Abstract:

The self-assembly of magnetic molecules on supporting surfaces may provide interesting nanodevices
in the fields of high-density information storage, information processing and
spintronics. We have evaporated wheel molecules of formula {Cr10(OMe)20(O2CCMe3)10}, ({Cr10}
from now-on) by direct sublimation in UHV on a Ag(110) single-crystal [1]. In powder form,
these molecules show ferromagnetic interactions [2][3], and therefore, they are good
candidates as magnetic molecules in a 2D array. In this work, the molecules are deposited onto
the Au(111) surface. STM images show a monolayer of {Cr10} molecules, self-organized in a
quasi-hexagonal 2D network.

We have studied several samples: a monolayer (1ML) and two kind of multilayers (14 ML) of
{Cr10} on Au(111) and Cu(111), as well as the {Cr10} molecular material in bulk, powder form.
XMCD was performed at the Cr L2,3 and K edges at T=2 K under magnetic field up to B of 6T and
17T respectively. Magnetization and susceptibility were measured by SQUID magnetometry on
multilayer and the powder sample up to B = 5 T. The magnetization of the three samples are
shown in Fig. 2.

Both M(H) and χ(T) for the powder sample suggest the {Cr10} wheels in a state of a total
molecule spin S=9 ground state, with overall ferromagnetic coupling (χT>0), which is lost by
depositing the molecules onto the Au(111) surface, both in the multi- and the monolayer
sample (χT<0). MonteCarlo 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: The self-assembly of 1ML {Cr10}
molecules on Au(111) modifies the Cr-Cr intramolecular exchange pathways and the local
anisotropy, leading to a dominant antiferromagnetic coupling.