Weiliang Gan

Highly Efficient Domain Walls Injection in Perpendicular Magnetic Anisotropy Nanowire

Electrical injection of magnetic domain walls in perpendicular magnetic anisotropy nanowire is crucial for data bit writing in domain wall-based magnetic memory and logic devices. Conventionally, the current pulse required to nucleate a domain wall is approximately ~1012 A/m2. Here, we demonstrate an energy efficient structure to inject domain walls. Under an applied electric potential, our proposed Π-shaped stripline generates a highly concentrated current distribution. This creates a highly localized magnetic field that quickly initiates the nucleation of a magnetic domain. The formation and motion of the resulting domain walls can then be electrically detected by means of Ta Hall bars across the nanowire. Our measurements show that the Π-shaped stripline can deterministically write a magnetic data bit in 15 ns even with a relatively low current density of 5.34 × 1011 A/m2. Micromagnetic simulations reveal the evolution of the domain nucleation – first, by the formation of a pair of magnetic bubbles, then followed by their rapid expansion into a single domain. Finally, we also demonstrate experimentally that our injection geometry can perform bit writing using only about 30% of the electrical energy as compared to a conventional injection line.

Deterministic Spin-Orbit Torque Induced Magnetization Reversal In Pt/[Co/Ni] n /Co/Ta Multilayer Hall Bars

Spin-orbit torque (SOT) induced by electric current has attracted extensive attention as an efficient method of controlling the magnetization in nanomagnetic structures. SOT-induced magnetization reversal is usually achieved with the aid of an in-plane bias magnetic field. In this paper, we show that by selecting a film stack with weak out-of-plane magnetic anisotropy, field-free SOT-induced switching can be achieved in micron sized multilayers. Using direct current, deterministic bipolar magnetization reversal is obtained in Pt/[Co/Ni]2/Co/Ta structures. Kerr imaging reveals that the SOT-induced magnetization switching process is completed via the nucleation of reverse domain and propagation of domain wall in the system.

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.

Spin orbit torques induced magnetization reversal through asymmetric domain wall propagation in Ta/CoFeB/MgO structures

The magnetization reversal induced by spin orbit torques in the presence of Dzyaloshinskii-Moriya interaction (DMI) in perpendicularly magnetized Ta/CoFeB/MgO structures were investigated by using a combination of Anomalous Hall effect measurement and Kerr effect microscopy techniques. By analyzing the in-plane field dependent spin torque efficiency measurements, an effective field value for the DMI of ~300 Oe was obtained, which plays a key role to stabilize Néel walls in the film stack. Kerr imaging reveals that the current-induced reversal under small and medium in-plane field was mediated by domain nucleation at the edge of the Hall bar, followed by asymmetric domain wall (DW) propagation. However, as the in-plane field strength increases, an isotropic DW expansion was observed before reaching complete reversal. Micromagnetic simulations of the DW structure in the CoFeB layer suggest that the DW configuration under the combined effect of the DMI and the external field is responsible for the various DW propagation behaviors.

Bi-directional high speed domain wall motion in perpendicular magnetic anisotropy Co/Pt double stack structures

We report bi-directional domain wall (DW) motion along and against current flow direction in Co/Pt double stack wires with Ta capping. The bi-directionality is achieved by application of hard-axis magnetic field favoring and opposing the Dzyloshinskii-Moriya interaction (DMI), respectively. The speed obtained is enhanced when the hard-axis field favors the DMI and is along the current flow direction. Co/Pt double stack is a modification proposed for the high spin-orbit torque strength Pt/Co/Ta stack, to improve its thermal stability and perpendicular magnetic anisotropy (PMA). The velocity obtained reduces with increase in Pt spacer thickness due to reduction in DMI and enhances on increasing the Ta capping thickness due to higher SOT strength. The velocity obtained is as high as 530 m/s at a reasonable current density of 1 × 1012 A/m2 for device applications. The low anisotropy of the device coupled with the application of hard-axis field aids the velocity enhancement by preventing Walker breakdown.

