Sarjoosing Goolaup

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.

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.

Observation of oscillatory resistance behavior in coupled Bernal and rhombohedral stacking graphene.

We report on the first observation of an anomalous temperature-dependent resistance behavior in coupled Bernal and rhombohedral stacking graphene. At low-temperature regime (<50 K) the temperature-dependent resistance exhibits a drop while at high-temperature regions (>250 K), the resistance increases. In the transition region (50-250 K) an oscillatory resistance behavior was observed. This property is not present in any layered graphene structures other than five-layer. We propose that the temperature-dependent resistance behavior is governed by the interplay of the Coulomb and short-range scatterings. The origin of the oscillatory resistance behavior is the ABCAB and ABABA stacking configurations, which induces tunable bandgap in the five-layer graphene. The obtained results also indicate that a perpendicular magnetic field opens an excitonic gap because of the Coulomb interaction-driven electronic instabilities, and the bandgap of the five-layer graphene is thermally activated. Potentially, the observed phenomenon provides important transport information to the design of few-layer graphene transistors that can be manipulated by a magnetic field.

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.

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.

Dependence of pinning on domain wall spin structure and notch geometry

In this work, we present a systematic investigation of the domain wall spin structure and notch geometries on the pinning field strength. We observed that for transverse domain wall, pinning is strongly dependent on the transversely varying energy profile of the wall. Domain walls are pinned at notches only when the notch provides a barrier to the higher energy component of the domain wall. For all notch shapes investigated, we observed that when the notch height/nanowire width > 0.3, the depinning field reaches a maximum and remains constant. We also note that the pinning of a domain wall at a notch is markedly sensitive to the angle of the notch with respect to the domain wall.

Dependence of domain wall stability on vortex chirality in asymmetric nanoring

We report on the direct observation of notch-free domain wall (DW) trapping and field history effect on the DW behavior in Ni80Fe20 asymmetric ring. We found that a 360° DW is trapped at the narrow arm while the ring adopts a vortex configuration. The stability of DW is dependent on the chirality of the vortex state and the external field direction. A 360° DW trapped in a clockwise vortex configuration is highly resistant to annihilation upon the application of +x field; the 360° DW trapped in an anticlockwise vortex breaks apart with a small +x field.

All-electrical deterministic single domain wall generation for on-chip applications

Controlling domain wall (DW) generation and dynamics behaviour in ferromagnetic nanowire is critical to the engineering of domain wall-based non-volatile logic and magnetic memory devices. Previous research showed that DW generation suffered from a random or stochastic nature and that makes the realization of DW based device a challenging task. Conventionally, stabilizing a Néel DW requires a long pulsed current and the assistance of an external magnetic field. Here, we demonstrate a method to deterministically produce single DW without having to compromise the pulse duration. No external field is required to stabilize the DW. This is achieved by controlling the stray field magnetostatic interaction between a current-carrying strip line generated DW and the edge of the nanowire. The natural edge-field assisted domain wall generation process was found to be twice as fast as the conventional methods and requires less current density. Such deterministic DW generation method could potentially bring DW device technology, a step closer to on-chip application.