Magnetization Distribution And Electronic Transport Of Several Systems With Noncollinear Magnetism

In recent decades,tremendous changes have been seen in spintronic fields especially since the giant magnetoresistance effect and spin transfer torque effect have been discovered.As a constituent element of spintronic device,ferromagnetic or antiferromagnet is essential to affect the properties of spintronic devices.In ferromagnetic or antiferromagnet,all magnetic moments are aligned along a line,which can be regarded as “collinear magnetism”.By tuning the magnetization of two ferromagnets and setting them as parallel and antiparallel,we can obtain both “off” and “on” states.Currently,the giant magnetoresistance,tunneling resistance and spin transfer torque effects have been extensively studied and have been predicted to be of wide application in the future.Besides collinear magnetism,noncollinear magnetism,i.e.the magnetic moments of atoms point to any arbitrary direction,occurs in some condition.Such noncollinear magnetism may bring new effect,such as magnetization frustration.Especially in magnetic tunnel junction,due to the fabrication condition,a transition metal oxide can form in the ferromagnet/insulator interface and its electronic transport properties have not been investigated.In order to shed light on the effect of noncollinear magnetism on magnetization distribution and electronic transport,we studied magnetization,transmission,current and tunnel magnetoresistance of several systems with noncollinear magnetism,e.g.graphene nanoribbon and magnetic tunnel junction,and analyzed the underlying physical mechanisms.This thesis is organized as follows:(1)In Charpter 1,we reviewed the history and current research status of spintronics,and the difference between collinear and noncollinear magnetism.(2)In Charpter 2,we introduce the method used in our work,i.e.the density functional combined with the nonequilibrium Green’s function method,in which the numerical atomic orbitals method are presented.Especially,the Hamiltonian under the noncollinear magnetism and the difference between collinear and noncollinear method are discussed.We then give some simple examples,i.e.gold atomic chain,topological insulator thin film and nickel atomic chain and presented the results of magnetization and electronic transport.(3)In Charpter 3,we discussed the magnetization and electronic transport of domain wall based on the graphene nanoribbon.We do this work because current research is focused on the collinear case and the study of noncollinear magnetism in electronic transport will be an interesting question.In this work,we consider the domain wall in which the magnetization of two graphene nanoribbon leads are made an angle.By fixing the lead magnetizations,the electronic structure in the central region is obtained in the self-consistency convergence.After the convergence is obtained,the trnasmission is calculated and the relation between the transmission and the angle in magnetization are discussed.The results show that the magnetization depends on the initial magnetization distribution of the central region while the transmission depends on the energy and the angle,which is consistent with the explanation provided by A.N.Chantis.(4)In Charpter 4,in order to tune electronic transport of graphene nanoribbon,we consider the magnetic tunnel junction made by the graphene/BN/graphene,which has been synthesized recently.In the work,we consider different tunnel junction with various C/BN interfaces and discuss the correlation between magnetization and interface structure and analyze the physical mechanism behind it.Moreover,we studied the effect of interface structure and barrier height on the transmission.Besides collinear case,we also studied the C/BN/C tunnel junction in the noncollinear magnetism and obtained the noncollinear magnetization and current.(5)In Charpter 5,motivated by recent magnetoresistance measurement by H.Yang in which the negative tunnel magnetoresistance was obtained in Fe/Mg O/Ni O/Fe tunnel junction,we studied the transmission,current and tunnel magnetoresistance of Fe/Mg O/Fe、Fe/Ni O/Fe、Fe/Mg O/Ni O/Fe tunnel junction.The analysis is focused on the electron tunneling in tunnel junction with various thickness and two dominating factors,i.e.spin-mixing effect and interface state.The comparison between experimental and theoretical results is given.(6)In Charpter 6,the conclusion and perspective of our work are presented.