Jean Anne Currivan-Incorvia

Logic circuit prototypes for three-terminal magnetic tunnel junctions with mobile domain walls

Spintronic computing promises superior energy efficiency and nonvolatility compared to conventional field-effect transistor logic. But, it has proven difficult to realize spintronic circuits with a versatile, scalable device design that is adaptable to emerging material physics. Here we present prototypes of a logic device that encode information in the position of a magnetic domain wall in a ferromagnetic wire. We show that a single three-terminal device can perform inverter and buffer operations. We demonstrate one device can drive two subsequent gates and logic propagation in a circuit of three inverters. This prototype demonstration shows that magnetic domain wall logic devices have the necessary characteristics for future computing, including nonlinearity, gain, cascadability, and room temperature operation.

360° domain walls: stability, magnetic field and electric current effects

The formation of 360° magnetic domain walls (360DWs) in Co and Ni80Fe20 thin film wires was demonstrated experimentally for different wire widths, by successively injecting two 180° domain walls (180DWs) into the wire. For narrow wires (≤50 nm wide for Co), edge roughness prevented the combination of the 180DWs into a 360DW, and for wide wires (200 nm for Co) the 360DW was unstable and annihilated spontaneously, but over an intermediate range of wire widths, reproducible 360DW formation occurred. The annihilation and dissociation of 360DWs was demonstrated by applying a magnetic field parallel to the wire, showing that annihilation fields were several times higher than dissociation fields in agreement with micromagnetic modeling. The annihilation of a 360DW by current pulsing was demonstrated.

Effects of Edge Taper on Domain Wall Structure and Current-Driven Walker Breakdown in a Ferromagnetic Thin Film Wire

Domain walls in a ferromagnetic thin film wire with rectangular cross section adopt transverse wall (TW) or vortex wall (VW) geometries depending on the magnetic material and the wire width and thickness. However, experimentally wires can have a trapezoidal cross section if they are made by liftoff using an undercut resist profile. Micromagnetic modeling shows that tapering of the wire not only promotes the formation of a TW over a VW, but also increases the critical current value and the domain wall velocity at which Walker breakdown occurs, providing a potential route to higher speed domain wall devices.

Edge-modulated perpendicular magnetic anisotropy in [Co/Pd]n and L10-FePt thin film wires

Thickness modulation at the edges of nanostructured magnetic thin films is shown to have important effects on their perpendicular magnetic anisotropy. Thin film wires with tapered edges were made from [Co/Pd]20 multilayers or L10-FePt films using liftoff with a double layer resist. The effect of edge taper on the reversal process was studied using magnetic force microscopy and micromagnetic modeling. In [Co/Pd]20 the anisotropy was lower in the tapered edge regions which switched at a lower reverse field compared to the center of the wire. The L10-FePt wires showed opposite behavior with the tapered regions exhibiting higher anisotropy.

Field and Current Driven Magnetic Domain Wall Motion in Disordered A1-FePt Nanowires

Arc-shaped magnetic FePt nanowires (10nm thick, 375nm wide) displaying in-plane anisotropy were fabricated. A vortex domain wall was formed in the middle of the arc after in-plane saturation in a field perpendicular to the wire. Translational domain wall propagation was observed at a field of 40 Oe applied parallel to the wire, while complete propagation to the end of the arc occurred at 100 Oe. Domain wall displacement at low velocity was achieved by current pulses of 2.5 1011 A m2 at zero applied magnetic field, comparable to the zero-field threshold current density of Permalloy. A field-assisted spin transfer torque drove the domain wall 1.8m to the end of the arc.

Micromagnetic modeling of domain wall motion in sub-100-nm-wide wires with individual and periodic edge defects

Reducing the switching energy of devices that rely on magnetic domain wall motion requires scaling the devices to widths well below 100 nm, where the nanowire line edge roughness (LER) is an inherent source of domain wall pinning. We investigate the effects of periodic and isolated rectangular notches, triangular notches, changes in anisotropy, and roughness measured from images of fabricated wires, in sub-100-nm-wide nanowires with in-plane and perpendicular magnetic anisotropy using micromagnetic modeling. Pinning fields calculated for a model based on discretized images of physical wires are compared to experimental measurements. When the width of the domain wall is smaller than the notch period, the domain wall velocity is modulated as the domain wall propagates along the wire. We find that in sub-30-nm-wide wires, edge defects determine the operating threshold and domain wall dynamics.

The Spatial Resolution Limit for an Individual Domain Wall in Magnetic Nanowires

Magnetic nanowires are the foundation of several promising nonvolatile computing devices, most notably magnetic racetrack memory and domain wall logic. Here, we determine the analog information capacity in these technologies, analyzing a magnetic nanowire containing a single domain wall. Although wires can be deliberately patterned with notches to define discrete positions for domain walls, the line edge roughness of the wire can also trap domain walls at dimensions below the resolution of the fabrication process, determining the fundamental resolution limit for the placement of a domain wall. Using a fractal model for the edge roughness, we show theoretically and experimentally that the analog information capacity for wires is limited by the self-affine statistics of the wire edge roughness, a relevant result for domain wall devices scaled to regimes where edge roughness dominates the energy landscape in which the walls move.

Effect of Magnetostatic Interactions on Stochastic Domain Wall Motion in Sub-100 nm Wide Nanowires

Magnetic field-driven domain wall motion is investigated in closely spaced sub-100 nm wide Co nanowires. Anticorrelations appear in the spatial distribution of pinning sites within weakly interacting nanowires, with a reduced probability of pinning adjacent to another pinning site over a mean correlation length similar to the line width roughness. In contrast, strong magnetostatic interactions between domain walls in adjacent nanowires eliminate the correlations, reducing the domain wall propagation distance for a given applied magnetic field.

Spintronic logic circuit and device prototypes utilizing domain walls in ferromagnetic wires with tunnel junction readout

We present a prototype of a switch that encodes information in the position of a magnetic domain wall (DW) in a ferromagnetic wire. The information is written using spin transfer torque and/or spin hall effect and read out using a magnetic tunnel junction (MTJ). We build prototypes and show that a single three-terminal device can perform buffer/inverter operations, which can be used as AND/NAND gates. We demonstrate one device can drive two subsequent gates, and we show bit propagation in a circuit of three inverters.