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The tunnel magnetoresistance (TMR) effect has been investigated for magnetic tunnel junctions with epitaxial Co2FeAl0.5Si0.5 Heusler electrodes with B2 and L21 structures on a Cr-bufferd MgO substrate. The epitaxially grown Co2FeAl0.5Si0.5 has B2 structure when annealed below 400°C, and has L21 structure for annealing above 450°C. The TMR ratio of 76% at room temperature and 106% at 5K were obtained for a MgO(001)∕Cr∕B2-type Co2FeAl0.5Si0.5∕Al oxide/Co75Fe25∕IrMn∕Ta. The TMR ratio is larger than that of magnetic tunnel junction with an L21-type electrode, which may be due to the smoother surface of the B2 structure and disordered L21 structure due to the Cr atom interdiffusion.

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Long-period structures with 32-and 36-fold periodicity to the original unit lattice distance were observed in a perovskite manganese oxide Bi 0.2 Ca 0.8 MnO 3 at around 130 K, in addition to the one with fourfold periodicity. Quantitative electron diffraction profiles were measured for the long-period structure, where the intensity of superlattice reflections was observed to increase with increasing scattering angle due to static strain. This result gave a reasonable explanation for occurrence of the charge ordering of Mn 3ϩ and Mn 4ϩ ions, which was responsible for the creation of the long-period structures in

We have successfully grown both L21 polycrystalline Co2CrAl and epitaxial L21-structured Co2FeAl films onto GaAs(001) substrates under an optimized condition. Both structural and magnetic analyses reveal the detailed growth mechanism of the alloys, and suggest that the Co2CrAl film contains atomically disordered phases, which decreases the magnetic moment per f.u., while the Co2FeAl film satisfies the generalized Slater–Pauling behavior. By using these films, magnetic tunnel junctions (MTJs) have been fabricated, showing 2% tunnel magnetoresistance (TMR) for the Co2CrAl MTJ at 5K and 9% for the Co2FeAl MTJ at room temperature (RT). Even though the TMR ratio still needs to be improved for future device applications, these results explicitly include that the Co2(Cr,Fe)Al full Heusler alloy is a promising compound to achieve half-metallicity at RT by controlling both disorder and surface structures in the atomic level by manipulating the Fe concentration.