Using the deposition technique we developed for double perovskite film growth, we have deposited of Co2FeAlxSi1-x Epitaxial films with pronounced Laue oscillations in x-ray diffraction and rocking curves as narrow as 0.0043°, demonstrating the state-of-the-art crystalline quality of the Heusler films.
XRD pole figures of (a) Co2FeAl0.5Si0.5 (022), (b) Co2FeAl0.5Si0.5 (111), (c) MgAl2O4 (111) peaks, and Phi scans of (d) Co2FeAl0.5Si0.5 (022) peaks and (e) Co2FeAl0.5Si0.5 (111) peaks. (f) AFM image of a Co2FeAl0.5Si0.5 film shows an rms roughness of 0.12 nm.
The Co2FeAl0.5Si0.5 films exhibit clear in-plane magnetocrystalline anisotropy as shown below in the hysteresis loops. This can be understood as the combination of a cubic magnetocrystalline anisotropy and an epitaxy-induced uniaxial anisotropy. For the cubic anisotropy, the easy axes are along the two equivalent in-plane Co2FeAl0.5Si0.5 [110] directions and the hard axes are along the two [100] directions. For the uniaxial anisotropy, the easy axis is along <110> and the hard axis is along <-110 >. Consequently, Co2FeAl0.5Si0.5<110> is the easiest axis, <-110 > is the second easy axis, and the hard axis is in-between the <110> and <-110 > axes, resulting in multistep switching in hysteresis loops near the second easy axis (<-110 >). The coercivity along the easiest axis is Hc = 8.5 Oe, which is among the smallest in Heusler films. The very small coercivity and abrupt magnetization reversal (~1 Oe) imply low density of defects which typically act as pinning sites during magnetic switching.
Magnetic hysteresis loops of a 45-nm Co2FeAl0.5Si0.5 film with an in-plane H applied at (a) 0, (b) 35, (c) 55, and (d) 90 degrees from the Co2FeAl0.5Si0.5 axis.
Reference:
B. Peters, A. Alfonsov, C. G. F. Blum, S. J. Hageman, P. M. Woodward, S. Wurmehl, B. Büchner, and F. Y. Yang, “Epitaxial films of Heusler compound Co2FeAl0.5Si0.5 with high crystalline quality grown by off-axis sputtering, Appl. Phys. Lett. 103, 162404 (2013).