Domain Walls In Ferromagnetic Nanowires Research Articles

Local spin transfer torque and magnetoresistance in domain walls with variable width

Use of a spin polarized current for the manipulation of magnetic domain walls in ferromagnetic nanowires has been the subject of intensive research for many years. Recently, due to technological advancements, creating nano-contacts with special characteristics is becoming more and more prevalent. We now present a full quantum investigation of the magnetoresistance and the spin transfer torque in a domain wall, which is embedded in a nano-contact of Ni$_{80}$Fe$_ {20}$, where the size of the domain wall becomes a relevant tunable parameter. The dependence on the domain wall width as well as the spatial dependence of the torque along the domain wall can be analyzed in complete detail. The magnetoresistance drops with increasing domain wall width as expected, but also shows characteristic modulations and points of resonant spin-flip transmission. The spin transfer torque has both significant in-plane and out-of-plane contributions even without considering relaxation. A closer inspection identifies contributions from the misalignment of the spin density for short domain walls as well as an effective gauge field for longer domain walls, both of which oscillate along the domain wall.

Synthetic ferrimagnet nanowires with very low critical current density for coupled domain wall motion

Domain walls in ferromagnetic nanowires are potential building-blocks of future technologies such as racetrack memories, in which data encoded in the domain walls are transported using spin-polarised currents. However, the development of energy-efficient devices has been hampered by the high current densities needed to initiate domain wall motion. We show here that a remarkable reduction in the critical current density can be achieved for in-plane magnetised coupled domain walls in CoFe/Ru/CoFe synthetic ferrimagnet tracks. The antiferromagnetic exchange coupling between the layers leads to simple Néel wall structures, imaged using photoemission electron and Lorentz transmission electron microscopy, with a width of only ~100 nm. The measured critical current density to set these walls in motion, detected using magnetotransport measurements, is 1.0 × 1011 Am−2, almost an order of magnitude lower than in a ferromagnetically coupled control sample. Theoretical modelling indicates that this is due to nonadiabatic driving of anisotropically coupled walls, a mechanism that can be used to design efficient domain-wall devices.

Trajectory and chirality of vortex domain walls in ferromagnetic nanowires with an asymmetric Y-branch

The motion and trajectory of vortex domain walls (VDWs) driven by magnetic field were investigated in Fe80Ni20 nanowires with an asymmetric Y-shape branch. By using the focused magneto-optical Kerr effect, we have probed the injection, pinning, and propagation of VDWs in the branch and in the wire beyond the branch entrance. Hysteresis cycles measured at these points show 3 and 4 jumps in the magnetization reversal, respectively. Micromagnetic simulations were carried out to obtain the number of jumps in the hysteresis cycles, and the magnetization process involved in each jump. Based on simulations and from the size of the jumps in the measured hysteresis cycles, one obtains the histogram of the domain wall type probability. While in the branch domain walls of different types are equiprobable, in the nanowire vortex domain walls with counter clockwise and clockwise chiralities and transverse-down domain walls are measured with probabilities of 65%, 25%, and 10%, respectively. These results provide an additional route to select the trajectory and chirality of VDWs in magnetic nanostructures.

Enhanced magnetoresistance and pinning–depinning processes of vortex domain walls in ferromagnetic nanowires

We have investigated the pinning–depinning processes of vortex domain walls (VDW) in Permalloy nanowires with asymmetric triangular notches by means of magnetoresistance measurements and micromagnetic simulations. Through a model based on the anisotropic magnetoresistance (AMR) and the magnetization structure obtained from simulations, the magnetoresistance when the VDW passes stretched through the notch was calculated. Besides the depinning field, as it is known, also the magnetoresistance drop depends on the domain wall chirality. Measurements show that the resistance drop for clockwise (CW) chirality is ≈180 m (38%) larger than for the counterclockwise (CCW) chirality. From the VDW resistance calculations it becomes clear which domain wall parameters, like domain wall structure, chirality and wall width, play a role on the magnetoresistance. These results offer an additional route to better controlling the VDW motion, which can be beneficial to applications.

Domain Wall Pinning by Antiferromagnets in Magnetic Nanowires

The use of antiferromagnets to pin domain walls in ferromagnetic nanowires was investigated using simulations. Pinning was effective for both in-plane and perpendicularly magnetized nanowires with transverse, Néel and Bloch domain walls. Domain walls could be depinned using external applied fields or electric currents and a pulsed current mode could be used to shift domain walls from one pinning site to the next.

