NM/FI/NI/FI/NM新型双自旋过滤隧道结的隧穿电导和隧穿磁电阻

在NM/FI/FI/NM型双自旋过滤隧道结(此处NM为非磁金属层，FI为铁磁绝缘体或半导体层)的基础上，我们提出一种NM/FI/NI/FI/NM新型双自旋过滤隧道结(此处NI表示非磁绝缘体或半导体层). 插入NI层的目的是为了避免原双自旋过滤隧道结中相邻FI层界面处磁的耦合作用所导致的对隧穿磁电阻的不利影响. 在自由电子近似的基础上，利用转移矩阵方法，对NM/FI/NI/FI/NM新型双自旋过滤隧道结的隧穿电导、隧穿磁电阻与FI层及NI层厚度的变化关系以及随偏压的变化关系进行了理论研究.计算结果表明，在NM/FI/NI/FI/NM新型双自旋过滤隧道结中仍可以得到很大的TMR值. 关键词： 双自旋过滤隧道结 隧穿磁电阻 非磁绝缘(半导)体间隔层     

The effect of the ferromagnetic metal layer on tunnelling conductance and magnetoresistance in double magnetic planar junctions

Based on the free-electron approximation,we investigate the effect of the ferromagnetic metal layer on the tunnelling magnetoresistance(TMR) and tunnelling conductance(TC)in the double magnetic tunnel junctions(DMTJs) of the structure NM/FM/I(S)/NM/I(S)/FM/NM,where FM,NM and I(S) represent the ferromagnetic metal,nonmagetic metal and insulator(Semiconductor),respectively,The FM,I(S)and inner NM layers are of finite thickness,while the thickness of the outer NM layer is infinite.The calculated results show that,due to the spin-dependent interfacial potential barriers caused by electronic band mismatch between the various magnetic and nonmagnetic layers,the dependences of the TMR and TC on the thicknesses of the FM layers exhibit oscillations,and a much higher TMR can be obtained for suitable thicknesses of FM layers.     

Universal behaviors of magnon-mediated spin transport in disordered nonmagnetic metal-ferromagnetic insulator heterostructures

We numerically investigate magnon-mediated spin transport through nonmagnetic metal/ferromagnetic insulator (NM/FI) heterostructures in the presence of Anderson disorder, and discover universal behaviors of the spin conductance in both one-dimensional (1D) and 2D systems. In the localized regime, the variance of logarithmic spin conductance σ²(lnG_T) shows a universal linear scaling with its average ⟨lnG_T⟩, independent of Fermi energy, temperature, and system size in both 1D and 2D cases. In 2D, the competition between disorder-enhanced density of states at the NM/FI interface and disorder-suppressed spin transport leads to a non-monotonic dependence of average spin conductance on the disorder strength. As a result, in the metallic regime, average spin conductance is enhanced by disorder, and a new linear scaling between spin conductance fluctuation rms(G_T) and average spin conductance G_T is revealed which is universal at large system width. These universal scaling behaviors suggest that spin transport mediated by magnon in disordered 2D NM/FI systems belongs to a new universality class, different from that of charge conductance in 2D normal metal systems.     

Spin diffusion in CPP spin valves

We present in this paper an analytic model to describe the effect of spin diffusion in CPP (current perpendicular to the plan) spin valves. Two ferromagnetic electrodes are separated by a nonmagnetic metal, organic or inorganic semiconducting spacer. We base our calculations on the evolution of the spin polarized density of states of the saturated ferromagnetic electrodes under an applied rotating magnetic field and the spin diffusion in the spacer. Without treating the mechanism of spin relaxation and dephasing in the spacer, we establish a general model with the cosine evolution of the magnetoresistance modulated by the effect of spin diffusion. Throughout our treatment we consider a tunnel junction at the interface between each ferromagnetic electrode and the spacer.     

