Magnetic anisotropy of Cr7Ni spin clusters on surfaces

We address here the problem of the experimental and theoretical determination of magnetic anisotropy in isolated molecular spin clusters. To this end, we consider the case of molecular Cr7Ni rings sublimated in UHV conditions and assembled in ordered fashion on Au(111) surface [1] and we investigate it by X-ray magnetic dichroism (XMCD) and theoretical calculations. Fixing the experimental conditions at T=8K and magnetic fi eld 5T, the angular-dependence of the dichroic signal reveals an easyaxis anisotropy for the Ni magnetization along the direction perpendicular to the ring whilst the magnetization of the whole Cr7Ni molecule results preferentially aligned within the ring plane. These features are well reproduced by spin Hamiltonian simulations wich essentially refl ect the character of the S=3/2 fi rst excited multiplet, which dominates at T=8K and 5T. DFT calculations show that local spin orbit interactions determine an easy axis anisotropy at the Ni site while the Cr magnetic moment turns out to be more isotropic. This is, to our knowledge, the fi rst direct observation of the interplay between the single ion and the overall magnetic anisotropy in complex (polynuclear) molecular systems.