The self-assembly of magnetic molecules on supporting surfaces provides potentially very interesting magnetic nano-devices. In a previous work we have evaporated wheel molecules of formula {Cr10(OMe)20(O2CCMe3)10}, abbreviated {Cr10} (Fig. 1a), by direct sublimation in UHV on a Ag(110) single-crystal. In powder form, the Cr-Cr interaction may be ferro- or antiferromagnetic, yielding to a paramagnetic molecule with ground state S=9 (implying that two Cr spins lie anti-parallel to the other eight, at T and H zero). For the powder the χT increases below 10 K, indicating average intracluster ferromagnetic behavior. In this work, the molecules are deposited onto the Au(111) surface. The STM image in Fig. 1b shows a monolayer of {Cr10} molecules, self-organized in a quasi-hexagonal 2D network. XMCD measurements were performed on the monolayer (1ML) and multilayer (14 ML) samples at the L2,3 edges of Cr at T=1.8 K and magnetic field up to μoH=6T. Magnetization and susceptibility were measured by SQUID magnetometry on the powder and the 14 ML samples (Fig. 2). In contrast to the powder sample, the 14 ML sample shows an overall intracluster antiferromagnetic coupling (χT decreases below 10 K). The M(H) XMCD determination of the {Cr10} 14 ML scales nicely to the SQUID measurement, with absolute values of the average Cr moment (mCr=1.89 μB). The M(H) XMCD for the 1ML sample is lower than the 14ML, evidencing a quenching effect of the substrate on mCr. Neither of the ML’s show any anisotropy. Monte-Carlo simulations for the {Cr10}, performed considering Cr(III) as individual S=3/2 spin entities coupled via Heisenberg interactions, allow to determine the exchange constants for the powder and 14 ML samples. In short, the self-assembly of 1ML {Cr10} molecules on Au(111) modifies the Cr-Cr intramolecular exchange pathways leading to a dominant antiferromagnetic intracluster coupling, and decreases the average Cr magnetic moment.

Resumen del trabajo presentado a la 21st International Conference on Magnetism (ICM), celebrada en San Francisco (US) del 15 al 20 de julio de 2018.

Peer Reviewed