Infoscience

Thesis

Growth and magnetism of 2D bimetallic nanostructures

This thesis addresses the growth and magnetic characterization of 2D bimetallic nanostructures deposited by atomic beam epitaxy (ABE) on Pt(111). These structures possess both tunable high perpendicular magnetic anisotropy (PMA) and magnetic moment. These properties make them appealing as model systems in order to learn how to control the properties of the futures media used in magnetic data storage. Our study combined two in situ measurements techniques : variable-temperature scanning tunneling microscopy (VT-STM) as a local probe allows insight on the morphology of nanostructures, while magneto-optic Kerr effect (MOKE) is a spatially integrating technique giving access to the variation of the overall magnetization of a sample. The first part of this work focuses on the growth of iron on the Pt(111) surface. Growth was investigated on the atomic scale as a function of the substrate temperature in the case of low coverages. We have fitted the mean cluster size as a function of the annealing temperature with mean field rate equations for diffusion-controlled growth. The activation parameters for monomer, dimer and trimer diffusion could be inferred from this procedure. The formation of monatomic Fe wires has also been evidenced on the temperature range 160 K–260 K. The origin of their formation was discussed. In a second part, we have made use of our knowledge on the growth of cobalt and iron on Pt(111) in order to fabricate "core-shell" Co nanostructures of which density, size and shape were controlled. Therefore, we could realized both compact and ramified structures within the size range 800–1800 atoms. The study of the mechanism of magnetization reversal of these model structures has revealed a strong size and shape dependence. This is due to the shape-induced non-uniformity of the local magnetization and the number of pinning centers. The conclusions are that ramified structures with arms longer than 150 Å reverse their magnetization by nucleation and domain-wall motion while compact structures reverse coherently their magnetization. The third part deals with the magnetic properties of one monolayer thick bimetallic "Co core-shell" nanostructures on Pt(111). The blocking temperature TB marks the transition between superparamagnetic and blocked states and is inferred from the magnetic anisotropy. Here, we performed magnetic zero-field susceptibility measurements so as to determine TB in our samples. From our experiments, we show the possibility to make up a fine tuning of the nanostructure magnetic anisotropy and overall magnetization. In the case of the FexCo1-x alloy, TB adopt a bell-shape with x and exhibit a maximum at x = 0:5. The various lateral and vertical interfaces between Co from one side and Fe, Pt or Pd from the other side are at the origin of substantial TB variation. Those variations are inferred from the symmetry breaking and the strong hybridization between the d orbitals of these elements.

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