Spin-injection mechanism of magnetization reversal and hysteresis of current in magnetic junctions

A novel mechanism is proposed for magnetization reversal by the current of magnetic junctions with two metallic ferromagnetic layers and thin separating nonmagnetic layer. The spin-polarized current flows perpendicularly to the interfaces between the ferromagnetic layers, in one of which the spins are pinned and in the other they are free. No domain structure is formed in the ferromagnetic layers. The current breaks spin equilibrium in the free layer, which manifests itself in the injection or extraction of spins. The nonequilibrium spins interact with the magnetization of the lattice due to the effective field of s-d exchange, which is current dependent. At currents exceeding a certain threshold value, this interaction leads to magnetization reversal. Two threshold currents for magnetization reversal have been obtained theoretically, which are reached as the current increases or decreases, respectively. Thus, the phenomenon of current hysteresis is found. The calculated results are in good agreement with experiments on magnetization reversal by current in three-layer junctions of composition Co(I)/Cu/Co(II) prepared in a pillar form.     

Change in the magnetic moment of a ferromagnetic nanoparticle under polarized current

The magnetization reversal of a ferromagnetic Fe₃O₄ nanoparticle with a volume of the order of several thousands of cubic nanometers under the influence of spin-polarized current has been investigated on a high-vacuum scanning tunneling microscope, where one of the electrodes is a magnetized iron wire needle and the second electrode is a ferromagnetic nanoparticle on a graphite substrate. The measured threshold current of magnetization reversal, i.e., the lowest value of the current corresponding to the magnetization reversal, is found to be I_thresh ≈ 9 nA. A change in the magnetization of a nanoparticle is revealed using the giant magnetoresistance effect, i.e., the dependence of the weak polarized current (I < I_thresh) on the relative orientation of the magnetizations of the electrodes.     

Persistent current in a magnetized Rashba ring

We theoretically study the persistent currents flowing in a Rashba quantum ring subjected to the Rashba spin-orbit interaction. By introducing uniform or nonuniform magnetization into the ring, we find that a nonzero persistent charge current circulates in the ring, which stems from the original equilibrium spin current due to the Rashba spin-orbit interaction. Because of broken time reversal symmetry, the two oppositely flowing spin-up and spin-down charge currents of the equilibrium spin current are no longer equal, and so a net persistent charge current can flow in the system. It is also found that the persistent current can be modulated by the Fermi energy, the Rashba spin-orbit interaction strength and the magnetization in the ring. Moreover, the magnetization perpendicular to the ring plane can optimize the current. The persistent current flowing in the ring is a manifestation of the nonzero equilibrium spin current existing in the ring.     

Modeling magnetization oscillations in ferromagnetic cobalt disks exposed to picosecond laser pulses

Magnetic reversal of a ferromagnetic single-domain nanoparticle in the form of a disk contained in a multilayer nanostructure irradiated by linearly and circularly polarized picosecond laser pulses is modeled. It is shown that heating of the disk by the laser light causes a change in its direction of magnetization. This is accompanied by oscillations in the magnetization that decay over 1–16 ns. The frequencies of these oscillations lie in the range of 0.5–26 GHz. The main reason for the magnetic reversal of the disk is a change in the magnetic anisotropy energy during heating. The laser pulses also create a spin-polarized current and an inverse Faraday magneto-optical field in the structure, which influence the duration, amplitude, and frequency of the magnetization oscillations.     

基于磁动力学方程的电流感应磁化翻转效应的宏观模型

提出了一个基于磁动力学方程的宏观唯象理论模型，对纳米级赝自旋阀结构的电流感应磁化翻转效应给出了明晰的物理解释：流入自由层的净自旋流和自由层内的自旋弛豫过程的共同作用，导致自由层总磁矩随时间的改变，甚至产生磁化方向的翻转.模型将“铁磁/非铁磁”界面的自旋相关散射，以及铁磁层中的自旋积累和弛豫过程，统一于宏观的磁动力学方程中.通过求解该方程的解析解，给出了赝自旋阀在电流激励下的磁化翻转条件和临界电流密度的表达式.对该效应的定性解释和数值模拟结果都和实验报道良好符合.根据模型分析了影响临界电流密度的诸因素，并指出提高器件性能的途径. 关键词： 电流感应磁化翻转 磁动力学方程 自旋电子学     

Asymmetric magnetization reversal on exchange biased CoO/Co bilayers

We study magnetic hysteresis loops after field cooling of a CoO/Co bilayer by MOKE and polarized neutron reflectivity. The neutron scattering reveals that the first magnetization reversal after field cooling is dominated by domain wall movement, whereas all subsequent reversals proceed essentially by rotation of the magnetization. In addition, off-specular diffuse scattering indicates that the first magnetization reversal induces an irreversible change of the domain state in the antiferromagnet.     

