First-principles Study Of The Magnetic Properties In Van Der Waals Magnetic Materials And Related Heterostructures

As the building block of spintronic applications,magnetic tunnel junctions（MTJs）have received considerable attention in the past two decades.Recently,two-dimensional（2D）van der Waals（vd W）intrinsic magnets have been successfully used to construct van der Waals magnetic tunnel junctions（vd W MTJs）with a high tunneling magnetoresistance（TMR）ratio.Except for the high TMR ratio,perpendicular magnetic anisotropy（PMA）is also an essential property for MTJs to realize high storage density and low critical switching current density for magnetic moment.The spontaneous polarization of 2D vd W ferroelectric materials can be switched between two opposite polarized states with energy degeneracy by applying an electric field.The vd W multiferroic heterostructures composed of 2D intrinsic magnets and 2D ferroelectrics are promising for achieving the nonvolatile electrical control of magnetism in 2D intrinsic magnets by electrically reversing the polarization direction of the 2D ferroelectrics.Different from the intrinsic magnetic monolayer MA₂Z₄（M=V,Nb,Ta;A=Si,Ge;Z=N,P,As）with only one magnetic M atom per unit cell,there are two magnetic M atoms per unit cell in monolayer M’M₂Z₄,which means that monolayer M’M₂Z₄ can provide higher sensitivity,higher efficiency and higher density for spintronics devices.Moreover,monolayer M’M₂Z₄which contains two magnetic M atoms per unit cell presents a similar structure to spin valves and MTJs,and the giant magnetoresistance（GMR）ratio or TMR ratio can be achieved in monolayer M’M₂Z₄.The main works and results are as follows:1.Using first principles calculations,we investigate the magnetic anisotropy,spin-dependent transport and TMR effect of the Fe₃Ge Te₂/Cr I₃（Sc I₃）/Fe₃Ge Te₂vd W MTJs formed by sandwiching a ferromagnetic（FM）monolayer Cr I₃ or non-magnetic monolayer Sc I₃ barrier between two vd W FM metal Fe₃Ge Te₂ electrode.Our calculations demonstrate that the Fe₃Ge Te₂/Cr I₃/Fe₃Ge Te₂vd W MTJs and the Fe₃Ge Te₂/Sc I₃/Fe₃Ge Te₂vd W MTJs possess a strong PMA.Due to no barrier for majority-spin transmission within half-metallic Cr I₃ barrier and the large difference between majority-and minority-spin conduction channels of the Fe₃Ge Te₂ electrode,a high TMR ratio of about 3100%is achieved in the Fe₃Ge Te₂/Cr I₃/Fe₃Ge Te₂ vd W MTJs.In contrast to the Fe₃Ge Te₂/Cr I₃/Fe₃Ge Te₂ vd W MTJs,the tunneling barrier of non-magnetic Sc I₃ suppresses the majority-spin transmission across Fe₃Ge Te₂/Sc I₃/Fe₃Ge Te₂ vd W MTJs and the suppression for majority-spin transmission in the case of the parallel state of magnetization of two Fe₃Ge Te₂electrodes is stronger than the suppression in the case of the antiparallel state of magnetization of two Fe₃Ge Te₂ electrodes,resulting in a smaller TMR ratio of about1200%in the Fe₃Ge Te₂/Sc I₃/Fe₃Ge Te₂ vd W MTJs.2.Using first principles calculations,we explore the ground state and magnetic structure of monolayer Mn Br₂ as well as the electronic structure and magnetoelectric coupling properties of the In₂Se₃/Mn Br₂/In₂Se₃ vd W multiferroic heterostructure formed by sandwiching monolayer Mn Br₂ between two intrinsically ferroelectric monolayerα-In₂Se₃ in different polarization states.Our calculations show that the ground state of monolayer Mn Br₂ is intrinsically antiferromagnetic（AFM）semiconductor and the magnetic structure of monolayer Mn Br₂is FM stripes of width two atom rows within the layer with AFM coupling between neighboring FM stripes,in which the AFM and FM couplings originate from superexchange interaction mediated by the p_σand p_πorbitals of the intervening Br in Anderson’s mechanism,respectively.More interestingly,the ground state of monolayer Mn Br₂ in the In₂Se₃/Mn Br₂/In₂Se₃ vd W multiferroic heterostructure can be reversibly switched between AFM and FM states by electrically modulating the ferroelectric polarization direction ofα-In₂Se₃ and the In₂Se₃/Mn Br₂/In₂Se₃ vd W multiferroic heterostructures are semiconducting for all polarized configurations.3.Using first principles calculations,we propose a new monolayer intrinsic magnet WV₂N₄ without three-dimensional parent material and investigate the stability,electronic structure and magnetic properties of monolayer WV₂N₄,as well as the influence of biaxial strain and external electric field on the electronic structure and magnetic properties of monolayer WV₂N₄.The calculated binding energy and phonon dispersion indicate that the fabrication of monolayer WV2N4 is feasible in the future and monolayer WV₂N₄ is dynamically stable.Furthermore,monolayer WV₂N₄ is FM metal,and the magnetic moment is mainly contributed by the d orbital of V atom.The intralayer magnetic coupling within the V atomic layer is FM coupling and the interlayer magnetic coupling between the upper and lower V atomic layers is weak FM coupling.The intralayer and interlayer FM couplings originate from the superexchange interaction mediated by the p orbital of the intervening N atom and the Ruderman-Kittel-Kasuya-Yosida interaction mediated by the spin-polarized conducting electrons,respectively.Under the biaxial strain from-3%to 3%,the interlayer magnetic coupling between the upper and lower V atomic layers is switched between FM and AFM due to the change of the distance between the upper and lower V atomic layers.When an external electric field is applied,the electrons of the d orbital of V atom at high potential increase and the corresponding magnetic moment becomes lager,while the electrons of the d orbital of V atom at low potential decrease and the corresponding magnetic moment becomes smaller,thus it is feasible to modulate the magnetic properties of monolayer WV₂N₄by applying electric field.