Transient domain wall displacement under spin-polarized current pulses

This paper investigates the non steady-state displacement of magnetic domain walls in a nanostrip submitted to a time-dependent spin-polarized current flowing along the nanostrip. First, numerical micromagnetic simulations show that a domain wall can move under application of a current pulse, and that the displacement resulting from a conversion of the domain wall structure is quantized. The numerical findings are subsequently explained in the framework of simplified analytic models, namely the 1D model and the point-core vortex model. We then introduce the concept of an angle linked to the magnetization of a general domain wall, and show that it allows understanding the transient phenomena quite generally. Simple analytic formulas are derived and compared to experiments. For this, charts are given for the key parameters of the domain wall mechanics, as obtained from numerical micromagnetic simulations. We finally discuss the limitations of this work, by looking at the influence of temperature elevation under current, presence of a non-adiabatic term, and of disorder.     

Controlled and reproducible domain wall displacement by current pulses injected into ferromagnetic ring structures

In a combined numerical and experimental study, we demonstrate that current pulses of different polarity can reversibly and controllably displace a magnetic domain wall (DW) in submicrometer permalloy (NiFe) ring structures. The critical current densities for DW displacement are correlated with the specific spin structure of the DWs and are compared to results of micromagnetic simulations including a spin-torque term. Using a notch, an attractive local pinning potential is created for the DW resulting in a highly reproducible spin structure of the DW, critical for reliable current-induced switching.     

Magnetic domain wall motion by current injection in CoPt nanowires consisting of notches

Magnetic domain wall behaviors in CoPt nanowires consisting of multiple pairs of notches were investigated by experimental measurements as well as by micromagnetic modeling. The nanowires were fabricated by ion-beam sputter deposition and e-beam lithography where one to three triangular shaped notches were installed at an interval of 1 μm. Based on the evaluated I–V characteristics, we observed that differential resistance curves showed two peaks with a local minimum at around zero current; the domain wall was trapped between the current ranges within these two peaks. As the number of notch was increased, the resistance of the nanowire became larger.     

Dynamics of a domain wall and spin-wave excitations driven by a mesoscopic current

The dynamics of a domain wall driven by a spin-polarized current in a mesoscopic system is studied numerically. Spin mixing in the states of the conduction electrons is fully taken into account. When the Fermi energy of the electrons is larger than the exchange energy (E(F) > J(sd)), the spin precession induces spin-wave excitations in the local spins which contribute towards the displacement of the domain wall. The resulting average velocity is found to be much smaller than the one obtained in the adiabatic limit. For E(F) < J(sd), the results are consistent with the adiabatic approximation except for the region below the critical current where a residual domain wall velocity is found.     

Vortex polarity switching by a spin-polarized current

The spin-transfer effect is investigated for the vortex state of a magnetic nanodot. A spin current is shown to act similarly to an effective magnetic field perpendicular to the nanodot. Then a vortex with magnetization (polarity) parallel to the current polarization is energetically favorable. Following a simple energy analysis and using direct spin-lattice simulations, we predict the polarity switching of a vortex. For magnetic storage devices, an electric current is more effective to switch the polarity of a vortex in a nanodot than the magnetic field.     

Exact solution of eddy current losses produced by a domain wall with nonzero thickness

The influence of the domain wall thickness on eddy current losses has been calculated. It is shown that, for samples with low magnetic anisotropy and small thickness, the reduction of magnetic losses with respect to the zero thickness domain wall must be seriously taken into account.     

Micromagnetic study of domain wall depinning driven by nanosecond current pulse in notched Permalloy nanowires

The complete understanding of domain wall (DW) dynamics is important in the design of future spintronic devices. The characteristics of faster time-scale and lower current amplitude to move DW along nanowire are crucial in fabrication upgrade. In this study, we have investigated depinning behavior of magnetic domain wall triggered by nanosecond current pulse in notched Permalloy nanowires by means of micromagnetic simulation. We introduced double-triangular notch as the constrictions in the nanowire. The non-adiabaticity of the spin-transfer-torque is considered in simulation by varying the non-adiabatic constant (β) value. We observed that the depinning current density (J_d) was not significantly affected by β for notch size (s) < 50 nm. Interestingly, we found that the depinning time (t_d) for β ≥ 0.04 was slightly constant for all the cases with s > 70 nm, where the DW structure was kept to be a transverse structure during the depinning process. The broadly applicable depinning behavior is considered to contribute to the development of high-speed memory storage devices based on magnetic domain wall.     

