EMERGENT MAGNETISM AT TRANSITION-METAL-NANOCARBON INTERFACES

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Using ALBA's synchrotron light, researchers have shed light on the origin of the magnetism arising at carbon/non-magnetic 3d,5d metal interfaces. These results may allow the manipulation of spin ordering at metallic surfaces using electro-optical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices.

Interfaces are key in solid state and quantum physics, controlling many fundamental properties and enabling emergent interfacial, bi-dimensional like phenomena. Therefore they offer potential opportunities for designing hybrid materials that profit from promising combinatory effects. In particular, the fine-tuning of spin polarization at metallo–organic interfaces opens a realm of possibilities, from the direct applications in molecular spintronics and thin-film magnetism to biomedical imaging or quantum computing. This interaction at the interface can control the spin polarization in magnetic field sensors, generate magnetization spin-filtering effects in non-magnetic electrodes or even give rise to magnetic ordering when non-magnetic elements such as diamagnetic copper or paramagnetic manganese are put in contact with carbon/fullerenes at such interfaces.

The research carried out by the group at University of Leeds, led by Oscar Cespedes, together with collaborators at the University of St. Andrews, University of Liverpool, the Paul Scherrer Institute and the ALBA Synchrotron, sheds light on the origin of the magnetism arising at carbon/non-magnetic 3d,5d metal interfaces.

These systems exhibit small magnetic anisotropy and coercivity together with a high Curie point. Low-energy muon spin spectroscopy in Cu and Sc–C60 multilayers show a quick spin depolarization and oscillations attributed to non-uniform local magnetic fields close to the metallo–carbon interface. The hybridization state of the carbon layers plays a crucial role and researchers observe an increased magnetization as sp3 orbitals are annealed into sp2−π graphitic states in sputtered carbon/copper multilayers.

The X-ray magnetic circular dichroism (XMCD) measurements performed at the BOREAS beamline of ALBA were essential to probe and confirm, in element specific measurements, the magnetism at the carbon K edge of C60 layers in contact with Sc films. These measurements evidenced spin polarization in the lowest unoccupied molecular orbital (LUMO) and higher π*-molecular levels, whereas the dichroism in the σ*-resonances was found to be small or non-existent. These results support the idea of an interaction mediated via charge transfer from the metal to the nanocarbon layer and dz–π hybridization.

Thin-film carbon-based magnets and related molecular spintronic devices may allow the manipulation of spin ordering at metallic surfaces using electro-optical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices.

IM-CarbonAtomsFullerenes_BOREAS
Fig: XMCD measurements at the Carbon K-edge at room temperature and at low temperature (2 Kelvin), evidencing the magnetism of C atoms that emerges when non-magnetic Carbon and Scandium layers are put in contact in C/Sc interfaces.

Reference: Emergent magnetism at transition-metal-nanocarbon interfaces PNAS 2017 114(22):5583-5588. doi: 10.1073/pnas.1620216114

The μSR experiments were performed at the Swiss Muon Source SμS at the Paul Scherrer Institut, Villigen, Switzerland. Authors thank the Engineering and Physical Sciences Research Council (EPSRC) in the United Kingdom for support through Grants EP/P001556/1, EP/J01060X/1, EP/ I004483/1, and EP/M000923/1. R.S. wishes to acknowledge EPSRC for a scholarship via the Grant EP/L015110/1. XAS/XMCD experiments were performed in the BOREAS beamline at the Alba synchrotron (Proposals ID2014071101 and ID2015091530).M.V. acknowledgesMineco Grant FIS2013-45469-C4-3-R. Use of the N8 High Performance Computing (HPC) (EPSRC EP/K000225/1) and ARCHER (via the UK Car–Parrinello Consortium, EP/K013610/1 and EP/P022189/1) HPC facilities is gratefully acknowledged. S.A. thanks Taibah University for support with a PhD scholarship).

 

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