BL24-CIRCE (PEEM endstation)
THE SURFACE MAGNETIC MOMENT OF MAGNETITE (001) Spin and orbital magnetic moment of reconstructed √2×√2R45 magnetite(001). Phys. Rev. B (Rapid Comm) 91 020408(R). (2015) Laura Martín-García1, Raquel Gargallo-Caballero1, Matteo Monti1, Michael Foerster2, José F. Marco1, Lucía Aballe2, and Juan de la Figuera1* Magnetite, an iron oxide with formula Fe3O4, is the oldest (and namegiving) magnetic material known to mankind. However, still today its surface magnetic properties are not fully known and understood. On one hand, the surface and interface magnetic moments may differ from the volume values. On the other hand, the surface and interface properties are key for using materials in modern spintronic and nanotechnology with steadily increasing surface to volume ratio, for instance in spin valves which are used as magnetic sensors in magnetic data storage like hard disk drives. A collaboration between researchers from the Instituto de Química Física “Rocasolano” and the ALBA Synchrotron, employed the high surface sensitivity and multi-technique capabilities of the CIRCE low-energy electron and photoelectron microscope, to characterize the magnetic moment at the magnetite surface and correlate it with the surface reconstruction, i.e. the specific local atomic structure of the surface. The (001) surface of a magnetite single crystal is studied by an approach combining photoemission electron microscopy with X-ray photoelectrons with in situ preparation monitored by low-energy electron microscopy and diffraction. The crystal surface is prepared by repeated cycles of sputtering and annealing in UHV with additional annealing in an 10−6 mbar oxygen atmosphere until a high quality low-energy electron diffraction pattern is observed, indicating that the surface is √2×√2R45° reconstructed1. Imaged with low energy electron microscopy, the surface of the crystal shows then typical square “mesas”, structures of 1–2 μm width, aligned along the <110> in-plane directions as shown in Figure 10 (a). In dark field imaging mode, two equivalent surface terminations but rotated 90º between them, are shown to cover the whole surface area (Figure 10 (b)).
AFFILIATION 1 Instituto de Química Física “Rocasolano” (CSIC), Madrid, Spain 2 ALBA Synchrotron, Cerdanyola del Vallès, Barcelona, Spain
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ALBA ACTIVITY Report
Illuminating the sample with polarized X-rays tuned to the iron absorption L edge and imaging the spatial distribution of the secondary electrons emitted by the X-ray absorption process, the magnetic domains are detected in complementary ways. While circular dichroism (XMCD, Figure 10(c)) is sensitive to the magnetic component along the X-ray beam direction, linear dichroism (XMLD Figure 10(d)) is sensitive to the magnetic component along the electric field direction. Combining these modes under different azimuthal sample rotation angles, the orientation of the magnetization was confirmed to be fully in the surface plane, along two inplane easy axis directions of the <110> type. These easy axes are different from the volume <111> type, but represent the projections of the volume easy axes onto the surface.