Magnetic Circular Dichroism Instrumentation at LNLS

Abstract

Magnetic Circular Dichroism means a dependence on the magnetic field applied to the sample, of its optical absorption coefficient for circularly polarized light. The effect can be seen comparing the absorption spectra with magnetic field B aligned parallel and antiparallel with the photon propagation vector k [1]. The fundamental physical explanation of this phenomenon invokes several quantum-mechanical principles such as electronic transitions between an occupied core state and an empty valence state in the presence of a magnetic field B, angular momenta of the electronic states and of the circularly polarized photons, conservation of energy and angular momentum and spin-orbit interaction. The detailed theory [2-4] is rather complex, but the final result is that, from the spectra gathered with a given choice of circular polarization but opposite directions of the magnetic field B, one can extract the separate contributions to the atomic magnetic moment, due to the intrinsic and orbital angular momenta of the electrons responsible for the light absorption process. Such quantities can be calculated directly from “first-principle” models by making hypotheses regarding the electronic quantum states. They depend critically on the chemical state and neighborhood of the atoms contributing to the absorption. It is recalled that the magnetic (and often the transport) properties of materials containing transition metal and or rare earth atoms are related to the incomplete 3d (in the transition metals) or 4f (in the rare earths) electronic shell. To study optical absorption involving these electrons, one needs photon energies in the range from 400 to 1500 eV, which are available only at synchrotron light sources. From a practical point of view, MCD is a powerful method for basic research in atomic and solid state physics. For instance, metal-insulator transitions related to the Ni 3d states in RNiO3 perovskites [5] have been investigated at LNLS using optical absorption. It is known that Ni perovskites exhibit a wealth of magnetic phase-transitions as a function of temperature [6] which could also be investigated by MCD in the same LNLS beamline, with the equipment described below. The experiment would be almost identical to [5] and perhaps allow for further elucidation of these complex materials. The instrumentation needed for MCD measurements in most standard magnetic materials includes therefore a tunable light source in the soft X-ray spectral range (“soft X-ray beamline” in a synchrotron facility), equipment to measure optical absorption, a device to apply a magnetic field B to the sample and a cooling/heating system if one wants to change the sample temperature. Since light in this spectral range is strongly absorbed by all solid materials, there can be no windows between the synchrotron source and the sample. The whole experiment has to be enclosed in the same ultra high vacuum needed for the operation of the synchrotron source. At the Brazilian synchrotron source LNLS [7] there are two beamlines [8,9] available for such experiments, capable of delivering circularly polarized light, and a third one is under construction. In section II we describe the MCD equipment without further discussion of the light source. In section III we show and discuss illustrative MCD spectra. In section IV we list conclusions and perspectives.

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Cite this paper

@inproceedings{Fonseca2007MagneticCD, title={Magnetic Circular Dichroism Instrumentation at LNLS}, author={Pamela Fonseca and A. R. B. de Castro and Giancarlo Tosin and James F. Citadini and R. Bası́lio}, year={2007} }