Fermi level tuning has been successfully demonstrated in Co-based full-Heusler alloy Co(2)FeAl(0.5)Si(0.5) (CFAS). The half-metallic band gap of CFAS was proved by the behavior of differential conductance of CFAS/(MgAl(2))O(x)/CoFe magnetic tunneling junctions with an unexplored crystalline (MgAl(2))O(x) barrier. CFAS exhibits the highest effective spin polarization (P_{eff}) at 300 K and the weakest temperature dependence of P_{eff} among all known half metals. Further study shows that P_{eff} of CFAS decays with increasing temperature (T) following T;{3/2} law perfectly, which indicates that the depolarization of CFAS is determined by spin wave excitation only.

Magnetoresistance ratio up to 330% at room temperature (700% at 10 K) has been obtained in a spin-valve-type magnetic tunnel junction (MTJ) consisting of a full-Heusler alloy Co2FeAl electrode and a MgO tunnel barrier fabricated on a single crystal MgO (001) substrate by sputtering method. The output voltage of the MTJ at one-half of the zero-bias value was found to be as high as 425 mV, which is the largest reported to date in MTJs using Heusler alloy electrodes. The present finding suggests that Co2FeAl may be one of the most promising candidates for future spintronics devices applications.

We report the investigation of structure and magnetic properties of full-Heusler alloy Co2FeAl0.5Si0.5 (CFAS) thin films grown on MgO-buffered MgO (001) substrates through magnetron sputtering. It was found that single-crystal CFAS thin films with high degree of L21 ordering and sufficiently flat surface could be obtained after postdeposition annealing. All the films show a distinct uniaxial magnetic anisotropy with the easy axis of magnetization along the in-plane [110] direction. These results indicate that the use of the MgO buffer for CFAS is a promising approach for achieving a higher tunnel magnetoresistance ratio, and thus for spintronics device applications.

Spin-dependent tunneling spectroscopy has been studied in fully epitaxial magnetic tunnel junctions with full-Heusler Co 2 FeAl 0.5 Si 0.5 ͑CFAS͒ alloys. We fabricated CFAS/MgO/CFAS structures with L2 1 -and B2-ordered CFAS layers and measured the bias voltage dependence of differential conductance G. We found for L2 1 -CFAS/ MgO/ L2 1 -CFAS structure symmetrical conductance curves with respect to polarity of the bias voltage for parallel ͑P͒ and antiparallel ͑AP͒ magnetization configurations and two characteristic crossovers in G between P and AP accompanied with a flat feature within Ϯ0.6 V in G ͑P͒. On the other hand, only one crossover was observed at a negative-bias voltage for L2 1 -CFAS/ MgO/ B2-CFAS structure. The direct tunneling that reflects the specific spin-dependent density of states of the half-metallic L2 1 -CFAS is proposed as a possible transport mechanism leading to the notable crossovers.

Magnetic tunnel junctions (MTJs) using polycrystalline Co2FeAl0.5Si0.5 (CFAS) electrodes with a MgO tunnel barrier were fabricated onto thermally oxidized Si substrates. Highly (001)-oriented and B2-ordered CFAS electrodes were obtained by optimizing growth conditions and postannealing temperature. The microfabricated MTJs exhibited relatively high tunnel magnetoresistance (TMR) ratios of 125% at room temperature and 196% at 7K. The large TMR obtained using oxidized Si substrates indicates that CFAS is promising for the practical applications.