Topological textures and their bifurcation processes in 2D ferromagnetic thin films

In this paper, by the use of the topological current theory, the topological structures and the dynamic processes in thin-film ferromagnetic systems are investigated directly from the viewpoint of topology. It is found that the topological charge of a thin-film ferromagnetic system can be changed by annihilation or creation processes of opposite polarized vortex–antivortex pairs taking place at space–time singularities of the normalized magnetization vector field of the system, the variation of the topological charge is integer and can further be expressed in terms of the Hopf indices and Brouwer degrees of the magnetization vector field around the singularities. Moreover, the change of the topological charge of the system is crucial to vortex core reversal processes in ferromagnetic thin films. With the help of the topological current theory and implicit function theorem, the processes of vortex merging, splitting as well as vortex core reversal are discussed in detail.     

Magnetization dynamics due to pure spin currents in magnetic double layers

The magnetization dynamics in magnetic double layers is affected by spin-pump and spin-sink effects. So far, only the spin pumping and its effect on the magnetic damping has been studied. However, due to conservation of angular momentum this spin current also leads to magnetic excitation of the layer dissipating this angular momentum. In this Letter we use time resolved magneto-optic Kerr effect to directly show the excitation due to the pure spin current. In particular, we observe magnetization dynamics due to transfer of angular momentum in magnetic double layers. In contrast to other experiments where a spin polarized charge current is passed through a nanomagnet, the effects discussed in this Letter are based on pure spin currents without net transfer of electric charge.     

Magnetization and polarized neutron reflectivity experiments on patterned exchange bias structures

Patterned Co/CoO thin film structures have been investigated by magnetization and polarized neutron reflectivity measurements in order to study the influence of finite size and shape anisotropy effects on the magnetization reversal. An anomaly was found in the upper branch of the hysteresis loops, probably caused by incomplete bias in the patterned structures. The asymmetry in magnetization reversal mechanism commonly found in the two branches of the hysteresis loops of unpatterned Co/CoO layers is altered in the patterned structures, consistent with the existence of interfacial domains.     

Pure spin currents generation in magnetic tunnel junctions by means of adiabatic quantum pumping

We study the spin polarized currents generation in a magnetic (ferromagnetic/ferromagnetic) tunnel junction by means of adiabatic quantum pumping. Using a scattering matrix approach, it is shown that a pure spin current can be pumped from one ferromagnetic lead into the adjacent one by adiabatic modulation of the magnetization and the height of the barrier at the interface in absence of external bias voltage. We numerically study the characteristic features of the pure spin current and discuss its behavior for realistic values of the parameters. We show that the generated pure spin current is robust with respect to the variation of the magnetization strength, a very important feature for a realistic device, and that the proposed device can operate close to the optimal pumping regime. An experimental realization of a pure spin current injector is also discussed.     

All-optical magnetic recording with circularly polarized light

We experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal previously believed impossible is the combined result of femtosecond laser heating of the magnetic system to just below the Curie point and circularly polarized light simultaneously acting as a magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.     

In-plane current-induced magnetization reversal of Pd/CoZr/MgO magnetic multilayers

High critical current density ($> 10^{6}$ A/cm$^{2})$ is one of major obstacles to realize practical applications of the current-driven magnetization reversal devices. In this work, we successfully prepared Pd/CoZr(3.5 nm)/MgO thin films with large perpendicular magnetic anisotropy and demonstrated a way of reducing the critical current density with a low out-of-plane magnetic field in the Pd/CoZr/MgO stack. Under the assistance of an out-of-plane magnetic field, the magnetization can be fully reversed with a current density of about 10$^{4}$ A/cm$^{2}$. The magnetization reversal is attributed to the combined effect of the out-of-plane magnetic field and the current-induced spin-orbital torque. It is found that the current-driven magnetization reversal is highly relevant to the temperature owing to the varied spin-orbital torque, and the current-driven magnetization reversal will be more efficient in low-temperature range, while the magnetic field is helpful for the magnetization reversal in high-temperature range.     

