ELABORATION, ETUDE DES PROPRIETES STRUCTURALES ET MAGNETIQUES DE COUCHES ET RESEAUX DE PLOTS SUBMICRONIQUES A BASE DE COBALT

摘要

In the first part of this doctoral work, we have studied the correlation between the magneticdomain structure and the macroscopic magnetic properties of epitaxial cobalt thin films withthicknesses varying from 10 to 500 nm. The growth was optimised to get the cobalt film in thehexagonal (0001) cristalline phase. In this cristallographic structure, the magnetocristallineanisotropy is very strong and along the c-axis. This strong perpendicular anisotropy forces thecobalt to split in domains in which the magnetization turns from fully in plane to fully out ofplane as the film thickness is increased from 10 to 50 nm. For thicknesses above 50 nm, thedomain structure depending on the magnetic history may consists of bubbles, stripes orlabyrinthine. The domain topology is independent on the film thickness while the periodicityof the domains varies from 100 to 400 nm and is proportional to the squareroot of the filmthickness. We have shown that these well characterized films appear to be model systems forthe study of magnetoresistance through magnetic wall as well as observation of domain wallresonance at high frequency.In the second part of this work, the structural and magnetic properties of square dot arrayswith dot lateral dimension of 0.5 μm and lattice periodicity of 1 μm have been studied. Thearrays 5 by 5 mm² were patterned from the epitaxial cobalt films using X-ray lithography. Wehave shown that the magnetization reorients from in-plane to perpendicular as a function ofdot height. Interestingly, the magnetic domains are shown to be geometrically constrained bythe side of the dots. For thick dots, the lateral extension of domains is comparable to the sizeof the dots leading to the constrain of the perpendicular component of magnetization. For the25 nm thick dots, the geometrical constrain of the parallel component of magnetization leadsto the stabilization of a vortex domain structure with circular rings. Starting from thisstructure, the magnetization curve displays one pronounced jump of magnetization whichimplies that the reversal of the magnetization occurs through the collapse of the vortexdomain structure. Moreover, magnetic phase transition occurs when the temperature islowered. The magnetization in each domain switches as a function of temperature frommainly in-plane into a fully perpendicular domain structure.