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Spin polarized electron transport\ud and\ud the anomalous Hall effect\ud in L10-ordered epitaxial Fe-alloys

By Klaus Michael Seemann

Abstract

Two epitaxial ferromagnets FePd and its isoelectronic partner FePt, which\ud order in the well known face-centered tetragonal L10-phase, have been fabricated\ud to (a) evaluate the current spin polarization in the ballistic and in the\ud diffusive electron transport regime, (b) to elucidate whether there is a discrepancy\ud in the current spin polarizations depending on the transport regime,\ud how high this discrepancy is, and finally (c) how the spin-orbit scattering\ud strength changes by replacing Pd by Pt. These are questions of fundamental\ud importance in the field of electron transport studies in magnetic materials,\ud in the recent years known as spintronics.\ud \ud The study began with experimental investigations on the structural, magnetic,\ud and electron transport properties of a L10-ordered epitaxial iron-platinum\ud (FePt) alloy layer fabricated by magnetron-sputtering on a single-crystal\ud MgO(001) substrate. At all temperatures ranging from 2 K to 258 K, a\ud diffusive spin current polarization of > 80% was found. To study the ballistic\ud transport regime, point-contact Andreev-reflection measurements at 4.2\ud K were performed and a value for the ballistic current spin polarization of\ud ∼ 42% was obtained (which compares very well with that of a polycrystalline\ud thin film of elemental Fe). This discrepancy is attributed to a difference\ud in the characteristic scattering times for oppositely spin-polarized electrons, such scattering times influencing the diffusive but not the ballistic current\ud spin polarization.\ud \ud One of the main parameters in the L10-phase ferromagnets is the degree\ud of crystallographic long range ordering. To investigate the influence of the\ud long range ordering on the current spin-polarization, epitaxial FePd magnetic\ud thin films of varying degrees of L10-phase chemical order were used.\ud By quantitatively accounting for changes in micromagnetism through simulation\ud and magnetic force microscopy, it is shown within the Levy-Zhang\ud spin-mistracking theory of domain wall resistance, that this is connected to a\ud change in the average degree of spin-dependence at the scattering centers that\ud control the diffusive transport in this system, suggesting a possible means of\ud producing highly spin-polarized currents in the diffusive electron transport\ud regime.\ud \ud Finally, both materials L10-FePd and -FePt were investigated with respect to\ud the anomalous Hall effect as a consequence of spin-orbit scattering. A linear\ud dependence between transverse and longitudinal resistivity in epitaxial L10-\ud ordered FePd and FePt films due to the anomalous Hall effect (AHE) has\ud been observed. In ordered ferromagnets such a linear relation may be anticipated\ud due to the skew-scattering mechanism; the absence of an additional\ud quadratic term suggests that side-jump scattering, which is dominant in less\ud well ordered material, has been effectively suppressed. One finds that on\ud exchanging Pd for the heavier, though isoelectronic Pt, the skew-scattering\ud deflection angle in an individual electronic scattering event dramatically increases\ud from 17 to 49 mrad. This increase corresponds well to a Z3/2 dependence\ud of the atomic spin-orbit interaction energy, contrary to commonly assumed Z4 dependence

Publisher: School of Physics and Astronomy (Leeds)
Year: 2009
OAI identifier: oai:etheses.whiterose.ac.uk:115

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