用外磁场控制电流感应磁化翻转效应中的临界电流方法

应用基于磁动力学方程的宏观唯象模型,研究了弱外磁场下纳米尺度赝自旋阀结构的电流感应磁化翻转效应.在统一考虑铁磁/非磁界面的自旋相关散射以及铁磁层中的自旋积累和弛豫过程后,给出了赝自旋阀结构在弱外磁场下的磁化翻转条件和临界电流.对该效应的数值计算解释了弱外磁场下赝自旋阀结构的电阻-电流回线的偏移,并给出了用外磁场控制电流感应磁化翻转效应中的临界电流方法. 关键词： 电流感应磁化翻转 外磁场 临界电流 赝自旋阀     

Magnetization reversal processes of single nanomagnets and their energy barrier

Micromagnetic simulations were performed to investigate the influence of geometry and magnetic anisotropy constant on energy barrier and magnetization reversal mechanism of individual bits important for the bit patterned media concept in magnetic data storage. It is shown that dependency of the energy barrier on magnetic and geometric properties of bits can be described by an analytical approach in the case of quasi-coherent magnetization rotation process. However, when the bit size exceeds a critical size, for which an incoherent magnetization reversal is preferred, the analytical approach becomes invalid and no self-consistent theory is available. By systematically investigating the influence of bit size on the magnetization reversal mode, it was found that the transition from quasi-coherent to incoherent magnetization reversal mode can still be described analytically if an activation volume is considered instead of the bit volume. In this case, the nucleation volume is an important parameter determining thermal stability of the bit. If the volume of the bit is larger than twice the activation volume, the energy barrier stays nearly constant; with further increase in bit size, no gain in thermal stability can be achieved.     

Photocurrent spin diode: Electrically controlled sign reversal of current polarization

We report a study of spin-dependent transport through a quantum dot irradiated by continuous circularly polarized light resonant to the electron-heavy hole transition. We use the nonequilibrium Green's function to calculate the spin accumulation, spin-resolved currents, and current polarization in the presence of an external bias and intradot Coulomb interaction. It is found that for a range of external biases sign reversal of the current polarization can be modulated. The system thus operates as a rectifier for spin current polarization. This effect follows from the interplay between the external irradiation and the Coulomb repulsion. The spin-polarized transport through a three-terminal device is also discussed. Spin current with high polarization could be obtained due to spin filter effect.     

Current-driven magnetization switching and domain wall motion in nanostructures—Survey of recent experiments

An overview of recent experimental studies and new routes in the field of current-driven magnetization dynamics in nanostructured materials is given. The review introduces the basic concepts (Landau–Lifshitz phenomenology, critical current, spin currents in relation to spin accumulation, adiabatic/non-adiabatic spin-torque) and describes the main results of recent experiments on current-driven magnetization reversal within vertical pillar-like nanostructures and current-driven domain wall motion within laterally confined specimens. While for the pillar systems a discussion is provided of how the introduction of layers with perpendicular magnetic anisotropy, tunnel barriers and exchange bias and(or) oxide layers can be used to reduce the critical current densities for current-induced switching, the role of perpendicular anisotropy, use of spin valve structures, diluted magnetic semiconductors and epitaxial materials to increase the domain wall velocities are reviewed in the case of current-driven domain wall movement within lateral systems.     

Two-stage magnetization reversal in exchange biased bilayers

MnF(2)/Fe bilayers exhibit asymmetric magnetization reversal that occurs by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other side of the loop. Here, we show by polarized neutron reflectometry, magnetization, and magnetotransport measurements that for samples with good crystalline "quality" the rotation is a two-stage process, due to coherent rotation to a stable state perpendicular to the cooling field direction. The result is remarkably asymmetrically shaped hysteresis loops.     

Study of surface magnetic properties in Co-rich amorphous microwires

Magneto-optical investigations carried out on a Co-rich glass-covered amorphous microwire is presented. The appearance of circular magnetic bistability and the influence of tensile stress and high-frequency electric current on the surface magnetization reversal have been studied. The change of the mechanism of the magnetization reversal induced by the high-frequency electric current is also discussed.     

Proposal for direct measurement of a pure spin current by a polarized light beam

The photon helicity may be mapped to a spin-1/2, whereby we put forward an intrinsic interaction between a polarized light beam as a "photon spin current" and a pure spin current in a semiconductor, which arises from the spin-orbit coupling in valence bands as a pure relativity effect without involving the Rashba or the Dresselhaus effect due to inversion asymmetries. The interaction leads to linear and circular optical birefringence, which are similar to the Voigt effect and the Faraday rotation in magneto-optics but nevertheless involve no net magnetization. The birefringence effects provide a direct, nondemolition measurement of pure spin currents.     

Magnetization reversal in granular thin films

The paper discusses the physics of magnetization reversal in granular magnetic films. It gives an overview of the key physical properties that determine the collective and macroscopically observable magnetization reversal behavior. In particular, the multitude of observable hysteresis loops is reduced to three key physical quantities, namely the single grain switching field distribution D(h_s), the inter-granular exchange coupling constant J_ex, and the magnetostatic interaction constant J_ms. By varying the relative influence of these quantities, many different shapes of hysteresis loops can occur, which is documented by experimental examples. The regime of partially and strongly correlated reversal is discussed in detail, and minor loop measurements are presented that show scaling behavior for strongly correlated magnetization reversal in the vicinity of hysteresis loop criticality.