SEM image of preferred orientation of 400 nm thick rod-shaped magnetic particles.

<p>The particles were fabricated using horizontal movement of the AAL stage and evaporation of gold/permalloy/gold tri-layers. The sample was lifted-off in TMAH solution, rinsed in DI water to remove the traces of TMAH and dried on a silicon surface in the absence of any external magnetic field. The observed preferred orientation of the particles is the result of shape anisotropy dominating the material properties. The rods’ alignment into the long-stranded pattern is the manifestation of the switching of easy axis along the length of the particles due to their shape.</p

Summary of coercivity and remnant magnetization of magnetic particles with permalloy layers 40–300 nm thick.

<p>Summary of coercivity and remnant magnetization of magnetic particles with permalloy layers 40–300 nm thick.</p

Rotation of rod-shaped particles (A) and cylinder-shaped particles (B) along the easy axis.

<p>The particles were printed to achieve specific geometries, which allows the manifestation of shape anisotropy. The samples were placed in the VSM with the easy axis parallel to the direction of the applied field and rotated in increments of 15 degrees to measure the change in coercivity as the sample moves from easy axis to hard and further to easy axis, making half-a-turn of 180 degrees. Cylindrical particles 300 nm in diameter and 400 nm tall exhibit a coercivity of approximately 47 Oe, while the rod-shaped particles 300 nm wide, 400 nm tall and 1.2 micron long exhibit a coercivity of 167 Oe.</p

SEM image of 5 µm and 3 µm magnetic particles on a silicon surface (top, right and left) and VSM graphs (bottom, left and right) of 40, 70, 100, and 200 nm thick magnetic particles on silicon surface.

<p>The 5 and 3 micron particles were printed using nickel mesh masks with 5 and 3-micron openings. Particle size standard deviation was approximately 7% of the mean based on ImageJ analysis. The evaporation of various thicknesses of magnetic layers creates the difference in the coercive field of the particles. The coercivity of the particles increases with increasing magnetic layer thickness. The VSM measurements were taken with the easy axis of the particles’ magnetization oriented along the direction of the applied magnetic field.</p

Coercivity curves for 5 µm, 3 µm, 1 µm, 300 nm, and rod-shaped particles.

<p>The coercivity (A) and the remnant magnetization (B) values were extracted from the VSM plots described above for different particle sizes and varying magnetic layer thicknesses.</p

Particle Fabrication Sequence.

<p>Spin-coating of PMGI and PMMA on a clean silicon wafer (a) is followed by exposure of the sample to a broad beam of helium ions through a stencil mask to form the pattern (b). During development, the exposed areas of PMMA wash away, and a subsequent etch in TMAH removes the PMGI layer underneath the PMMA openings (c). The particles of interest are evaporated as stacked layers of 10 nm gold, 10 nm of permalloy, and 10 nm gold (d). A lift-off procedure removes the evaporated metal on top of the PMMA layer (e) and the PMGI layer is etched in TMAH solution (f) to release the particles.</p

SEM image of 1micron (A), 300 nm (B), rod-shaped particles (C) and their corresponding VSM measurements (D, E, and F).

<p>The shape anisotropy dominates the material properties of the 300 nm thick evaporated particles, resulting in higher coercivity and lower remnant magnetization. The results of VSM measurements of rod-shaped particles show the highest coercivity among all particles, measuring as high as 37 Oe and remnant magnetization decrease from 0.7 to 0.3 with increase of layer thickness.</p

A schematic of helium beam-induced protein patterning on PEG.

<p>Panels A and B show the electron images of 35 µm mesh and 300 nm masks, respectively. Top-down helium beam exposure through a mask (C) of grafted PEG on surface allowed proteins preferentially to attach on irradiated regions (D) to form patterns.</p

Elemental XPS spectra of beam protected (unexposed) and irradiated (exposed at different doses) PEG before and after protein incubation.

<p>Before (“unexp”) and after (“exp”) helium beam exposure, carbon C1s signals (A) show characteristic alkyl and ether peaks at 284.6 eV and 286.6 eV binding energies, respectively. The presence of oxygen O1s signals (B) at 532 eV and the absence of nitrogen N1s signals (C) at 400 eV also were observed. Subsequent incubation with avidin shows additional C1s peak at 288 eV (D), similar O1s signals at 532 eV (E) and existence of N1s peak at 400 eV (F) binding energies for beam exposed PEG.</p

ATR-FTIR spectra of beam protected (unexposed) and irradiated (exposed) PEG at different helium beam doses.

<p>Spectra of grafted PEG subjected to different beam doses and incubated in PBS for 1 hour show the characteristic ether and methoxy peaks at 1120 cm⁻¹ and 2870 cm⁻¹, respectively, which also are present in unexposed PEG.</p