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.