Voltage-controlled domain wall traps in ferromagnetic nanowires

Electrical control of magnetism has the potential to bring about revolutionary new spintronic devices, many of which rely on efficient manipulation of magnetic domain walls in ferromagnetic nanowires. Recently, it has been shown that voltage-induced charge accumulation at a metal-oxide interface can influence domain wall motion in ultrathin metallic ferromagnets, but the effects have been relatively modest and limited to the slow, thermally activated regime. Here we show that a voltage can generate non-volatile switching of magnetic properties at the nanoscale by modulating interfacial chemistry rather than charge density. Using a solid-state ionic conductor as a gate dielectric, we generate unprecedentedly strong voltage-controlled domain wall traps that function as non-volatile, electrically programmable and switchable pinning sites. Pinning strengths of at least 650 Oe can be readily achieved, enough to bring to a standstill domain walls travelling at speeds of at least ~20 m s(-1). We exploit this new magneto-ionic effect to demonstrate a prototype non-volatile memory device in which voltage-controlled domain wall traps facilitate electrical bit selection in a magnetic nanowire register.

Position-dependent spontaneous motion of the magnetic domain wall in ferromagnetic nanowires

We have investigated the dependence of the spontaneous dynamics of the magnetic domain wall in ferromagnetic nanowires on the initial position of the domain wall without any external field. Using micromagnetic simulations, we have observed in detail a spontaneous magnetic domain wall motion for various wire widths and thicknesses. We have found that the average speed of the spontaneous domain wall motion significantly depends on the wire width and thickness, as well as the starting position of the domain wall motion. We conclude that the spontaneous magnetic domain wall motion should be considered very carefully in practical design of spintronic devices based on magnetic domain wall motion.

Stability of precessing domain walls in ferromagnetic nanowires

We show that recently reported precessing solution of Landau-Lifshitz-Gilbert equations in ferromagnetic nanowires is stable under small perturbations of initial data, applied field and anisotropy constant. Linear stability is established analytically, while nonlinear stability is verified numerically.

Micromagnetic Study of Forced Oscillation of Magnetic Domain Wall in Ferromagnetic Nanowires with Variation of Damping Constant

We have investigated a DW motion driven by AC external field with a variation of damping parameter α in Landau–Lifshitz–Gilbert equation as well as a variation of an oscillating force frequency by means of a micromagnetic simulation. Straight ferromagnetic wires having a DW at the wire center is considered, where the DW is assumed to behave like a free quasiparticle. With variation of strength and frequency of the external AC field, we have observed that relative phase delays between the forced DW motion and the AC field depends on the damping constant. The overall behavior is explainable with a single-degree-of-freedom model of a damped forced oscillation without a restoring force.

Interaction of Antiparallel Transverse Domain Walls in Ferromagnetic Nanowires

The interaction of antiparallel transverse domain walls in ferromagnetic nanowires was investigated via micromagnetic simulation with systematic variations of the external field strength as well as the wire thickness. The interaction of antiparallel transverse walls after domain wall collision exhibited damped multiple collisions due to the rigid structure of the antiparallel transverse walls. The detailed process during the multiple collisions was analyzed via the Fast Fourier Transform technique, along with a careful examination of the inner spin structures of the colliding domain walls. It was found that a frequency peak of multiple collisions shifted to a higher peak position as the external field strength increases. With a stronger field strength of around a few hundred mT, it was found that two antiparallel transverse walls were finally annihilated with formation of complex antivortex structures.

Magnetic imaging of the pinning mechanism of asymmetric transverse domain walls in ferromagnetic nanowires

The pinning of asymmetric transverse magnetic domain walls by constrictions and protrusions in thin permalloy nanowires is directly observed using the Fresnel mode of magnetic imaging. Different domain wall (DW)/trap configurations are initialized using in situ applied magnetic fields, and the resulting configurations are imaged both at remanence and under applied fields. The nature of the chirality dependent pinning potentials created by the traps is clearly observed. The effect of the asymmetry of the DW is discussed. Micromagnetic simulations are also presented, which are in excellent agreement with the experiments.

Domain-Wall Motion in Ferromagnetic Nanowires Driven by Arbitrary Time-Dependent Fields: An Exact Result

We address the dynamics of magnetic domain walls in ferromagnetic nanowires under the influence of external time-dependent magnetic fields. We report a new exact spatiotemporal solution of the Landau-Lifshitz-Gilbert equation for the case of soft ferromagnetic wires and nanostructures with uniaxial anisotropy. The solution holds for applied fields with arbitrary strength and time dependence. We further extend this solution to applied fields slowly varying in space and to multiple domain walls.

