X-Ray Magnetic Circular Dichroism Spectroscopy and Microscopy
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MRS BULLETIN/OCTOBER 1995
which shows a negative spike at the L3 edge and a positive spike at the L2 edge. The information contained in the XMCD spectrum is element-specific since different elements have different core-level binding energies. It also directly gives the local vector magnetization of a
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Photon Energy (eV)
Figure 1. X-ray absorption and x-ray magnetic circular dichroism (XMCD) at the 1-2,3 core-level edges of Ni taken with helicity directions of the photons and that of the majority spin 3d-valence electrons parallel and antiparallel to each other.
sample, and, in combination with sum rules described later, can give information on the orbital and spin contributions to the magnetic moment. The L2,3 absorption edges of the 3d transition metals lie in the soft x-ray energy region at photon energies of approximately 500-900 eV. The ALS is an ultrahigh brightness source that has been optimized for the photon-energy range of 100-1,500 eV, which encompasses these L2,3 edges and also the M4/5 edges of the magnetically interesting rare-earth elements. Circularly polarized soft x-rays from a high brightness synchrotronradiation source, such as the ALS, open up the possibility of the use of XMCD microscopy to measure local magnetic properties of materials. We will address such prospects toward the end of this article. Synchrotron Radiation Synchrotron radiation is produced by the acceleration of a relativistic electron as it is constrained to stay in a circular orbit by a magnetic-dipole field. The spectrum thus created is a continuum extending from the far infrared to the x-ray region. The distribution of light within this range depends on the electron energy and the magnetic-dipole field. For the ALS, with parameters of 1.5 GeV and 1.07 T, respectively, the peak of the distribution is around 500 eV. The relativistic nature of the electron beam also causes the light to be radiated within a small range of angles (the instantaneous opening angle), typically a fraction of a milliradian. When viewed in the plane of the electron orbit, the light is plane-polarized in the orbit plane, but when viewed out of the plane, there is an appreciable circular component. Alternation between left and right helicity is achieved by switching between aboveand below-plane. The third generation of synchrotronradiation sources such as the ALS is specifically designed to optimize the brightness of the radiation. This is accomplished in two ways: first, by reducing the angle and position space occupied by the electron beam (reduction of emittance), and second, by the use of undulator radiation sources. An undulator consists of an array of permanent magnets arranged to give an alternating vertical magnetic field with a period of a few centimeters. These devices are located in a straight section of the storage ring between dipole magnets. As the electron beam passes through such a device, it oscillates in the horizontal plane and radiates. If the angular oscillation is
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X-Ray Magnetic Circular Dichroism Sp
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