磁性多层膜的巨磁电阻随铁磁和非磁层厚度变化的唯象理论计算

根据唯象理论，并采用以铁磁─非磁混合层代替铁磁／非磁层界面的理论方法，计算了Ｆｅ／Ｃｒ多层膜的巨磁电阻随铁磁和非磁层厚度的变化关系与实验结果做了比较，发现它们符合得较好．还绘出了巨磁电阻随铁磁和非磁层厚度变化的二元函数图 关键词：     

一维铁磁/有机系统的基态与自旋分布研究

文中用一维紧束缚模型描述铁磁金属,用一维非简并的Su-Schrieffer -Heeger (SSH)模型描述共轭聚合物,研究了在一维铁磁/共轭聚合物系统和一维CMR材料/ 聚合物系统中的电子转移和自旋转移.发现在聚合物部分没有自旋的双极化子比有自旋的极化子具有较低的能量而容易产生.然而在铁磁CMR材料/聚合物系统中极化子的产生能低于聚合物中极化子的产生能,增加了有机物中自旋极化输运的可能性.     

Ferromagnetic resonance spin pumping in CoFeB with highly resistive non-magnetic electrodes

The relative contribution of spin pumping and spin rectification from the ferromagnetic resonance of CoFeB/non-magnetic bilayers was investigated as a function of non-magnetic electrode resistance. Samples with highly resistive electrodes of Ta or Ti exhibit a stronger spin rectification signal, which may result in over-(or under-) estimation of the spin Hall angle of the materials, while those with low resistive electrodes of Pt or Pd show the domination of the inverse spin Hall effect from spin pumping. By comparison with samples of single FM layer and an inverted structure, we provide a proper analysis method to extract spin pumping contribution.     

Tunneling magnetoresistance of the double spin-filter junctions at nonzero bias

A recent theoretical estimation indicated that the NM/FI/FI/NM double spin-filter junction (DSFJ, here the NM and FI represent the nonmagnetic electrode and the ferromagnetic insulator (semiconductor) spacer, respectively) could have very high tunneling magnetoresistance (TMR) at zero bias. To meet the requirement in research and application of the magnetoresistance devices, we have calculated the dependences of tunneling magnetoresistance of DSFJ on the bias (voltage), the thicknesses of ferromagnetic insulators (semiconductors) and the average barrier height. Our results show that except its very high value, the TMR of DSFJ does not decrease monotonously and rapidly with rising bias, but increase slowly at first and decrease then after having reached a maximum value. This feature is in distinct contrast to the ordinary magnetic tunnel junction FM/NI/FM (FM and NI denote the ferromagnetic electrode and the nonmagnetic insulator (semiconductor) spacer, respectively), and is of benefit to the use of DSFJ as a magnetoresistance device.     

Perspectives of electrically generated spin currents in ferromagnetic materials

Spin-orbit coupling enables charge currents to give rise to spin currents and vice versa, which has applications in non-volatile magnetic memories, miniature microwave oscillators, thermoelectric converters and Terahertz devices. In the past two decades, a considerable amount of research has focused on electrical spin current generation in different types of nonmagnetic materials. However, electrical spin current generation in ferromagnetic materials has only recently been actively investigated. Due to the additional symmetry breaking by the magnetization, ferromagnetic materials generate spin currents with different orientations of spin direction from those observed in nonmagnetic materials. Studies centered on ferromagnets where spin-orbit coupling plays an important role in transport open new possibilities to generate and detect spin currents. We summarize recent developments on this subject and discuss unanswered questions in this emerging field.     

Bias-controlled spin memory and spin injector scheme in the tunneling junction with a single-molecule magnet

A bias-controlled spin-filter and spin memory is theoretically proposed, which consists of the junction with a singlemolecule magnet sandwiched between the nonmagnetic and ferromagnetic(FM) leads. By applying different voltage pulses Vwriteacross the junction, the spin direction of the single-molecule magnet can be controlled to be parallel or anti-parallel to the magnetization of the FM lead, and the spin direction of SMM can be "read out" either by the magneto-resistance or by the spin current with another series of small voltage pulses V_(probe). It is shown that the polarization of the spin current is extremely high(up to 100%) and can be manipulated by the full-electric manner. This device scheme can be compatible with current technologies and has potential applications in high-density memory devices.     

