C60/Cr4O5/Fe(001) (b) 1 ML C60/Fe(001). The XMCD spectra are multiplied by 5. 


In order to understand the striking results obtained when inserting Cr4O5 at the C60/Fe(001) interface, we have performed a theoretical analysis based on first-principle density functional theory (DFT) simulations. Figure 3, presenting the section view of the spin density at the C60/Cr4O5 interface, shows that the molecule is indeed spin polarized and the spin polarization of the ionized atom is opposite to that of the nearest Cr atoms in the substrate. Furthermore, the spin polarization of the molecule changes sign when changing energy range, as observed for the surface states. These properties are in agreement with those observed in the XMCD spectra, which are characterized by the presence of peaks of opposite sign.

Figure 3. a) First-principle density functional theory (DFT) simulations of the XMCD spectra of the Cr4O5 -C60/Fe(001) interface. b): section view of the spin density at the C60/Cr4O5 interface.

In conclusion, the insertion of a two dimensional magnetic oxide directly influences both morphologic and electronic properties of the interface. Moreover, spin-polarized hybridized states were enhanced in the case of the C60/Cr4O5 interface.Theoretical simulations supported the experimental results and emphasized the fact that tailoring the surface density of states of the substrate would allow one to tailor the magnetic properties of a spinterface. Spinterfaces characterized by the presence of a two-dimensional magnetic oxide can thus be considered as reliable candidates for the design and development of organic spintronics systems.



 

This research was conducted by the following research team:

Alberto Brambilla1, Andrea Picone1, Dario Giannotti1, Alberto Calloni1, Giulia Berti1, Gianlorenzo Bussetti1, Lamberto Duò1, Marco Finazzi1, Franco Ciccacci1, Simona Achilli2, Guido Fratesi2, Mario I. Trioni3, Giovanni Vinai4, Piero Torelli4, Giancarlo Panaccione4
 
 
Dipartimento di Fisica, Politecnico di Milano, Milano, Italy
2 Dipartimento di Fisica, Università degli Studi di Milano, Milano, Italy
CNR - National Research Council of Italy, ISTM, Milano, Italy
Laboratorio TASC, IOM-CNR, Trieste, Italy


Contact persons:

Alberto Brambilla, email: alberto.brambilla@polimi.it
Piero Torelli, email: piero.torelli@elettra.eu

 

Reference

A. Brambilla, A. Picone, D. Giannotti, A. Calloni, G. Berti, G. Bussetti, S. Achilli, G. Fratesi, M. I. Trioni, G. Vinai, P. Torelli, G. Panaccione, L. Duò, M. Finazzi and F. Ciccacci, "Enhanced Magnetic Hybridization of a Spinterface through Insertion of a Two-Dimensional Magnetic Oxide Layer", Nano Letters 17, 7440 (2017)  DOI: 10.1021/acs.nanolett.7b03314

 

Last Updated on Wednesday, 17 January 2018 16:28