Current induced domain wall motion in GaMnAs close to the Curie temperature

Domain wall dynamics produced by spin transfer torques is investigated in (Ga, Mn)As ferromagnetic semiconducting tracks with perpendicular anisotropy, close to the Curie temperature. The domain wall velocities are found to follow a linear flow regime which only slightly varies with temperature. Using the D?ring inequality, boundaries of the spin polarization of the current are deduced. A comparison with the predictions of the mean field k·p theory leads to an estimation of the carrier density whose value is compatible with results published in the literature. The spin polarization of the current and the magnetization of the magnetic atoms present similar temperature variations. This leads to a weak temperature dependence of the spin drift velocity and thus of the domain wall velocity. A combined study of field- and current-driven motion and deformation of magnetic domains reveals a motion of domain walls in the steady state regime without transition to the precessional regime. The ratio between the non-adiabatic torque β and the Gilbert damping factor α is shown to remain close to unity.     

Roles of nonequilibrium conduction electrons on the magnetization dynamics of ferromagnets

The mutual dependence of spin-dependent conduction and magnetization dynamics of ferromagnets provides the key mechanisms in various spin-dependent phenomena. We compute the response of the conduction electron spins in a spatial and time varying magnetization M(r,t) in the time-dependent semiclassical transport theory. We show that the induced nonequilibrium conduction spin density in turn generates four spin torques acting on the magnetization-with each torque playing a different role in magnetization dynamics. By comparing with recent theoretical models, we find that one of these torques which has not been previously identified is crucial to consistently interpreting experimental data on domain wall motion.     

Current-induced domain wall motion in magnetic nanowires with spatial variation

We model current-induced domain wall motion in magnetic nanowires with the variable width. Employing the collective coordinate method we trace the wall dynamics. The effect of the width modulation is implemented by spatial dependence of an effective magnetic field. The wall destination in the potential energy landscape due to the magnetic anisotropy and the spatial nonuniformity is obtained as a function of the current density. For a nanowire of a periodically modulated width, we identify three (pinned, nonlinear, and linear) current density regimes for current-induced wall motion. The threshold current densities depend on the pulse duration as well as the magnitude of wire modulation. In the nonlinear regime, application of ns order current pulses results in wall displacement which opposes or exceeds the prediction of the spin transfer mechanism. The finding explains stochastic nature of the domain wall displacement observed in recent experiments.     

Indirect control of antiferromagnetic domain walls with spin current

The indirect controlled displacement of an antiferromagnetic domain wall by a spin current is studied by Landau-Lifshitz-Gilbert spin dynamics. The antiferromagnetic domain wall can be shifted both by a spin-polarized tunnel current of a scanning tunneling microscope or by a current driven ferromagnetic domain wall in an exchange coupled antiferromagnetic-ferromagnetic layer system. The indirect control of antiferromagnetic domain walls opens up a new and promising direction for future spin device applications based on antiferromagnetic materials.     

Gyroscopic dynamics of antiferromagnetic vortices in the orthoferrite domain wall

The method of generation of antiferromagnetic vortices on the supersound domain wall in the orthoferrites was proposed. Moving antiferromagnetic vortices were accompanied by the solitary deflection waves. These waves were used for investigation of generation and nonlinear dynamics of the antiferromagnetic vortices on a moving domain wall with the help of two- and three-fold digital high-speed photography and Faraday rotation in the orthoferrites plates cut perpendicular to the optical axis. The full velocity of antiferromagnetic vortex nonlinearly increases and saturates on the spin velocity level c. The vortices with smallest topological charges saturate earlier than with big one. The vortices velocity along the domain wall u increases up to the maximum and goes to the dependence u²+v²=c². Vortex dynamics is quasirelativistic on quasirelativistic domain wall. The theory of gyroscopic force in the domain wall of orthoferrites was elaborated by Zvezdin et al. and was confirmed our earlier experimental results.     

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.     

Spin mixing and spin-current asymmetry measured by domain wall magnetoresistance

By making a straightforward reformulation of the Levy-Zhang spin-mistracking model, we show that it is possible to extract the spin asymmetry of a current from measurements of domain wall resistance. Experiments on epitaxial films of L1(0) FePd are reported, showing that, while the micromagnetic structure of the sample is stable, the resistivity and the domain wall resistance change by a factor approximately 3 between helium and room temperature. The temperature dependence of the spin asymmetry of the current has been determined over a wide range in a single material.     

