Magnetic Phase Transitions Probed with Spin-Polarized Photoelectron Diffraction
Scientists have verified unusual surface magnetic behavior in gadolinium (Gd) by studying magnetic phase transitions in a novel way. Their technique, spin-polarized photoelectron diffraction, has shown that surface atoms in Gd lose their long-range magnetic order at 60 to 80 degrees higher temperature than bulk atoms.
Spin-polarized photoelectron diffraction (SPPD) takes advantage of multiplet splitting in Gd. In this phenomenon, spin-up and spin-down photoelectrons ejected from the same core level have different kinetic energies, making it possible to measure their intensities separately. Because spin-up and spin-down photoelectrons scatter differently from neighbor atoms with magnetic moment, the ratio of the two intensities is affected by the magnetic order of the sample. Comparison of the red and green spectral lines above shows that the ratio of spin-up (7S) to spin-down (9S) intensities has decreased with the rise in temperature. This change corresponds to the initial downward trend in the asymmetry plot below, which in turn correlates to a small increase in magnetic order.
To track changes in magnetic order, the researchers defined the asymmetry (A) between spin-up and spin-down intensities as a function of the intensity ratio for a maximum temperature (Tmax), at which they know that no magnetic order remains. As the temperature approaches Tmax, the asymmetry approaches zero, but the change is not uniform. Rapid changes (peaks) occur at Curie temperatures (Tc), where ferromagnetic atoms become paramagnetic (i.e., where the atoms lose their long-range magnetic order). This plot shows the change in asymmetry for 4s electrons ejected from bulk and surface Gd(0001) atoms grown as a thin layer on tungsten (110). The vertical lines mark the average Curie temperatures found for both 4s and 5s electrons. The surface Curie temperature averaged 60 to 80 degrees K higher than the bulk Curie temperature. The small nitrogen peak (N 1s) is due to a low-level impurity (about 1% of an atomic layer).
Experiment performed by C.S. Fadley (principal investigator), E.D. Tober, R.X. Ynzunza, and Z. Wang (University of California Davis and Berkeley Lab); F.J. Palomares (Instituto de Ciencia de Materiales de Madrid); and Z. Hussain (Berkeley Lab) using the advanced photoelectron spectrometer/diffractometer at Beamline 9.3.2.
Funding: U.S. Office of Naval Research and Office of Basic Energy Sciences of the U.S. Department of Energy.
Publications about this experiment:
C.S. Fadley et al., Progress in Surface Science, 54, 341-386
(1997).
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