Indra Purnama

Guided current-induced skyrmion motion in 1D potential well

Magnetic skyrmions are particle-like magnetization configurations which can be found in materials with broken inversion symmetry. Their topological nature allows them to circumvent around random pinning sites or impurities as they move within the magnetic layer, which makes them interesting as information carriers in memory devices. However, when the skyrmion is driven by a current, a Magnus force is generated which leads to the skyrmion moving away from the direction of the conduction electron flow. The deflection poses a serious problem to the realization of skyrmion-based devices, as it leads to skyrmion annihilation at the film edges. Here, we show that it is possible to guide the movement of the skyrmion and prevent it from annihilating by surrounding and compressing the skyrmion with strong local potential barriers. The compressed skyrmion receives higher contribution from the spin transfer torque, which results in the significant increase of the skyrmion speed.

Helical domain walls in constricted cylindrical NiFe nanowires

Reducing the magnetic shape anisotropy of a cylindrical NiFe nanowire allows the formation of two vortices with opposite chirality at the two ends. At relatively low aspect ratio these two vortices are connected via a gradual rotation of the magnetization over a short region, which forms a three-dimensional helical domain wall. Micromagnetic simulations reveal that it is possible to control the number of helical domain walls in the cylindrical nanowire by geometrical constrictions engineering. A technique to create constricted Ni95Fe5/Ni87Fe13 multilayered nanowires is demonstrated, and magnetic force microscopy imaging was carried out to confirm the prediction of simulated helical domain walls.

Current‐induced coupled domain wall motions in a two‐nanowire system

In two closely spaced nanowires system, where domain walls exist in both of the nanowires, applying spin‐polarized current to any of the nanowire will induce domain wall motions in the adjacent nanowire. The zero‐current domain wall motion is accommodated by magnetostatic interaction between the domain walls. As the current density is increased, chirality flipping is observed in the adjacent nanowire where no current is applied. When current is applied to both nanowires, the coupled domain wall undergoes oscillatory motion. Coupling breaking is observed at a critical current density which varies in a non‐linear manner with respect to the interwire spacing.

Coupled Néel domain wall motion in sandwiched perpendicular magnetic anisotropy nanowires

The operating performance of a domain wall-based magnetic device relies on the controlled motion of the domain walls within the ferromagnetic nanowires. Here, we report on the dynamics of coupled Néel domain wall in perpendicular magnetic anisotropy (PMA) nanowires via micromagnetic simulations. The coupled Néel domain wall is obtained in a sandwich structure, where two PMA nanowires that are separated by an insulating layer are stacked vertically. Under the application of high current density, we found that the Walker breakdown phenomenon is suppressed in the sandwich structure. Consequently, the coupled Néel domain wall of the sandwich structure is able to move faster as compared to individual domain walls in a single PMA nanowire.

Depinning assisted by domain wall deformation in cylindrical NiFe nanowires

We report on transverse domain wall (DW) depinning mechanisms at the geometrical modulations in NiFe cylindrical nanowires. The DW depinning field and current density always follow opposite trends with diameter modulation. For current driven DW, the depinning current density decreases with increasing notch depth. This interesting behavior arises due to a combination of DW deformation and rotation at the pinning site. With increasing anti-notch height, two distinct depinning mechanisms are observed for both field and current driven DW. Above a critical height, the DW transformation from transverse to vortex configuration leads to a change in the potential barrier. For field-driven, the barrier is lowered, whereas for current-driven, the barrier increases. The increase in the potential barrier for the current driven DW is due to the appearance of an intrinsic pinning within the anti-notch.

Remote Walker breakdown and coupling breaking in parallel nanowire systems

In a multiple nanowire system, we show by micromagnetic simulations that a transverse domain wall in a current-free nanowire can undergo a remote Walker breakdown when it is coupled to a nearby current-driven domain wall. Moreover, for chirality combination with the highest coupling strength, the remote Walker breakdown preceded the current-induced Walker breakdown. The Walker breakdown limit of such coupled systems has also been shifted towards higher current densities, where beyond these, the coupling is shown to be broken.

Crossover in domain wall potential polarity as a function of anti-notch geometry

We have carried out a systematic study on domain wall (DW) pinning at an anti-notch in a Ni80Fe20 nanowire. Micromagnetic studies reveal that the potential polarity experienced by the DW at the anti-notch is a function of both DW chirality and anti-notch geometry. A transition in the potential disruption experienced by the DW is observed when the anti-notch height-to-width ratio (HAN/WAN) is 2. This transition is due to the relative orientation of the spins in the anti-notch with respect to the transverse component of the DW. When the anti-notch acts as a potential barrier, the DW undergoes damped oscillations prior to coming to an equilibrium position. The equilibrium position is a strong function of the anti-notch dimensions when the HAN/WAN ratio <2 and is constant for HAN/WAN ? 2. The effect of the relative orientation between the spins in the anti-notch and the transverse component of the DW on the shape of the potential is discussed.

Multi-vortex states in magnetic nanoparticles

We demonstrate a fabrication technique to create cylindrical NiFe magnetic nanoparticles (MNPs) with controlled dimensions and composition. MNPs thicker than 200 nm can form a double vortex configuration, which consists of a pair of vortices with opposite chirality. When MNPs thicker than 300 nm are relaxed after saturation, it forms a frustrated triple vortex state which produces a higher net magnetization as verified by light transmissivity measurements. Therefore, a greater magnetic torque can be actuated on a MNP in the triple vortex state.

Collective Motions Assisted by Magnetostatic Interactions in Coupled Domain Wall System

We have investigated the coupling between a transverse head-to-head domain wall and a transverse tail-to-tail domain wall in closely spaced Ni80Fe20 nanowires. Micromagnetic simulation shows that the two domain walls are coupled to each other. When spin-polarized current is applied to any of the nanowires, the two domain walls move in the same direction. The Walker breakdown limit in such system is shifted to higher current density. We have also observed that the coupling is broken for some combinations of initial magnetizations of the two domain walls.

Dynamics of three-dimensional helical domain wall in cylindrical NiFe nanowires

We report on a micromagnetic study on the dynamics of current-driven helical domain wall (DW) in cylindrical NiFe nanowires. The helical DW is a three-dimensional transition region between magnetizations with clockwise and anticlockwise vortex orientations. A minimum current density is needed to overcome an intrinsic pinning to drive the helical DW, and the propagation along the nanowire is accompanied by a rotational motion. As the driving current strength is increased, the rotation ceases while the DW propagates at an increased velocity. However, a velocity barrier is experienced which results in the decrease of the DW mobility. Throughout its motion, the propagated helical DW maintains a stable profile without showing any sign of structural breakdown even at relatively high driving current.