Diffusive and ballistic current spin polarization in magnetron-sputtered <i>L</i>1₀-ordered epitaxial FePt

We report on the structural, magnetic, and electron transport properties of an L10-ordered epitaxial iron-platinum alloy layer fabricated by magnetron sputtering on a MgO (001) substrate. The film studied displayed a long-range chemical order parameter of S~0.90, and hence has a very strong perpendicular magnetic anisotropy. In the diffusive electron transport regime, for temperatures ranging from 2 to 258 K, we found hysteresis in the magnetoresistance mainly due to electron scattering from magnetic domain walls. At 2 K, we observed an overall domain wall magnetoresistance of about 0.5%. By evaluating the spin current asymmetry &#945;=&#963;&#8593;/&#963;&#8595;, we were able to estimate the diffusive spin current polarization. At all temperatures ranging from 2 to 258 K, we found a diffusive spin current polarization of greater than 80%. To study the ballistic transport regime, we have performed point-contact Andreev-reflection measurements at 4.2 K. We obtained a value for the ballistic current spin polarization of ~42% (which compares very well with that of a polycrystalline thin film of elemental Fe). We attribute the discrepancy 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