FERROMAGNETIC AND ANTIFERROMAGNETIC COUPLING OF SPIN MOLECULAR INTERFACES WITH HIGH THERMAL STABILITY

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Researchers from the physics department of the Università "La Sapienza" in Rome, Centro S3 of Modena and ALBA, have demonstrated that magnetic coupling of metal-organic molecules to a magnetic substrate mediated by a graphene layer can be tuned in strength and direction by choosing the symmetry of the molecular orbitals that is largely preserved thanks to the graphene layer. The results have been published in the journal Nano Letters.

Cerdanyola del Vallès, 28th May 2018. Paramagnetic molecules become potential building blocks in spintronics when their magnetic moments are stabilized against thermal fluctuations, for example, by a controlled interaction with a magnetic substrate. Spin molecular interfaces with preserved magnetic activity and exhibiting magnetic remanence at room temperature (RT) can open the route to engineer highly spin-polarized, nanoscale current sources. The need to fully control the organic spin interface and the tuning of ferromagnetic (FM) or antiferromagnetic (AFM) coupling to achieve a stable conductance has motivated a vast experimental interest.

In this work, published in the journal Nano Letters, researchers led by prof. Maria Grazia Betti from the physics department of the Università "La Sapienza" in Roma, Centro S3 of Modena and ALBA, were able to demonstrate that a class of metal-organic molecules, in particular metal-phthalocyanine of Fe and Cu, can be magnetically coupled up to room temperature to a Cobalt layer by interposing a graphene layer. The experimental results, carried out at the BOREAS beamline (Fig.1a), are in close agreement with ab-initio theoretical calculations (Fig.1b) and demonstrate that by choosing the metal configuration in the organic molecule, more in detail the symmetry of the spin-carrying orbital, it is possible to tune the strength and the direction of the magnetic coupling to be either antiparallel or parallel. The peculiar coupling mechanism is determined by the different super-exchange path involving the Cobalt, graphene and molecular orbitals. The graphene layer between the active magnetic cobalt layer and the metal-organic molecule protects the molecular orbitals symmetries (Fig.1c,d), eventually opening the possibility to finely tune the magnetic interaction determined by the different super-exchange paths.

The choice of an effective super-exchange path can ensure the stability against thermal fluctuations, even at RT, and it can be further optimized with a fine control of the relative orientation of the easy magnetization axes at the spin interface. In perspective, the magnetic remanence at RT of these archetypal spin interfaces, once paired with a tunable magnetic substrate, opens the possibility to produce future organic-based operational spintronic devices.

 

Figure 1: a,b) Antiferromagnetic/Ferromagnetic coupling as deduced by element-specific hysteresis loops of  a FePc and CuPc (respectively) to a Cobalt layer with perpendicular magnetic anisotropy intercalated below graphene. c,d) orbital-porjection of the spin-density for the FePc and CoPc interface reflecting the different symmetry of the molecular orbitals involved in the ferromagnetic and antiferromagnetic interaction.

 

Reference: Ferromagnetic and Antiferromagnetic Coupling of Spin Molecular Interfaces with High Thermal Stability. Giulia Avvisati, Claudia Cardoso, Daniele Varsano, Andrea Ferretti, Pierluigi Gargiani, and Maria Grazia Betti. 
Nano Letters 2018 18 (4), 2268-2273. DOI: 10.1021/acs.nanolett.7b04836

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