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

Two epitaxial ferromagnets FePd and its isoelectronic partner FePt, which
order in the well known face-centered tetragonal L10-phase, have been fabricated
to (a) evaluate the current spin polarization in the ballistic and in the
diffusive electron transport regime, (b) to elucidate whether there is a discrepancy
in the current spin polarizations depending on the transport regime,
how high this discrepancy is, and finally (c) how the spin-orbit scattering
strength changes by replacing Pd by Pt. These are questions of fundamental
importance in the field of electron transport studies in magnetic materials,
in the recent years known as spintronics.

The study began with experimental investigations on the structural, magnetic,
and electron transport properties of a L10-ordered epitaxial iron-platinum
(FePt) alloy layer fabricated by magnetron-sputtering on a single-crystal
MgO(001) substrate. At all temperatures ranging from 2 K to 258 K, a
diffusive spin current polarization of > 80% was found. To study the ballistic
transport regime, point-contact Andreev-reflection measurements at 4.2
K were performed and a value for the ballistic current spin polarization of
∼ 42% was obtained (which compares very well with that of a polycrystalline
thin film of elemental Fe). This discrepancy is attributed to a difference
in the characteristic scattering times for oppositely spin-polarized electrons, such scattering times influencing the diffusive but not the ballistic current
spin polarization.

One of the main parameters in the L10-phase ferromagnets is the degree
of crystallographic long range ordering. To investigate the influence of the
long range ordering on the current spin-polarization, epitaxial FePd magnetic
thin films of varying degrees of L10-phase chemical order were used.
By quantitatively accounting for changes in micromagnetism through simulation
and magnetic force microscopy, it is shown within the Levy-Zhang
spin-mistracking theory of domain wall resistance, that this is connected to a
change in the average degree of spin-dependence at the scattering centers that
control the diffusive transport in this system, suggesting a possible means of
producing highly spin-polarized currents in the diffusive electron transport
regime.

Finally, both materials L10-FePd and -FePt were investigated with respect to
the anomalous Hall effect as a consequence of spin-orbit scattering. A linear
dependence between transverse and longitudinal resistivity in epitaxial L10-
ordered FePd and FePt films due to the anomalous Hall effect (AHE) has
been observed. In ordered ferromagnets such a linear relation may be anticipated
due to the skew-scattering mechanism; the absence of an additional
quadratic term suggests that side-jump scattering, which is dominant in less
well ordered material, has been effectively suppressed. One finds that on
exchanging Pd for the heavier, though isoelectronic Pt, the skew-scattering
deflection angle in an individual electronic scattering event dramatically increases
from 17 to 49 mrad. This increase corresponds well to a Z3/2 dependence
of the atomic spin-orbit interaction energy, contrary to commonly assumed Z4 dependence.