Molecular Beam Epitaxy And Physical Properties Of Magnetic Thin Films

Spintronics is one of the new fields with potential for practical applications,after the discovery of giant magnetoresistance(GMR)and tunneling magnetoresistance(TMR)effect.Spintronic devices rely on thin layers of magnetic materials,for they are designed to control both the charge and the spin current of the electrons.Half-metallic ferromagnets(HMFs)have only one spin channel for conduction at the Fermi level,while they have a band gap in the other spin channel,which is significant for spin injection,spin filtering and spin transfer torque devices.Moreover,the realization of spin transport at the interface of semiconductor and magnetic materials is another key building block for spintronic devices.However,lower magnetization of conventional ferromagnetic(FM)ultra-thin films hinders their applications in spintronics.Most recently,the discovery of intrinsic ferromagnetism in two-dimensional(2D)materials has generated increasing interests.And the substrate-induced effects of 2D materials are limited for the van der Waals(vdW)force.Molecular beam epitaxy(MBE)growth is particularly significant as it provides the opportunity to obtain nominally stoichiometric single-crystalline films,explore the role of physical dimensionality as well as fabricate heterostructures and superlattices in a way compatible with conventional microelectronics techniques.In this article,I have investigated the magnetic performance and magnetic modulation of HMFs and 2D FM thin films grown by MBE with the main results as follows:(1)Direct observation of high spin polarization in Co2FeAl thin films.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.This suggests that a bulk magnetization is achieved for Co2FeAl thin films with a thickness of at least 4 uc.(2)The atomic-scale magnetism at Co2FeAl/GaAs interface.The atomic-scale magnetism of Co2FeAl Heusler alloys has long been an outstanding question,and with the thickness down to the nanometer scale,this becomes even more sophisticated.Here,we report a direct measurement of the Co2FeAl epitaxial thin films on the GaAs(001)substrate with the in-situ MOKE and the synchrotron-based X-ray magnetic circular dichroism(XMCD)techniques.Strong uniaxial magnetic anisotropy has been observed from all thicknesses of the Co2FeAl 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 occurs.This anti-parallel spin component can be responsible for the significantly reduced magnetic moment and the low spin polarization near the Fermi level of the Co2FeAl.(3)Room-temperature intrinsic ferromagnetism of epitaxially grown CrTe2 monolayer films.We report the growth and properties of single-and few-layers CrTe2,a van der Waals(vdW)material,on bilayer graphene by MBE.Intrinsic 2D ferromagnetism with a Curie temperature,TC,up to 300 K,an atomic magnetic moment of?0.21?B/Cr and perpendicular magnetic anisotropy(PMA)Ku of 4.89×105 erg/cm3 in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device(SQUID)and XMCD.This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with?0.2 eV at ? point using angle-resolved photoemission spectroscopy.Remarkably,the FM order is preserved with the film thickness down to 3 layers,benefiting from the strong PMA and weak interlayer coupling.(4)Large topological Hall Effect(THE)in vdW CrTe2/Bi2Te3 bilayers.To realize the non-coplanar spin textures with scalar spin chirality,apart from the broken time-reversal symmetry,a significant Dzyaloshinskii-Moriya interaction(DMI)is required.Here,the 2D ferromagnets and topological insulators hybrid system,CrTe2/Bi2Te3,with an atomically sharp interface was fabricated.And its Hall-transport response was studied.Remarkably,hump and dip features superposing on top of anomalous Hall loops were observed,which has been identified as arising from the topologically non-trivial chiral spin texture stabilized by DMI.The temperature and magnetic field dependence of THE signals were mapped and attributed to the Neel type skyrmions.