Magnetic coupling governed by defects
In recent years, novel magnetic textures in low-dimensional systems have created great excitement in the field of magnetism. In particular, the discovery of skyrmions — whirling magnetic bubbles with a well-defined chirality — in ultrathin films has opened a completely new research avenue. Their topologically-driven stability and their fast motion under current pulses have rendered skyrmions highly relevant for technological applications. Along with their own merits, these early research efforts also revealed potential problems in manipulating these magnetic objects. Specifically, their current-induced motion exhibits unwanted deviations due to the skyrmion Hall effect. On this issue, synthetic antiferromagnets (SAF) have proven extremely useful in suppressing such deviations. Thus, contriving SAFs and understanding their magnetic interactions have come to the forefront in magnetism research.
The two main pathways by which SAF systems can be realized are either by combining rare-earths and transition-metals or by interposing a non-magnetic spacer between two ferromagnets. Concerning the latter, recent reports suggest that a monolayer graphene spacer layer may mediate antiferromagnetic coupling between elemental 3d transition-metal films. However, the fabrication of these heterostacks is hampered by the morphological quality of graphene, which is difficult to control. This raises questions regarding the magnetic properties of the heterostructures. In our highlighted work, we focused on this key relationship between the structural quality of graphene and the magnetic coupling it mediates between adjacent Fe and Co layers.
As a model graphene-based SAF, an Fe/graphene/Co heterostack was grown epitaxially on a Re(0001) single-crystal surface. The spatially-resolved structural, chemical and magnetic properties were probed by X-ray photoemission electron microscopy and low-energy electron microscopy using the Spectroscopic PhotoEmission and Low Energy Electron Microscope (SPELEEM) at the Nanospectroscopy beamline of the Elettra synchrotron. The spectromicroscopy measurements were complemented by X-ray Absorpiton Spectroscopy (XAS) at SOLARIS synchrotron and density-functional theory (DFT) calculations at ICTP and SISSA. The combined experimental and theoretical study showed that Fe grows in clusters over graphene, and that the Fe clusters preferentially sit on graphene vacancy sites. Across the pristine defect-free graphene lattice, the Fe clusters couple antiferromagnetically to the underlying Co film via a robust super-exchange coupling independent of the exact adsorption configuration.
The first discovery in our study was that an Fe cluster placed on a graphene vacancy couples ferromagnetically to the Co underlayer. In the experimental model, depending on the density of graphene vacancies, the overall Fe magnetization (i.e., averaged over all clusters) varied from antiferromagnetically to ferromagnetically oriented with respect to that of Co. The second discovery regarded the passivation of vacancies using nonmagnetic atoms and molecules prior to Fe deposition. Upon such passivation, the ferromagnetic coupling channels were suppressed and the resulting Fe layer showed a pronounced antiferromagnetic coupling to Co. Fig. 1 illustrates the passivation. DFT calculations in Fig. 1a-1b shows an Fe cluster, when placed over an Ag-passivated graphene vacancy, with magnetization opposite to that of the Co underlayer. The vacancy passivation due to the presence of Ag atoms are seen in the x-ray photoemission data in Fig. 1c; the resulting Fe and Co magnetic domains, imaged by XMCD-PEEM, appear with opposite contrast, demonstrate that antiferromagnetic coupling is restored, see Fig. 1d.
Figure 1: Effect of passivating graphene defects on the magnetic coupling in Fe/Ag(0.15ML)/gr/Co. DFT calculation a) atomic structure and b) spin density plot of Fe13 cluster on triple vacancy defect with a Ag3 cluster passivating the defect. c) C 1s x-ray photoemission spectra are shown before and after the Ag passivation of graphene. d) XMCD-PEEM images acquired at the Co and Fe edges displaying antiferromagnetically-coupled out-of-plane magnetic domains.
Combining defect passivation with controlled defect creation using local electron/X-ray beams, our study opens up the possibility of using lithographic tools to engineer patterns in which the magnetic coupling can be tuned locally. We expect that spatially resolved SAF patterns will constitute an attractive platform for skyrmion motion and manipulation.
This research was conducted by the following research team:
Carlo Alberto Brondin1,2, Maha Hsouna3,4, Francesca Genuzio2, Matteo Jugovac2, Marcin Zając5, Ewa Partyka-Jankowska5, Stefano Bonetti1, Andrea Locatelli2, Nataša Stojić3, Tevfik Onur Menteş2
1 Università Ca’ Foscari di Venezia, dipartimento di scienze molecolari e nanosistemi, Venice, Italy
2 Elettra Sincrotrone Trieste S.C.p.A., Trieste, Italy
3 Abdus Salam International Centre for Theoretical Physics, Trieste, Italy
4 The International School for Advanced Studies, Trieste, Italy
5 SOLARIS National Synchrotron Radiation Centre, Krakow, Poland
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Reference
C.A. Brondin, M. Hsouna, F. Genuzio, M. Jugovac, M. Zając, E. Partyka-Jankowska, S. Bonetti, A. Locatelli, N. Stojić, T.O. Menteş, “Defect-controlled magnetic coupling in graphene-based synthetic antiferromagnets”, Carbon 249, 121169 (2026); DOI: 10.1016/j.carbon.2025.121169.