Influence of current on field-driven domain wall motion in permalloy nanowires from time resolved measurements of anisotropic magnetoresistance

The motion of magnetic domain walls in permalloy nanowires is investigated by real-time resistance measurements. The domain wall velocity is measured as a function of the magnetic field in the presence of a current flowing through the nanowire. We show that the current can significantly increase or decrease the domain wall velocity, depending on its direction. These results are understood within a one-dimensional model of the domain wall dynamics which includes the spin transfer torque.     

Domain wall resistance in epitaxial Fe wires

We studied the magnetoresistance behavior of epitaxial Fe wires grown on GaAs(1 1 0) with varying widths at room temperature. Single nanowires show a wire width (w) dependence of the coercive field, which increases with 1/w for decreasing wire widths. This enables the pinning of a single domain wall in the connection area of two wires with different widths. Magnetoresistance measurements of such wire structures clearly reveal resistance contributions arising from a domain wall. The presence of the domain wall is proven by photoemission electron-microscopy with synchrotron radiation. Moreover, micromagnetic simulations are performed to determine the spin orientations, especially within the domain wall. This permits us to calculate the anisotropic magnetoresistance caused by the domain wall. Taking this into account, we determine the intrinsic domain wall resistance, for which we found a positive value of 0.2%, in agreement with theoretical predictions.     

Domain wall creep in epitaxial ferroelectric Pb(Zr(0.2)Ti(0.08)O(3) thin films

Ferroelectric switching and nanoscale domain dynamics were investigated using atomic force microscopy on monocrystalline Pb(Zr(0.2)Ti(0.8))O(3) thin films. Measurements of domain size versus writing time reveal a two-step domain growth mechanism, in which initial nucleation is followed by radial domain wall motion perpendicular to the polarization direction. The electric field dependence of the domain wall velocity demonstrates that domain wall motion in ferroelectric thin films is a creep process, with the critical exponent mu close to 1. The dimensionality of the films suggests that disorder is at the origin of the observed creep behavior.     

Thermal effects on domain wall depinning from a single notch

The statistical behavior of the domain wall depinning from a notch placed in a thin ferromagnetic wire is studied by means of a stochastic one-dimensional model which considers the wall as a rigid object inside a parabolic potential at room temperature. This analysis reveals the key role of thermal fluctuations on the current and field-induced domain wall depinning, and it allows for direct comparison with experiments in order to gain information on the nonadiabaticity degree of the spin torque.     

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.     

Classical dependence of the domain wall velocity on the magnetic field in garnet ferrite films with orthorhombic magnetic anisotropy

The dependence of the domain wall velocity V on the acting magnetic field H is investigated for bismuth-containing single-crystal garnet ferrite films with orthorhombic magnetic anisotropy. It is shown that this dependence includes both the initial linear portion and a saturation portion and exhibits a complex behavior. This behavior is explained within the model of domain wall motion with spin wave radiation.     

Domain dynamics and magneto-electronics in magnetic thin films and nanowires

Recent theoretical results are highlighted that illustrate some of the interesting phenomena associated with magnetic domain boundary walls. Two problems will be discussed: dynamics associated with domain wall propagation, and effects related to spin transport through domain walls. For the first problem, an example of wall interaction and motion through a random potential will be discussed with reference to the general problem of roughening transitions. Images of domain dynamics in thin films of ion irradiated Co reveal a de-roughening transition associated with long range magnetostatic interactions between pairs of domain walls. A scaling theory of this transition is described in which a curious type of dynamic hysteresis can occur. For the second problem, results from calculations of ballistic charge and spin transport through domain boundary walls are discussed in terms of an effective circuit model.     