Magnetic nanoparticles for magnetomechanical cell destruction and magnetic hyperthermia agents

Recently, magnetic nanoparticles are gaining interest for use in magnetic biomedical applications, such as magnetomechanical cell destruction and magnetic hyperthermia. Biofunctionalized NiFe microdiscs, with the application of a low-frequency alternating magnetic field, have been used to demonstrate magnetomechanical cancer-cell destruction by generating an oscillatory motion that transmits a mechanical force to the cell [1]. A magnetization reversal can also occur in magnetic nanoparticles due to a high-frequency alternating magnetic field resulting in the production of thermal energy, which is expressed by the specific absorption rate (SAR) [2]. The heating ability of magnetic nanoparticles shows great potential for a non-invasive and powerful therapy technique for biomedical applications, such as magnetic hyperthermia. By focusing the magnetic nanoparticles at the tumor site, the temperature at the targeted region can be raised to 42-46 °C, which will greatly lower the viability of cancer cells. The advantage of these methods over the conventional cancer therapy is the localization of treatment of the cancer tumor, which minimizes the detrimental side effects experienced by the patient.

Observation of large planar Hall effect on the current induced spin-orbit effective fields in Ta/MgO/CoFeB/Ta

Current induced spin-orbit effective magnetic fields in metal/ferromagnet/ oxide trilayer provide an alternative switching mechanism that is more efficient than the conventional spin torque transfer.

SOT induced magnetization reversal in Pt/[Co/Ni] 2 /Co/Ta multilayer Hall bars without external magnetic field

Currend-induced spin-orbit torque (SOT) has attracted extensive attention as an efficient method of controlling the magnetization of ferromagnets.

Spin-orbit torque induced magnetization anisotropy modulation in Pt/(Co/Ni)4/Co/IrMn heterostructure

In this work, we show that domain wall (DW) dynamics within a system provide an alternative platform to characterizing spin-orbit torque (SOT) effective fields. In perpendicularly magnetized wires with a Pt/(Co/Ni)4/Co/IrMn stack structure, differential Kerr imaging shows that the magnetization switching process is via the nucleation of the embryo state followed by domain wall propagation. By probing the current induced DW motion in the presence of in-plane field, the SOT effective fields are obtained using the harmonic Hall voltage scheme. The effective anisotropy field of the structure decreases by 12% due to the SOT effective fields, as the in-plane current in the wire is increased.

2D transport of superparamagnetic microbeads on a ferromagnetic hexagonal nanolattice

Summary form only given. Surface-functionalized superparamagnetic (SPM) beads have been widely used to detect and manipulate chemical and biological agents in lab-on-a-chip systems. Recently, it has been shown that by exploiting the stray field generated by domain walls in magnetic nanostructures, it is possible to capture and couple a SPM bead to a domain wall. To store the captured bead, the domain walls can be pinned by fabricating geometrical defects on the nanotracks. Furthermore, the position of the coupled SPM bead can be pinpointed by measuring the magnetoresistance across nanotrack sections. However, studies on such systems have so far been limited to 1D transport. In this work, we develop a novel structure to manipulate SPM beads across a substrate surface. While domain wall trajectory under a constant field has been studied, we found that by applying a bias field in the direction of a branch, the domain wall can be forced to propagate in the direction of the selected branch. Replicating the structure into a lattice of 120° V-branches similar to a honeycomb pattern, we can finally shuttle the SPM beads across a 2D surface. By changing the width and thickness of the nanowire, the pinning potential energies of different domain boundaries were investigated. To verify the simulation results, a unit cell of the hexagonal lattice was fabricated using a combination of electron beam lithography and magnetron sputtering. A V-branch fabricated to study the amount of bias field needed to force the domain wall to propagate in the intended direction is shown. Our results show that it is indeed possible to shuttle SPM beads across a 2D surface with the application of a pulsed magnetic field with varying in-plane directions.