Theoretical Study Of Spin-polarized Transport Properties Of Magnetic Molecular Junctions And Magnetic Tunnel Junctions

Spintronic devices enable more efficient information storage,transfer and processing by exploiting the electron’s spin degree of freedom.The key scientific issues in the rapidly developing field of spintronics are spin injection,spin-polarized transport,spin orientation detection and regulation.Recently,the research activities in spintronics focused on giant magnetoresistance effect,tunneling magnetoresistance effect,spin transfer torque effect and so on.Among them,the magnetic tunnel junctions based on the tunneling magnetoresistance effect are widely used in spintronic devices.Combining molecular electronics with spintronics,molecular spintronics emerges in recent years.Based on single molecule,various functional spintronic devices with low energy consumption have been successfully demonstrated via bottom-up methodology.In this doctoral dissertation,by performing extensive density functional theory calculations combining with non-equilibrium Green’s function technique,we explore the spin-dependent transport properties of molecular junction based on a mononuclear spin crossover(SCO)Fe(Ⅱ)complex.We observe molecular switching and spin-filtering effects via applying an external electric field.In addition,nearly perfect thermal spin filtration and negative differential thermal resistance are observed after applying temperature gradients.We propose and realize a new class of molecular materials named bipolar magnetic molecules(BMMs).In BMMs based molecular junction,a 100%spin polarized current with switchable spin direction is achievable by electrical gating.We also design spin thermoelectronic devices based on BMMs,in which the thermal spin filtering and pure spin current are observed by tuning the applied voltage gate.Finally,we construct two sandwich structure magnetic tunnel junctions,both of them exhibit giant tunneling magnetoresistance.The main research activities in this doctoral dissertation are summarized as follows:(1)Due to the magnetic bistability between the low-spin(LS)and high-spin(HS)states,switched by diverse external stimuli,such as temperature,pressure,light,and electric field,spin crossover(SCO)complexes have become one of the most promising candidates for designing molecular spintronic devices.Here,we explore the spindependent transport properties of molecular junction based on a mononuclear SCO Fe(Ⅱ)complex by applying electric bias voltage or temperature gradient.Under small bias voltage,our results clearly reveal that the current through the molecular junction with the HS state is significantly larger than that of the LS state,resulting in a molecular switching effect in the examined junction.As for the molecular junction with the HS state,the current is dominated by the spin-down electrons,leading to a robust spinfiltering effect,which is not sensitive to the contact structures.Moreover,according to the calculated spin-dependent thermal currents through the molecular junction with the HS state by applying temperature gradient,two interesting spin-caloritronic properties including nearly perfect thermal spin filtration and negative differential thermal resistance are observed.These theoretical findings suggest that the examined SCO Fe(Ⅱ)complex holds potential application in molecular spintronics and spin caloritronics.(2)Electrical control of spin transport at single molecule level is highly desired for molecular spintronics.Here,we propose and realize a new class of molecular materials named bipolar magnetic molecules(BMMs),which possess unique energy level arrangement,i.e.the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)come from two different spin channels.When BMMs are used to assemble single molecule devices with non-magnetic electrodes,a 100%spin-polarized current can be generated by reversibly adjusting the position of device Fermi level approaching either HOMO or LUMO via changing the polarity of the applied voltage gate.Through rational design and careful screening,we finally find nine potential BMMs,and the corresponding calculated electronic structures and transport properties confirm the feasibility of the BMMs conception.(3)To explore the possibility of BMMs for application in spin caloritronics,based on two screened Vphen2NCS2(phen=1,10-Phenanthroline)and Vbtz2NCS2(btz =2,2’-Bi-4,5-dihydrothiazine)in Chapter 4,we propose two spin thermoelectronic devices.Theoretical calculation results show that perfect thermal spin filtration and ideal spin Seebeck effect(SSE)are achievable in the molecular junctions by applied temperature gradients under different gate voltages,originating from the special energy level arrangement of the BMMs,i.e.HOMO and LUMO are mainly contributed by two different spin channels.When the device Fermi level is adjusted to a position between HOMO and LUMO by the applied gate voltage,pure spin current is observed through the molecular junction.When the device Fermi level is adjusted approaching either HOMO or LUMO via changing gate voltage,the thermal current dominated by the spin-down or the spin-down electrons is obtained.This work provides an instructive theoretical strategy for tuning spin thermoelectric properties at single-molecule level in experiments.(4)Since magnetic tunnel junction with a large tunneling magnetoresistance has attracted great attention due to its importance in the spintronics applications,Here,we propose two periodic magnetic tunnel junctions(MTJ)with sandwich structure.A nonpolar SrTiO3 barrier layer is sandwiched between two Heusler alloy Co2MnSi electrodes in the first MTJ.Theoretical results clearly reveal that the transmission coefficient of the magnetic tunnel junction in parallel magnetization configuration(PC)at the Fermi level is several orders of magnitude larger than that in the antiparallel magnetization configuration(APC),resulting in a huge tunneling magnetoresistance up to 106,which originates from the coherent spin-polarized tunneling,due to the half-metallic nature of Co2MnSi electrodes and the significant spin polarization of the interfacial Ti-3d orbital.The second MTJ is designed based on two-dimensional ferromagnetic Fe3GeTe2,in which Fe3GeTe2/h-BN/Fe3GeTe2 van der Waals(vdW)heterostructure is sandwiched between two Au electrodes.At zero bias voltage,the calculated value of the tunneling magnetoresistance ratio is about 240%,which is slightly larger than the experimental value.Interestingly,based on different magnetization states,simple logic devices can be designed with the second MTJ.Clearly,the above first-principles studies provide helpful theoretical insight for design and control the spin-polarized transport performance in molecular spintronics,spin caloritronics,and spintronics in future experiments.