电流调控磁涡旋的极性和旋性

提出了一种特殊自旋阀结构, 其极化层(钉扎层)磁矩沿面内方向, 自由层磁矩成磁涡旋结构. 自由层在形状上设计成左右两边厚度不同的阶梯形圆盘. 微磁学模拟研究发现, 通过调控所施加的高斯型脉冲电流的大小、方向和脉冲宽度, 可以实现磁涡旋的不同旋性、不同极性的组态控制. 分析了该结构中电流调控磁涡旋旋性和极性的物理原因和微观机理.     

Influence of a transport current on a domain wall in an antiferromagnetic metal

We consider the influence of an electric current on the position of a domain wall in an antiferromagnetic metal. We first microscopically derive an equation of motion for the Néel vector in the presence of current by performing, in the transport steady state, a linear-response calculation in the deviation from collinearity of the antiferromagnet. This equation of motion is then solved variationally for an antiferromagnetic domain wall. We find that, in the absence of dissipative or non-adiabatic coupling between magnetization and current, the current displaces the domain wall by a finite amount and that the domain wall is then intrinsically pinned by the exchange interactions. In the presence of dissipative or non-adiabatic current-to-domain-wall coupling, the domain wall velocity is proportional to the current and is no longer pinned.     

Hysteresis mediated by a domain wall motion

The position of an interface (domain wall) in a medium with random pinning defects is not determined unambiguously by the instantaneous value of the driving force, even on average. Employing the general theory of the interface motion in a random medium, we study this hysteresis, different possible shapes of the hysteresis loop, and the dynamical phase transitions between them. Several principal characteristics of the hysteresis, including the coercive force and the curves of dynamical phase transitions obey scaling laws and display a critical behavior in the vicinity of the mobility threshold. At finite temperature the threshold is smeared and a new range of thermally activated hysteresis appears. At a finite frequency of the driving force there exists a range of the non-adiabatic regime in which not only the position, but also the average velocity of the domain wall, displays hysteresis.     

Pressure exerted on a domain wall by a current in a spin valve: Stoner ferromagnet model

The influence of an electric current flowing through a spin-valve perpendicular to its layers on a domain wall located in the free layer of the spin valve is studied. It is demonstrated that the nonequilibrium spin distribution generated by the current gives rise to a pressure exerted on the domain wall. This pressure is proportional to the current squared, and, for typical values of the magnetic parameters and a current density of 10⁷–10⁸ A/cm², its effect is similar to that of a magnetic field of several oersteds to several tens of oersteds. The magnitude and sign of the pressure are strongly dependent on the geometric and physical parameters of the device. The problem is solved using the model of itinerant-electron ferromagnetism. The relation of the discovered effect to experimental data on magnetization reversal induced by a spin-polarized current in such structures is discussed.     

Faster 360° domain wall motion in nanostrip induced by spin-polarized current with out-of-plane magnetic field

By micromagnetic simulation, we show that faster propagation of 360° domain wall in magnetic nanostrips under spin-polarized currents in conjunction with out-of-plane magnetic fields can be obtained. Without magnetic field, the annihilation process of 360° domain wall is irreversible when spin-polarized current velocity above about 220 m/s. The annihilation of 360° domain wall can be suppressed by an out-of -plane magnetic field and domain wall speed can exceed 1500 m/s at large current density. This is different from the case exhibited in 180° domain wall. The underlying mechanism is investigated by changing the state of 360° domain wall and the direction of out-of-plane field.     

Deroughening of domain wall pairs by dipolar repulsion

As a magnetic domain wall propagates under small fields through a random potential, it roughens as a result of weak collective pinning, known as creep. Using Kerr microscopy, we report experimental evidence of a surprising deroughening of wall pairs in the creep regime, in a 0.5 nm thick Co layer with perpendicular anisotropy. A bound state is found in cases where two rough domains nucleated far away from one another and first growing under the action of a magnetic field eventually do not merge. The two domains remain separated by a strip of unreversed magnetization, characterized by flat edges and stabilized by dipolar fields. A creep theory that includes dipolar interactions between domains successfully accounts for (i) the domain wall deroughening as the width of the strip decreases and (ii) the quasistatic and dynamic field dependence of the strip width s.     

