C. Murapaka

Transverse Domain Wall Profile for Spin Logic Applications

Domain wall (DW) based logic and memory devices require precise control and manipulation of DW in nanowire conduits. The topological defects of Transverse DWs (TDW) are of paramount importance as regards to the deterministic pinning and movement of DW within complex networks of conduits. In-situ control of the DW topological defects in nanowire conduits may pave the way for novel DW logic applications. In this work, we present a geometrical modulation along a nanowire conduit, which allows for the topological rectification/inversion of TDW in nanowires. This is achieved by exploiting the controlled relaxation of the TDW within an angled rectangle. Direct evidence of the logical operation is obtained via magnetic force microscopy measurement.

Effect of magnetic field on the electronic transport in trilayer graphene.

The perpendicular magnetic field dependence of the longitudinal resistance in trilayer graphene at various temperatures has been systematically studied. For a fixed magnetic field, the trilayer graphene displays an intrinsic semiconductor behavior over the temperature range of 5-340 K. This is attributed to the parabolic band structure of trilayer graphene, where the Coulomb scattering is a strong function of temperature. The dependence of resistance on the magnetic field can be explained by the splitting of Landau levels (LLs). Our results reveal that the energy gap in the trilayer graphene is thermally activated and increases with √B.

Reconfigurable logic via gate controlled domain wall trajectory in magnetic network structure

An all-magnetic logic scheme has the advantages of being non-volatile and energy efficient over the conventional transistor based logic devices. In this work, we present a reconfigurable magnetic logic device which is capable of performing all basic logic operations in a single device. The device exploits the deterministic trajectory of domain wall (DW) in ferromagnetic asymmetric branch structure for obtaining different output combinations. The programmability of the device is achieved by using a current-controlled magnetic gate, which generates a local Oersted field. The field generated at the magnetic gate influences the trajectory of the DW within the structure by exploiting its inherent transverse charge distribution. DW transformation from vortex to transverse configuration close to the output branch plays a pivotal role in governing the DW chirality and hence the output. By simply switching the current direction through the magnetic gate, two universal logic gate functionalities can be obtained in this device. Using magnetic force microscopy imaging and magnetoresistance measurements, all basic logic functionalities are demonstrated.

Direct observation of domain wall evolution at a bifurcation in magnetic network structures

We report on the magnetization dynamics at a bifurcation in a dual-branch magnetic network structure. When a transverse domain wall (DW) propagates through the network, interaction with an edge defect at the bifurcation leads to the transformation of the DW from transverse to vortex. The topological charge is conserved as the DW moves through the bifurcation, and this charge conservation is intrinsically linked to a −1/2 topological defect in the system. Magnetic force microscopy (MFM) imaging enables the direct observation of defect displacement during DW transformation, which induces a selective switching in the branch of the network structure.

In-plane current induced domain wall nucleation and its stochasticity in perpendicular magnetic anisotropy Hall cross structures

Hall cross structures in magnetic nanowires are commonly used for electrical detection of magnetization reversal in which a domain wall (DW) is conventionally nucleated by a local Oersted field. In this letter, we demonstrate DW nucleation in Co/Ni perpendicular magnetic anisotropy nanowire at the magnetic Hall cross junction. The DWs are nucleated by applying an in-plane pulsed current through the nanowire without the need of a local Oersted field. The change in Hall resistance, detected using anomalous Hall effect, is governed by the magnetic volume switched at the Hall junction, which can be tuned by varying the magnitude of the applied current density and pulse width. The nucleated DWs are driven simultaneously under the spin transfer torque effect when the applied current density is above a threshold. The possibility of multiple DW generation and variation in magnetic volume switched makes nucleation process stochastic in nature. The in-plane current induced stochastic nature of DW generation may find applications in random number generation.

Remote driving of multiple magnetic domain walls due to topological interaction

We present a method to drive multiple domain walls in the absence of direct current application in a coupled nanowire system. The domain walls were driven by a combination of remote coupling and exchange repulsion force from the domain wall compressions. The domain walls were compressed as they were unable to annihilate each other due to having similar topological charges. The compressions are present between the subsequent domain walls, which allow them to be driven as a group in the coupled nanowire system.

Direct observation of deterministic domain wall trajectory in magnetic network structures.

Controlling the domain wall (DW) trajectory in magnetic network structures is crucial for spin-based device related applications. The understanding of DW dynamics in network structures is also important for study of fundamental properties like observation of magnetic monopoles at room temperature in artificial spin ice lattice. The trajectory of DW in magnetic network structures has been shown to be chirality dependent. However, the DW chirality periodically oscillates as it propagates a distance longer than its fidelity length due to Walker breakdown phenomenon. This leads to a stochastic behavior in the DW propagation through the network structure. In this study, we show that the DW trajectory can be deterministically controlled in the magnetic network structures irrespective of its chirality by introducing a potential barrier. The DW propagation in the network structure is governed by the geometrically induced potential barrier and pinning strength against the propagation. This technique can be extended for controlling the trajectory of magnetic charge carriers in an artificial spin ice lattice.

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.

Coupled domain wall oscillations in magnetic cylindrical nanowires

We report on transverse domain wall (DW) dynamics in two closely spaced cylindrical nanowires. The magnetostatically coupled DWs are shown to undergo an intrinsic oscillatory motion along the nanowire length in addition to their default rotational motion. In the absence of external forces, the amplitude of the DW oscillation is governed by the change in the frequency of the DW rotation. It is possible to sustain the DW oscillations by applying spin-polarized current to the nanowires to balance the repulsive magnetostatic coupling. The current density required to sustain the DW oscillation is found to be in the order of 105 A/cm2. Morover, our analysis of the oscillation reveals that the DWs in cylindrical nanowires possess a finite mass.

Metastable magnetic domain in bifurcated nanowire network probed by domain wall magnetoresistance

We investigate the nanowire network with one input and two outputs for the magneto-resistive signal processing by angle dependent field. The effect of topological defect in the bifurcated nanowire and the angle dependent field leads to a pinned domain wall (DW) and subsequently the local magnetic domain reversal at the junction of triple nanowire network. A transverse DW propagates in a selected part of branch in a network with an alternating magnetic field. The asymmetric MR curve demonstrates how the magnetization configuration affects to the change of resistance due to the interplay between the local magnetization and the propagation of DW at the selected nanowire. The MR measurement reveals that the bifurcated branch is able to obtain the distinct signals via the bifurcated current paths in the magnetic nanowire network. In the experiment, the resistance-sensitive signals related to local magnetization reversal and DW were collected at a low field. The low external field induces the deformation of domain magnetization configuration and DW in the bifurcated branches in nanowire network without any injected DW through a selected input nanowire branch. Then, the MR curve obtained from the nanowire network was compared with micromagnetic simulation configurations.