Optical detection of spin transport in nonmagnetic metals

We determine the dynamic magnetization induced in nonmagnetic metal wedges composed of silver, copper, and platinum by means of Brillouin light scattering microscopy. The magnetization is transferred from a ferromagnetic Ni80Fe20 layer to the metal wedge via the spin pumping effect. The spin pumping efficiency can be controlled by adding an insulating interlayer between the magnetic and nonmagnetic layer. By comparing the experimental results to a dynamical macroscopic spin-transport model we determine the transverse relaxation time of the pumped spin current which is much smaller than the longitudinal relaxation time.     

The observation of inherent spin Seebeck effect in Rh/YIG hybrid structure

Rhodium (Rh) is a 4d metal possessing a large spin orbit coupling strength and spin-Hall conductivity with a very small magnetic susceptibility, implying an insignificant magnetic proximity effect (MPE). We report here the observation of longitudinal spin Seebeck effect (LSSE) using Rh as a normal metal. A Rh film was sputtered on nanometer thick YIG films of highly crystalline nature and extremely low magnetic damping to obtain Rh/YIG hybrid structure. A clear thermal voltage V_th (SSE voltage) was obtained when a temperature gradient was applied on the Rh/YIG hybrid. The Rh film showed a very weak anomalous Hall resistance and the magneto-resistive testing clearly ruled out the magnetization of the Rh films via MPE. The anisotropic magnetoresistance (AMR) revealed a clear spin hall magnetoresistance (SMR) signal in Rh film implying a purely intrinsic spin current generation, free from any parasitic magnetic effects. The work can open a new window in the study of pure and uncontaminated spin current, generated in ferromagnetic insulators, using Rh as spin current detector.     

Tunable exchange bias in La1.5Sr0.5CoMnO6 double perovskite doped with nonmagnetic Ga ions

The crystal structure, electronic structure, and magnetic behaviors of nonmagnetic Ga ions doped double perovskite La_1.5Sr_0.5CoMnO₆ single phase crystals have been investigated. Different from the traditional magnetic dilution effect of nonmagnetic doping, Ga doping in La_1.5Sr_0.5CoMnO₆ enhances the ferromagnetic (FM) exchange interaction of Co³⁺-O-Mn³⁺. Moreover, both conventional and spontaneous exchange bias (EB) effects can be tuned by modulating the Ga doping content, which is accompanied by the variation of the Co^3+/4+ and Mn^3+/4+ and the effective magnetic moment. The EB field and magnetization can be improved by nonmagnetic Ga³⁺ doping with content lower than 0.2. The evolution of conventional and spontaneous EB effects in La_1.5Sr_0.5Co_1-xGa_xMnO₆ can be understood in terms of the unidirectional interface anisotropic coupling between FM/anti-FM, and/or FM/spin glass, which is affected by antisite disorder, spin glass, and the uncompensated coupling between Co and Mn.     

Tunable interlayer exchange coupling energy by modification of Schottky barrier potentials

We theoretically demonstrate that the interlayer exchange coupling (IEC) energy can be manipulated by means of an external bias voltage in a F₁/NM/F₂/S (F₁: ferromagnetic, NM: nonmagnetic metallic, F₂: ferromagnetic, S: semiconductor layers) four-layer system. It is well known that the IEC energy between two ferromagnetic layers separated by nanometer thick nonmagnetic layer depends on the spin-dependent electron reflectivities at the interface in F₁/NM/F₂ trilayer system. We apply such dependence to the F₁/NM/F₂/S four-layer system, where the reflectivity of NM/F₂ interface also depends on F₂/S interface due to the multiple reflection of an electron like optics. Finally, the IEC energy depends on the spin-dependent electron reflectivity not only at the interfaces of F₁/NM/F₂, but also at the interface of F₂/S. Naturally the Schottky barrier is formed at the interface between metallic ferromagnetic layer and semiconductor, the Schottky barrier height and thickness can be tailored by an external bias voltage, which causes the change of the spin-dependent reflectivity at F₂/S interface. We show that the IEC energy between two ferromagnetic layers can be controlled by an external bias voltage due to the electron-optics nature using a simple free-electron-like one-dimensional model.     