Detection of the static and kinetic pinning of domain walls in ferromagnetic nanowires

Two distinct pinning mechanisms named as kinetic and static pinning of magnetic domain wall (DW) are experimentally resolved. Both the pinning situations are realized at an artificial notch on U-shaped Permalloy nanowires, depending on the initial DW states, moving or pinned. The kinetic depinning field—a critical field for a moving DW to be trapped at a notch—is revealed to be distinguishably smaller than the static depinning field—a critical field to depin a trapped DW at the notch. Based on one-dimensional collective model, the discrepancy is explained by the tilting angle of the moving DW.

Direct observation of the spin structures of vortex domain walls in ferromagnetic nanowires

The spin structures of vortex domain walls forming in a ferromagnetic ${\text{La}}_{0.6}{\text{Sr}}_{0.4}{\text{MnO}}_{3}$ nanowire have been directly observed using Lorentz microscopy. We visualized the in-plane anisotropic structures, which exhibited twofold rotational symmetry and revealed that the simultaneous reversal of the vortex chirality and the magnetization directions of separated domains occurs at all the pinning sites. The observations indicate that the chirality is determined by the magnetization directions and the magnetic anisotropy induced at the pinning sites.

Dynamics of Domain Walls in Magnetic Nanostrips

We express the dynamics of domain walls in ferromagnetic nanowires in terms of collective coordinates, generalizing Thiele's steady-state results. For weak external perturbations the dynamics is dominated by a few soft modes. The general approach is illustrated on the example of a vortex wall relevant to recent experiments with flat nanowires. A two-mode approximation gives a quantitatively accurate description of both the steady viscous motion of the wall in weak magnetic fields and its oscillatory behavior in moderately high fields above the Walker breakdown.

Current-induced torques in magnetic metals: Beyond spin-transfer

Current-induced torques (CITs) on ferromagnetic (FM) nanoparticles and on domain walls in FM nanowires are normally understood in terms of transfer of conserved spin angular momentum between spin-polarized currents and the magnetic condensate. In a series of recent articles, we have discussed a microscopic picture of CITs in which they are viewed as following from exchange fields produced by the misaligned spins of current carrying quasiparticles. This picture has the advantage that it can be applied to systems in which spin is not approximately conserved. More importantly, this point of view makes it clear that CITs can also act on the order parameter of an antiferromagnetic (AFM) metal, even though this quantity is not related to total spin. In this informal and intentionally provocative review we explain this picture and discuss its application to antiferromagnets.

Microscopic theory of current-driven domain wall motion

The dynamics of a domain wall in ferromagnetic nanowires under electric current is theoretically studied. The driving mechanism is shown to depend on wall thickness. Thick walls, as in metallic wires, are propelled by the spin torque arising from spin polarization of current, while thin walls, as in nanocontacts and magnetic semiconductors, are driven by the pressure from charge current. The condition for small-current operation, which is a key issue when integrated into devices, is discussed.

Electronic transport through domain walls in ferromagnetic nanowires: Coexistence of adiabatic and nonadiabatic spin dynamics

We study the effect of a domain wall on the electronic transport in ferromagnetic quantum wires. Due to the transverse confinement, conduction channels arise. In the presence of a domain wall, spin up and spin down electrons in these channels become coupled. For very short domain walls or at high longitudinal kinetic energy, this coupling is weak, leads to very few spin flips, and a perturbative treatment is possible. For very long domain wall structures, the spin follows adiabatically the local magnetization orientation, suppressing the effect of the domain wall on the total transmission, but reversing the spin of the electrons. In the intermediate regime, we numerically investigate the spin-dependent transport behavior for different shapes of the domain wall. We find that the knowledge of the precise shape of the domain wall is not crucial for determining the qualitative behavior. For parameters appropriate for experiments, electrons with low longitudinal energy are transmitted adiabatically while the electrons at high longitudinal energy are essentially unaffected by the domain wall. Taking this co-existence of different regimes into account is important for the understanding of recent experiments.

The Effect of Magnetization Defects on DW Motion in Ferromagnetic Nanowires

The propagation of domain walls in ferromagnetic nanowires offers the possibility of ultrafast logic devices and high density data storage. The understanding and control of domain wall motion has become more desirable recently as these devices come closer to realization. Domain walls can only be propagated by small magnetic fields or their movement becomes hindered by the precessional motion of the magnetic dipoles, also called the Walker breakdown effect1. Through the use of Landau-Lifshitz Gilbert Simulation, we report a method of eliminating the Walker breakdown. This investigation also offers some insight into how these proposed devices might perform in the high energy radiation of space for extended periods of time.Â