Auger-PhotoElectron Coincidence Spectroscopy on magnetic thin films : a study of multiplet spin structure and electron correlation

Abstract

Besides magnetism is a largely investigated eld, atomic level origin of magnetic properties in condensed matter still remain not completely understood. In recent years it has been claimed that macroscopic magnetic manifestations are strongly dictated by the atomic scale behaviour, hence the study of the local properties seems to be the key for understanding magnetism. Unfortunately several and controversial experimental results, obtained using different techniques which rely on the exsistance of the long-range order, have contributed in preventing to obtain a unique vision of local magnetic properties. Among all, a still largely debated issue is understanding whether or not the magnetic order, existing at long-range below the transition temperature, is lost crossing the critical point even at short-range, that means on the atomic scale. This thesis work has been devoted to the investigation of the local properties of mag- netic thin lms, examinating the nal state spin multiplet contributions exploiting the unique capability of the Auger Photo-Electron Coincidence Spectroscopy (APECS) of access the multiplet spin con guration in a spin-selective fashion once performed in an angle-resolved (AR-APECS) mode. Because of its local sensitivity, which extends on the scale of the atomic wavefunction, APECS is a suitable tool to investigate magnetism from a local viewpoint. By using linearly polarised synchrotron radiation and the unique experimental cham- ber of the ALOISA beamline, AR-APECS has been performed to investigate the M23V V Auger transition both on ferromgnetic (FM) and antiferromagnetic (AFM) systems. The attention has been focused on 3d transition metals and their oxides. For such systems the valence 3d levels are responsible for their magnetic properties, hence valence electron correlation contains also information on magnetic behaviour. The M23V V Auger partial yield in coincidence with 3p photoelectrons from NiO/Ag(001) has been collected both below and above the N eel temperature of the lm. Combining the dipole and Auger selection rules with the angular distribution of the emitted electrons AR-APECS gives the possibility to access the local spin con guration of the Auger nal state spin coupling. Electron-pairs have been collected upon changing kinematics conditions allowing to probe different nal spin states and achieving the so called high-spin (triplet) versus low-spin (singlet) selectivity. Two different geometries have been took into account, the AN in which the photoelectron is collected aligned and the Auger electron not aligned (at an angle larger than 20◦) with respect to the light polarisation vector and the NN in which both the electrons are collected in directions not aligned with it. Such geometries have led to a magnetic dichroism when the sample was probed at a temperature lower than the N eel value, but this asymmetry has collapsed once the critical point has been exceeded. The presence of magnetic moments affects the AR- APECS dichroic effect (DEAR-APECS) via matrix elements. AR-APECS cross-section for spin-polarised atomic targets contains three relevant kinematic vectors, the momenta of the two emitted electrons and the magnetisation. Changes in one of these vectors is responsible for changes in the AR-APECS spectra. Because the only difference affecting the measurements below and above the N eel point is the temperature of the sample, which in turns involves only the magnetisation, the observed collapse of the dichroism in AR-APECS is an evidence for the loss of local magnetic order. An analogous behaviour, i.e. a strongly reduction of the asymmetry in AR-APECS spectra crossing the transition temperature, has been observed also in the case of FM systems. The M23V V Auger electrons in coincidence with 3p photoelectrons have been collected both for Fe/Ag and Ni/Cu, probing their ferromagnetic phase. For Ni the coincidence spectra have been collected for two different lm thicknesses and also above the Curie point. In this case the occurrance of DEAR-APECS collecting Auger electrons emitted in the decay of the M23 core-hole, in the same two AN and NN geometries, results different with respect to the case of AFM, revealing a different degree of electron correlation. For these metals DEAR-APECS shows itself as an asymmetry in the main line, but more relevantly, as a feature which could be associated to a high-binding energy resonance. The spectrum of FM systems requires both the self-convolution of the density of states and the Cini-Sawatzky theory to be fully interpreted, thus revealing the nature of both the band-like main line transition and the presence of an atomic-like localised state in the valence band which is characterised by a large correlation. TheM23V V Auger transition of Fe has been collected also with the FM layer in contact with an AFM phase, namely CoO, showing differences with respect to what observed in the case of a non magnetic substrate. Changes in the band structure imply changes in magnetic and transport properties. In the proposed experiments AR-APECS has shown itself as a suitable tool to monitor the magnetic phase transition from a local viewpoint without rely on crystal periodicity or thermodynamics arguments. The results reported just above are an hint for the collapse of the local magnetisation and constitute a valuable contribute in solving the issue about the collapse of the magnetic order crossing the transition temperature. The different degree in electron correlation affecting both FM and AFM systems has been revealed, particularly showing the presence of a localised state in the valence band of metals samples, which existance have to be included in the theories about FM systems. Supported by a dedicated theory these experiments should be able to unravel the nature of the M23V V Auger transition for both FM and AFM systems, but unfortunately, a uni ed theory describing AR-APECS in magnetic systems has not been developed yet, but these experimental results are very encouraging to a theoretical challenge.