We have studied the Co2FeAl thin films with different thicknesses epitaxially grown on GaAs (001) by molecular beam epitaxy. The magnetic properties and spin polarization of the films were investigated by in-situ magneto-optic Kerr effect (MOKE) measurement and spin-resolved angle-resolved photoemission spectroscopy (spin-ARPES) at 300 K, respectively. High spin polarization of 58% (±7%) was observed for the film with thickness of 21 unit cells (uc), for the first time. However, when the thickness decreases to 2.5 uc, the spin polarization falls to 29% (±2%) only. This change is also accompanied by a magnetic transition at 4 uc characterized by the MOKE intensity. Above it, the film’s magnetization reaches the bulk value of 1000 emu/cm3. Our findings set a lower limit on the thickness of Co2FeAl films, which possesses both high spin polarization and large magnetization.
The atomic-scale magnetism of Co 2 FeAl Heusler Alloy has long been an outstanding question, and with the thickness down to nanometer scale, this become even more sophisticated. Here we report a direct measurement of the Co 2 FeAl epitaxial thin films on the GaAs(001) substrate with in-situ magneto-optic Kerr effect (MOKE) and the synchrotron-based X-ray magnetic circular dichroism (XMCD) techniques. Strong uniaxial magnetic anisotropy (UMA) have been observed from all thicknesses of the Co 2 FeAl thin films between 3 unit cells (uc) and 20 uc. A critical thickness of 3 uc has been identified, below which an anti-parallel spin component of the Co atoms occur. This antiparallel spin component can be responsible for the significantly reduced magnetic moment and the low spin-polarization near the Fermi level of the Co 2 FeAl.
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