The Study Of Static Magnetic Properties Of Half-metallic Co₂FeAl Ultra-thin Films Grown By MBE

Due to its theoretically 100%spin polarization around the Fermi lever at room temperature,low damping parameters and high Curie temperature,half-metallic Heusler alloy Co₂FeAl has shown a remarkable potential application in spintronics devices and has drawn great attention.In this work,high quality single-crystal Co₂FeAl ultra-thin films have been successfully epitaxially grown on GaAs（001）using Molecular Beam Epitaxy（MBE）,and the thickness dependent spin polarization and magnetic anisotropy of the Co₂FeAl films are further studied.Electron beam and K-cell evaporators were used to evaporate Co,Fe and Al.Evaporation rates were adjusted to keep the ratio of atoms of Co,Fe and Al as 2:1:1.By employing Real-time in-situ Reflection High-Energy Electron Diffraction（RHEED）to monitor the epitaxy process of the films,a suitable growth temperature of the substrates were studied.The thickness of films was controlled by growth duration.Ultra-thin films with thickness below 12 nm have been grown with the substrate temperature 250℃ and 300℃,of which the thickness dependent spin polarization and magnetic anisotropy have been measured using in-situ Spin Angle-Resolved Photoelectron Spectroscopy（SARPES）and in-situ Magnetic Optical Kerr Effect（MOKE）setup.Furthermore,X-ray Diffraction（XRD）and Atomic Force Microscopy（AFM）were employed to characterize the atomic order and surface smoothness of the samples.The results are as follows:（1）Single-crystal Co₂FeAl ultra-thin films have been successfully epitaxially grown on GaAs（001）and the suitable growth temperature is from 200 ℃-300℃.In-situ RHEED patterns suggest the growth mode was Layer plus Island Mode.The epitaxial relation is Co₂FeAl[110]||GaAs[110]and Co₂FeAl（001）||GaAs（001）.（2）The samples are of high quality,postense（002）peak and（004）peak,implying the part ordered B2 phase.Up to 58%sessing high spin polarization.XRD shows（±7%）spin polarization have been measured by SARPES.（3）Spin polarization of samples grown at 250℃,gradually declines from 58%（±7%）to 46%(±5%with the thickness decreasing from 21 uc（unit cell）to 6uc,and further decreases to 29%（±2%）with the film thickness reaches 2.5uc.（4）Samples grown at 250℃and 300℃ both show a strong uniaxial magnetic anisotropy,but unusual inverted magnetic hysteresis loops are observed in the latter samples,which is due to the magnetic anisotropy energy basin from[110]to[100]and the asymmetric high barrier around[110]direction.The preferable stacking of Co on GaAs along[110]with Co-Ga bonding may be responsible for this barrier.