Spin-relaxation and magnetoresistance in FM/SC/FM tunnel junctions

The effect of spin relaxation on tunnel magnetoresistance (TMR) in a ferromagnet/superconductor/ferromagnet (FM/SC/FM) double tunnel junction is theoretically studied. The spin accumulation in SC is determined by balancing of the spin-injection rate and the spin-relaxation rate. In the superconducting state, the spin-relaxation time τ_s becomes longer with decreasing temperature, resulting in a rapid increase of TMR. The TMR of FM/SC/FM junctions provides a useful probe to extract information about spin-relaxation in superconductors.     

Spin-orbit torque from spin-flipping scattering at ferromagnetic metal/topological insulator interface

Recent experiments report large damping-like spin-orbit torque in a magnetic bilayer that consists of a topological insulator (TI) layer and a ferromagnetic metal (FM) layer. Here we examine the bilayer theoretically with particular attention to roles of conduction electrons in FM on the spin-orbit torque in this structure. We use electron scattering approach to address electron spin accumulation at the interface between TI/FM caused by the conduction electrons. While topological surface states are not well defined in this bilayer, we find that large damping-like spin-orbit torque can still arise through spin-flipping scattering of the conduction electrons at the TI-FM interface. The resulting damping-like spin-orbit torque is comparable in magnitude to that of the field-like spin-orbit torque. The ratio between the components of the spin-orbit torque relies on various details of the system. The result is compared with recent experimental results and other theoretical works.     

Spectroscopy of phonons and spin torques in magnetic point contacts

Phonon spectroscopy is used to investigate the mechanism of current-induced spin torques in nonmagnetic/ferromagnetic (N/F) point contacts. Magnetization excitations observed in the magneto-conductance of the point contacts are pronounced for diffusive and thermal contacts, where the electrons experience significant scattering in the contact region. We find no magnetic excitations in highly ballistic contacts. Our results show that impurity scattering at the N/F interface is the origin of the new single-interface spin torque effect.     

Generation and manipulation of spin current via a hybrid four-terminal single-molecule junction

We present a new device which consists of a molecular quantum dot (MQD) attached to a normal-metal, two ferromagnetic (FM), and a superconducting leads. The spin-related Andreev reflection (AR) current and the spin-dependent single-particle tunneling current through the normal-metal terminal are obtained, and it is found that the spin current exhibits the transistor-like behavior. The joint effects of the coherent spin flip and the angle between magnetic moments of the two FM leads on the spin current are also studied, these results provide the possibility to manipulate the spin current with the system parameters.     

Charge Hall effect generated by spin-polarized current injected into Rashba spin orbit coupling media

Using the Keldysh Green’s function method, we study theoretically the electron accumulation induced by the inverse spin Hall effect in a spin valve structure in which a clean quantum wire formed from a 2D electron gas (2DEG) with Rashba/Dresselahaus spin orbit interaction (SOI) is connected to two ferromagnet electrodes. In a nonequilibrium situation when a spin current with an out-of plane (the 2DEG plane) spin polarization is driven through the SOI region by an external voltage, non-equilibrium electron accumulation or a Hall voltage forms at the two lateral sides of the quantum wire and exhibits an oscillation along the wire like the Rashba spin precession; the magnetization directions of FMs affect the Hall voltage and their parallel or antiparallel alignment along the normal direction of the 2DEG plane is most favorable to the Hall voltage. In an equilibrium situation, two planar magnetizations which are not collinear can generate an electron accumulation/a Hall voltage too. When one of the FM electrodes is replaced by a normal metal (NM), the electron accumulation is still present along the wire and its magnitude remains nearly unchanged in the biased case, whereas in the unbiased case it is reduced significantly and even vanishes.     

Electrical control of the direction of spin accumulation

We are able to continuously change the direction of polarization of spin accumulation in a nonmagnetic metal by varying the currents injected by two ferromagnetic spin injectors. From measurements made at a distance from the injection area, we find a cosvarphi variation of the spin signal. This confirms that the angle of polarization of the nonlocal spin polarization with respect to the magnetization of the fixed spin detector is continuously varied as we change the injection currents. We give an explanation for the origin of this simple cosvarphi variation of the spin signal.