Half-metallicity in highly L2₁-ordered CoFeCrAl thin films

The structural, magnetic, and electron-transport properties of Heusler-ordered CoFeCrAl thin films are investigated experimentally and theoretically. The films, sputtered onto MgO and having thicknesses of about 100 nm, exhibit virtually perfect single-crystalline epitaxy and a high degree of L2₁ chemical order. X-ray diffraction and transmission-electron microscopy show that the structure of the films is essentially of the L2₁ Heusler type. The films are ferrimagnetic, with a Curie temperature of about 390 K, and a net moment of 2 μ_B per formula unit. The room temperature resistivity is 175 μΩ cm; the carrier concentration and mobility determined from the low temperature (5 K) measurement are 1.2 × 10¹⁸ cm^-3 and 33 cm²/V s, respectively. In contrast to the well-investigated Heusler alloys such as Co₂(Cr_1-xFe_x)Al, the CoFeCrAl system exhibits two main types of weak residual A2 disorder, namely, Co-Cr disorder and Fe-Cr disorder, the latter conserving half-metallicity. Point-contact Andreev reflection yields a lower bound for the spin polarization, 68% at 1.85 K, but our structural and magnetization analyses suggest that the spin polarization at the Fermi level is probably higher than 90%. The high resistivity, spin polarization, and Curie temperature are encouraging in the context of spin electronics.