Spin polarization of Co2FeSi full-Heusler alloy and tunneling magnetoresistance of its magnetic tunneling junctions

The authors report spin polarization (P) and tunneling magnetoresistance (TMR) of epitaxially grown Co2FeSi thin films on a MgO (001) substrate. A Heusler-type L2(1) structure was observed in the samples sputter deposited at 473 K or above. The P value of the ordered film was measured as 0.49 +/- 0.02 by the point contact Andreev reflection (PCAR) technique. The TMR values obtained from the magnetic tunneling junction (MTJ) using the Co2FeSi electrode and Al-oxide barrier were 67.5% at 5 K and 43.6% at 298 K, respectively. The P value estimated from the TMR using Julliere's model matches the spin polarization measured by the PCAR very well, indicating that the TMR value from the MTJ is governed by the intrinsic value of P of the electrode material for incoherent tunneling.