Etude du magnétisme local par analyse du dichroïsme en microscopie électronique en transmission – DICHROMET

Aim: The aim of this research project is to develop at the CEMES an original method to measure magnetic dichroism in a transmission electron microscope (TEM), in order to study the local electronic and magnetic structure of nanomaterials. We aim to reach a nanometer scale probe and a signal/noise ratio large enough to quantify the magnetic moments by improving the method recently proposed by a team in Vienna (P.Schattschneider et al., Nature, 441 (2006) 486). We will use the SACTEM, a top quality microscope whose objective lens is corrected for spherical aberrations, and will rely on comparison with ab initio calculations. Expected results: The development of this new analysis method at the CEMES will give access to the study of the electronic and magnetic structure at interfaces, particularly in magnetic layers for spintronic applications (giant or tunnel magnetoresistive devices). These materials are already studied in the lab by HREM, EELS and Lorentz microscopy. Close to interfaces, local strain and interdiffusion can induce noticeable and localised changes of the crystalline structure and of the electronic structure. These structural and chemical changes modify locally the magnetic properties (increase or decrease of the magnetic moment). The first step would be to measure locally the magnetic moment modification of the atoms. The second step would be to understand the link between the modifications of the electronic structure close to the interfaces and the spin-dependent transport properties in spintronic devices. Methodology: These new experiments will be performed on the Tecnai F20 TEM FEG at the CEMES. This microscope is fitted with a field emission gun, a spherical aberration corrector and a GIF (gatan imaging filter) Tridiem system that allows to get the energy filtered images. The experimental part will be sustained by ab-initio calculations of the local electronic structure and of the magnetic dichroism; we will also use specific sum rules to quantify magnetic moments. A first part of the project will be dedicated to the optimisation of this new experimental method and to its validation on 'model' bulk samples by comparison with X-ray magnetic circular dichroism (XMCD) measurements performed at the synchrotron. The second part will be focused on the study of multilayers to use the method at a nanometre scale. All the measurements will be compared with calculations of the local electronic structure and magnetic moments and with calculations of the expected dichroic signal (calculation of the fine structure of the L2 and L3 edges for iron, nickel and cobalt in metals and oxides).