Edge-modulated perpendicular magnetic anisotropy in [Co/Pd][subscript n] and L1[subscript 0]-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][subscript 20] multilayers or L1[subscript 0]-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][subscript 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 L1[subscript 0]-FePt wires showed opposite behavior with the tapered regions exhibiting higher anisotropy.National Science Foundation (U.S.) (Award ECCS1101798

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

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.National Science Foundation (U.S.) (Contract ECCS-1101798)United States. Dept. of Energy. Office of Science. Graduate Fellowship Progra

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