Real-space observation of current-driven domain wall motion in submicron magnetic wires

We report direct observation of current-driven magnetic domain wall (DW) displacement by using a well-defined single DW in a microfabricated magnetic wire with submicron width. Magnetic force microscopy visualizes that a single DW introduced in a wire is displaced back and forth by positive and negative pulsed current, respectively. The direct observation gives quantitative information on the DW displacement as a function of the intensity and the duration of the pulsed current. The result is discussed in terms of the spin-transfer mechanism.     

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.     

Influence of exchange coupling on current-driven domain wall motion in a nanowire

In this study, the effect of exchange stiffness constant on current-driven domain wall motion in nanowires with in-plane magnetic anisotropy (IMA) and perpendicular magnetic anisotropy (PMA) has been investigated using micromagnetic simulation. The critical current density in a nanowire with IMA decreases as the exchange stiffness constant decreases because the domain wall width at the upper edge of the nanowire narrows according to the decrease of the exchange stiffness constant. On the other hand, the critical current density in a nanowire with PMA slightly decreases contrary to that of IMA although the domain wall width reasonably decreases as the exchange stiffness constant decreases. The slight reduction rate of the critical current density is due to the increase of the effective hard-axis anisotropy of PMA nanowire.     

Theory of current-driven domain wall motion: spin transfer versus momentum transfer

A self-contained theory of the domain wall dynamics in ferromagnets under finite electric current is presented. The current has two effects: one is momentum transfer, which is proportional to the charge current and wall resistivity (rho(w)); the other is spin transfer, proportional to spin current. For thick walls, as in metallic wires, the latter dominates and the threshold current for wall motion is determined by the hard-axis magnetic anisotropy, except for the case of very strong pinning. For thin walls, as in nanocontacts and magnetic semiconductors, the momentum-transfer effect dominates, and the threshold current is proportional to V(0)/rho(w), V0 being the pinning potential.     

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 activated depinning of a narrow domain wall from a single defect

We describe the field induced depinning process of a magnetic domain wall (DW) from a single bidimensional nanometric defect. The DW propagates in a wire lithographed on a film with strong perpendicular anisotropy. We observe a statistical distribution of the relaxation time consistent with a Néel-Brown picture of magnetization reversal. This indicates that the nanometric DW can be considered as an ideal monodomain particle switching over a single energy barrier. Such a stochastic character of DW depinning has to be taken into account for spintronic applications.     

Temperature estimation in a ferromagnetic Fe-Ni nanowire involving a current-driven domain wall motion

We observed a magnetic domain wall (DW) motion induced by the spin-polarized pulsed current in a nanoscale Fe(19)Ni(81) wire using a magnetic force microscope. High current density, which is of the order of 10(11) A m(-2), was required for the DW motion. A simple method to estimate the temperature of the wire was developed by comparing the wire resistance measured during the DW motion with the temperature dependence of the wire resistance. Using this method, we found the temperature of the wire was proportional to the square of the current density and became just beneath at the threshold Curie temperature. Our experimental data qualitatively support this analytical model that the temperature is proportional to the resistivity, thickness, width of the wire and the square of the current density, and also inversely proportional to the thermal conductivity.     

Magnetic soft x-ray microscopy of the domain wall depinning process in permalloy magnetic nanowires

Full-field magnetic transmission x-ray microscopy at high spatial resolution down to 20 nm is used to directly observe field-driven domain wall motion in notch-patterned permalloy nanowires. The depinning process of a domain wall around a notch exhibits a stochastic nature in most nanowires. The stochasticity of the domain wall depinning sensitively depends on the geometry of the nanowire such as the wire thickness, the wire width, and the notch depth. We propose an optimized design of the nanowire for deterministic domain wall depinning field at a notch.     

Electric current effect on the energy barrier of magnetic domain wall depinning: origin of the quadratic contribution

The energy barrier of a magnetic domain wall trapped at a defect is measured experimentally. When the domain wall is pushed by an electric current and/or a magnetic field, the depinning time from the barrier exhibits perfect exponential distribution, indicating that a single energy barrier governs the depinning. The electric current is found to generate linear and quadratic contributions to the energy barrier, which are attributed to the nonadiabatic and adiabatic spin-transfer torques, respectively. The adiabatic spin-transfer torque reduces the energy barrier and, consequently, causes depinning at lower current densities, promising a way toward low-power current-controlled magnetic applications.     

