Rare Earth and Bimetallic Transition Metal Islands at Surfaces

This thesis reports on the growth and on the magnetic characterization of nanostructures made of 3d-4d transition metals and rare earths. In all the experiments the nanostructures have been grown by atomic beam epitaxy (ABE) in ultra high vacuum conditions (UHV) by performing growth steps with the precise selection of external parameters as: the deposition flux F, the sample deposition temperature Tdep and the sample annealing temperature Tann . The morphological and magnetic properties of the nanostructures have been characterized by performing two in situ measurement techniques in our experimental setup, namely: scanning tunneling microscopy (STM) and magneto-optical Kerr effect (MOKE). The first aims of the thesis consisted in the formation of a supperlattice of rare earth on the graphene moiré pattern induced by the lattice mismatch of a single graphene layer grown on Ir(111) crystal surface. We report the first cluster supperlattice made of rare earths, namely Sm, where clusters nucleate in registry with the moiré pattern, forming a superlattice for deposition temperatures between 80 K and 110 K. Within this temperature range, the Sm supperlattice shows long-range order extending over several tens of nanometers and a cluster size distribution competing with the finest superlattices grown by ABE. Sm cluster spatial order is preserved, up to a coverage of 0.5 ML yielding a mean cluster size of 50 atoms, while for higher coverages coalescence starts. Moreover, Sm clusters with a mean size of 9 atoms are thermally stable up to Tann = 130 K from where on the order is lost and the density reduces progressively by cluster coalescence. Similar experiments carried out for Dy in the same deposition temperature range show the absence of cluster arrays. The second part focused on the enhancement of the MAE of bimetallic nanostructures, grown on Pt(111), having a Co-core by forming atomically sharp interlines and interfaces with three 4d elements, namely Ag, Pd and Rh. Our approach is based on the experimental measurement of the island magnetic susceptibility x(T) and morphology by means of MOKE and STM, respectively, combined with magnetic simulation analysis in order to quantify the different contributions to the island magnetic anisotropy. The monolayer capping for all studied elements contributes positively to the out-of-plane MAE of Co islands, with Pd giving the largest increase in the magnetic hardness. In addition, the capping with two Pd monolayers of pure Co islands maximizes the blocking temperature Tb . However, the lateral decoration of Co islands with all elements contributes negatively to MAE, with the Co/Pd shell giving the smallest effect and Rh the largest. Morphologically, Ag was found to produce a partial decoration of the Co island edges while Rh completely decorates laterally Co islands forming a rim of irregular shape without nucleating on top of Co islands for the used deposition parameters. Simulations of the magnetization reversal process by using coherent rotation (CR) and/or domain wall nucleation and propagation (DW) reversal models including different contributions to the MAE for atoms located at the interface (Ks) and at interline (Kp) and an exchange interaction between Co and 4d transition metals have reproduced the experimental x'(T) and x''(T) curves.

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