Control of atomic ionization by two-color few-cycle pulses

We have studied the ionization of Rydberg atoms by few-cycle radio-frequency pulses and used two-color fields to control the ionization dynamics. We show that the number of times that electrons are emitted during a pulse can be limited and that the duration of the electron emission can be shortened. These results, once they are transposed to the optical domain, may inspire new strategies for the production of single attosecond pulses.     

Controlled domain wall motion by current into patterned-U Ni80Fe20 wires

The current-induced domain wall motion was observed experimentally in the case of the domain wall trapped at the semicircular arc within the U shape Ni₈₀Fe₂₀ wire. The measurement of the current-induced domain wall motion was achieved by adding a biased field before switching field and a critical current density was measured. We found two magnetic domain structures in the U pattern. At zero fields, the vortex domain wall nucleated at the semicircular arc of the U pattern. Continuous magnetic state without wall was investigated in near-switching field.     

Direct imaging of stochastic domain-wall motion driven by nanosecond current pulses

Magnetic transmission x-ray microscopy is used to directly visualize the influence of a spin-polarized current on domain walls in curved permalloy wires. Pulses of nanosecond duration and of high current density up to 1.0x10(12) A/m(2) are used to move and to deform the domain wall. The current pulse drives the wall either undisturbed, i.e., as composite particle through the wire, or causes structural changes of the magnetization. Repetitive pulse measurements reveal the stochastic nature of current-induced domain-wall motion.     

Current-induced domain wall motion

The present understanding of domain wall motion induced by spin-polarized electric current is assessed by considering a subset of experiments, analytical models, and numerical simulations based on an important model system: soft magnetic nanowires. Examination of this work demonstrates notable progress in characterizing the experimental manifestations of the “spin-torque” interaction, and in describing that interaction at a phenomenological level. At the same time, an experimentally verified microscopic understanding of the basic mechanisms will require substantial future efforts, both experimental and theoretical.     

Control of polarity of heteroepitaxial ZnO films by interface engineering

Control of polarity of heteroepitaxial ZnO films has been examined by interface engineering. ZnO films were grown by plasma-assisted molecular beam epitaxy on Ga-polar GaN template and c-plane sapphire substrates. Polarity of all the samples is determined by coaxial impact collision ion scattering spectroscopy. Zn- and O-polar ZnO films have successfully grown by Zn- and O-plasma pre-exposures on Ga-polar GaN templates prior to ZnO growth. High-resolution transmission electron microscopy revealed formation of a single-crystalline monoclinic Ga₂O₃ interface layer by O-plasma pre-exposure on Ga-polar GaN templates, while no interface layer was observed for Zn pre-exposed ZnO films. The polarity of ZnO films grown under oxygen ambient on c-plane sapphire with MgO buffer is revealed as O-polar. Fabrication of polarity inverted ZnO heterostructure has been studied: polarity of ZnO films on Ga-polar GaN templates was changed from Zn-polar to O-polar by inserting a MgO layer. High-resolution transmission electron microscopy revealed atomically flat interfaces at both lower and upper ZnO/MgO interfaces and no inversion domain boundaries were detected in the upper ZnO layer.     

Anisotropy of domain wall resistance

The resistive effect of domain walls in FePd films with perpendicular anisotropy was studied experimentally as a function of field and temperature. The films were grown directly on MgO substrates, which induces an unusual virgin magnetic configuration composed of 60 nm wide parallel stripe domains. This allowed us to carry out the first measurements of the anisotropy of domain wall resistivity in the two configurations of current perpendicular and parallel to the walls. At 18 K, we find 8.2% and 1.3% for the domain wall magnetoresistance normalized to the wall width (8 nm) in these two respective configurations. These values are consistent with the predictions of Levy and Zhang.     

Linear photogalvanic current excited in sillenite crystals by femtosecond laser pulses

We observe electric pulses generated in sillenite crystals (Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀) by 100-fs laser pulses at the wavelength of 400 nm (below the band gaps of both crystals). The peak value of the current pulses scales linearly with the intensity of laser pulses up to ∼45 GW/cm². The direction of the induced current depends on the polarization state of the laser pulse. This polarization dependence and features of the current detection via charge accumulation at the sample electrodes allow us to conclude that the electric pulses are generated due to the linear photogalvanic effect.