Magneto-optical and micromagnetic simulation study of the current-driven domain wall motion in ferromagnetic (Ga,Mn)As

We have studied current-driven domain wall motion in modified Ga_0.95Mn_0.05As Hall bar structures with perpendicular anisotropy by using spatially resolved polar magneto-optical Kerr effect microscopy and micromagnetic simulation. Regardless of the initial magnetic configuration, the domain wall propagates in the opposite direction to the current with critical current of 1-2×10⁵ A/cm². Considering the spin-transfer torque term as well as various effective magnetic field terms, the micromagnetic simulation results are consistent with the experimental results. Our simulated and experimental results suggest that the spin-torque rather than Oersted field is the reason for current-driven domain wall motion in this material.     

Nonequilibrium domain pattern formation in mesoscopic magnetic thin film elements assisted by thermally excited spin fluctuations

The dynamic behavior in the evolving pattern of thermally assisted, nonequilibrium domains in magnetic thin-film elements undergoing ultrafast 180 degrees magnetization reversal was studied. Magnetization reversal enters a fully dynamic regime when the external field conditions are changed much faster than the sample is able to respond. The dynamic pathway develops a complexity not seen in quasistatic reversal but still retains a high level of order with well-developed dynamic domain patterns formed in response to subnanosecond transitions of the external applied magnetic field.     

Dynamics of a domain wall coupled to thermally agitated spins: A model of a rare earth ferrite-garnet

The nonlinear mobility of a domain wall in an idealized model of a RE ferrite-garnet is considered, the thermally agitated RE sublattice being described by the Landau-Lifshitz-Bloch equation [4] with longitudinal relaxation terms. It is shown that in some field intervals the DW velocity is small and governed by the longitudinal relaxation of the RE ions. This effect may be observed in sufficiently low magnetic fields near the compensation point of ferrites. If the dynamics of the RE spins is dissipationless, there is a non-analytic contribution to the DW damping due to the wake in the RE subsystem.     

Controlled pinning and depinning of domain walls in nanowires with perpendicular magnetic anisotropy

We investigate switching and field-driven domain wall motion in nanowires with perpendicular magnetic anisotropy comprising local modifications of the material parameters. Intentional nucleation and pinning sites with various geometries inside the nanowires are realized via a local reduction of the anisotropy constant. Micromagnetic simulations and analytical calculations are employed to determine the switching fields and to characterize the pinning potentials and the depinning fields. Nucleation sites in the simulations cause a significant reduction of the switching field and are in excellent agreement with analytical calculations. Pinning potentials and depinning fields caused by the pinning sites strongly depend on their shapes and are well explained by analytical calculations.     

Theory of thermally assisted electron hopping in amorphous solids

The reaction¹⁶O(n, α)¹³C was studied with 13.9 MeV neutrons using two different counter telescopes. Absolute differential cross sections were measured for the transitions to the ground state and to the 3.08 and 3.68+3.85 MeV levels of¹³C. The results are interpreted in terms of a direct reaction mechanism.     

Nonmonotonic effects of perpendicular magnetic anisotropy on current-driven vortex wall motions in magnetic nanostripes

In a magnetic nanostripe, the effects of perpendicular magnetic anisotropy(PMA) on the current-driven horizontal motion of vortex wall along the stripe and the vertical motion of the vortex core are studied by micromagnetic simulations.The results show that the horizontal and vertical motion can generally be monotonously enhanced by PMA. However, when the current is small, a nonmonotonic phenomenon for the horizontal motion is found. Namely, the velocity of the horizontal motion firstly decreases and then increases with the increase of the PMA. We find that the reason for this is that the PMA can firstly increase and then decrease the confining force induced by the confining potential energy. In addition, the PMA always enhances the driving force induced by the current.     

Analytic theory of wall configuration and depinning mechanism in magnetic nanostructure with perpendicular magnetic anisotropy

We present an analytic theory of the domain wall depinning in magnetic nanostructure with perpendicular magnetic anisotropy. The variational principle reveals that the wall is bent in the form of a circular arc which intersects the structure boundaries perpendicularly. The radius is inversely proportional to the magnetic field. With increasing the field the radius shrinks, followed by depinning from the constriction when the arc is not geometrically allowed. The depinning field is proportional to the sine of the constriction angle and the inverse of the constriction width. The validity of the theory is confirmed by comparison with the micromagnetic simulation.