Solitary deflection waves on the supersonic domain wall in yttrium orthoferrite

The moving antiferromagnetic vortices are accompanied by solitary deflection waves. These waves allow to investigate generation and nonlinear dynamics of the antiferromagnetic vortices on the moving domain wall with the help of the two- and three-fold digital high speed photography. On the quasi-relativistic domain wall the vortex dynamics is quasi-relativistic with the limiting velocity c=20 km/s, which is equal to the spin-wave velocity. The solitary deflection waves dynamics can be explained assuming existence of the gyroscopic force. A theory for the gyroscopic force in the orthoferrite domain wall is elaborating by A.K. Zvezdin et al. currently. We present a comparison of the theoretical and experimental results on the dynamics of the solitary deflection waves, which accompany the antiferromagnetic vortices in the domain wall of orthoferrites.     

Spin-transfer torques in antiferromagnetic metals from first principles

In spite of the absence of a macroscopic magnetic moment, an antiferromagnet is spin-polarized on an atomic scale. The electric current passing through a conducting antiferromagnet is polarized as well, leading to spin-transfer torques when the order parameter is textured, such as in antiferromagnetic noncollinear spin valves and domain walls. We report a first principles study on the electronic transport properties of antiferromagnetic systems. The current-induced spin torques acting on the magnetic moments are comparable with those in conventional ferromagnetic materials, leading to measurable angular resistances and current-induced magnetization dynamics. In contrast to ferromagnets, spin torques in antiferromagnets are very nonlocal. The torques acting far away from the center of an antiferromagnetic domain wall should facilitate current-induced domain wall motion.     

Investigation of magnetoimpedance effect on electrodeposited NiFe/Cu wire using inductance spectroscopy

In this report, inductance spectroscopy (IS) has been used as a tool to investigate the thickness dependence of magnetoimpedance (MI) on electrodeposited NiFe thin films. An MI value as high as 140% has been observed under an applied magnetic field of 76 Oe at 300 kHz frequency for a film thickness of 6.8 μm. This result is in sharp contrast to earlier reports in literature showing monotonous increase in MI as a function of thickness. Maximum of MI was found at an optimum film thickness whose position varies with frequency. These reports exhibiting strong frequency dependence of MI prompted us to investigate the underlying physics using IS. The origin of MI lies in the combined effect of domain wall motion and spin rotation, which contributes to permeability. A parallel inductance and resistance (LR) circuit in series with series LR circuit model has been proposed as an equivalent electrical model to describe the property of these coated wires. The circuit elements have been linked with the phenomenon of domain wall motion and spin rotation. The experimental results obtained appear to be consistent with the proposed equivalent circuit model.     

Giant current-driven domain wall mobility in (Ga,Mn)As

We study theoretically hole current-driven domain wall dynamics in (Ga,Mn)As. We show that the spin-orbit coupling causes significant hole reflection at the domain wall, even in the adiabatic limit when the wall is much thicker than the Fermi wavelength, resulting in spin accumulation and mistracking between current-carrying spins and the domain wall magnetization. This increases the out-of-plane nonadiabatic spin-transfer torque and consequently the current-driven domain wall mobility by 3 to 4 orders of magnitude. Trends and magnitude of the calculated domain wall current mobilities agree with experimental findings.     

Thermally assisted current-driven domain-wall motion

Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive Langevin equations that describe the nonzero-temperature dynamics of a rigid domain wall. We derive an expression for the average drift velocity of the domain wall r(dw) as a function of the applied current, and find qualitative agreement with recent magnetic semiconductor experiments. Our model implies that at any nonzero-temperature r(dw) initially varies linearly with current, even in the absence of nonadiabatic spin torques.     

Stress dependence of the domain wall dynamics in the adiabatic regime

In this work, we determine the domain wall velocity in the low field region and study the domain dynamics in as-cast and annealed bi-stable amorphous glass-covered Fe_77.5Si_7.5B₁₅ microwires. In particular, from the relation between the domain wall velocity and magnetic field in the adiabatic regime, the power-law critical exponent β, the critical field H₀ and the domain wall damping η were obtained. It has been verified that the main source of domain wall damping is the eddy current and spin relaxation, both with a strong relation with the magnetoelastic energy. This energy term is changed by the axial applied stress, which, by its time, modifies the damping mechanisms. It was also verified that the domain wall damping terms present different behavior at low (mainly eddy currents) and high applied stress (spin relaxation).