Nanoscale Magnetic Materials for Energy-Efficient Spin Based Transistors

In this dissertation, I study the physical behavior of nanoscale magnetic materials and build spin-based transistors that encode information in magnetic domain walls. It can be argued that energy dissipation is the most serious problem in modern electronics, and one that has been resistant to a breakthrough. Wasted heat during computing both wastes energy and hinders further technology scaling. This is an opportunity for physicists and engineers to come up with creative solutions for more energy-efficient computing. I present the device we have designed, called domain wall logic (DW-Logic). Information is stored in the position of a magnetic domain wall in a ferromagnetic wire and read out using a magnetic tunnel junction. This hybrid design uses electrical current as the input and output, keeping the device compatible with charge- based transistors. I build an iterative model to predict both the micromagnetic and circuit behavior of DW- Logic, showing a single device can operate as a universal gate. The model shows we can build complex circuits including an 18-gate Full Adder, and allows us to predict the device switching energy compared to complementary metal-oxide semiconductor (CMOS) transistors. Comparing 15 nm feature nodes, I find DW-Logic made with perpendicular magnetic anisotropy materials, and utilizing both spin torque transfer and the Spin Hall effect, could operate with 1000× reduced switching energy compared to CMOS. I fabricate DW-Logic device prototypes and show in experiment they can act as AND and NAND gates. I demonstrate that one device can drive two subsequent devices, showing gain, which is a necessary requirement for fanout. I also build a clocked ring oscillator circuit to demonstrate successful bit propagation in a DW-Logic circuit and show that properly scaled devices can have improved operation. Through building the devices, I develop a novel fabrication method for patterning sub-25 nm magnetic wires with very low (~ 2 nm) average edge roughness. I apply the fabrication method to measuring the Spin Hall angle in epitaxially grown thin films and to studying the repeatability of domain wall motion in narrow wires. I also present a number of modeling results, including the effect of edge roughness on both magnetic tunnel junctions and domain walls.Physic

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

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

The formation of 360° magnetic domain walls (360DWs) in Co and Ni[subscript 80]Fe[subscript 20] 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.National Science Foundation (U.S.) (Award ECCS 1101798

Combining Graphoepitaxy and Electric Fields toward Uniaxial Alignment of Solvent-Annealed Polystyrene–<i>b</i>–Poly(dimethylsiloxane) Block Copolymers

We report a combined directing effect of the simultaneously applied graphoepitaxy and electric field on the self-assembly of cylinder forming polystyrene-<i>b</i>-poly­(dimethylsiloxane) block copolymer in thin films. A correlation length of up to 20 μm of uniaxial ordered striped patterns is an order of magnitude greater than that produced by either graphoepitaxy or electric field alignment alone and is achieved at reduced annealing times. The angle between the electric field direction and the topographic guides as well as the dimensions of the trenches affected both the quality of the ordering and the direction of the orientation of cylindrical domains: parallel or perpendicular to the topographic features. We quantified the interplay between the electric field and the geometry of the topographic structures by constructing the phase diagram of microdomain orientation. This combined approach allows the fabrication of highly ordered block copolymer structures using macroscopically prepatterned photolithographic substrates