User‐programmed meso‐ to microscale 2D shapes using magnetic nanoparticles as building blocks with magnetic‐field‐directed self‐assembly are created. The assembly templates are magnetically recorded using perpendicular magnetic recording (PMR) media. The results demonstrate that PMR can template user‐designed features in two dimensions down to 30 nm in size, i.e., the nanoparticle diameter. It has been also shown that the nanoparticles assemble onto transitions between oppositely magnetized regions in the medium. At these transitions, the magnetic field gradients are extremely large (25 MT m−1 2 nm above the medium) and change rapidly with height (≈1015 T/m/m within 20 nm of the surface). It is found that 30 nm diameter particles assemble into 1–2 layers with feature widths ranging from 30 to 350 nm. It is hypothesized that large lateral growth can occur because the magnetic forces parallel to the disk extend up to 150 nm on either side of a recorded transition, falling off more slowly with distance than the vertical magnetic forces. Once lateral growth saturates, a second layer of nanoparticles begins to assemble on top of the first layer, suggesting strong potential for controlling layer‐by‐layer assembly through appropriate design of the medium and its resulting field gradient profiles.
We report magnetic properties of electrospun multiferroic nanofibers assembled into linear aggregates with an external magnetic field, and measured with the magneto-optical Kerr effect (MOKE) in a non-specular or scattering geometry (ScMOKE). Compared with bulk magnetometry, ScMOKE’s sensitivity to subtle differences between aggregates offers a route to determine local multiferroic coupling in disordered nanomaterials. CoFe2O4-BaTiO3 nanofiber agglomerates are assembled prior to measurement by suspending and aligning the fibers in a transparent air-cured polyvinyl alcohol solution. We detect the polarization change in light scattered from the fibers, collected at an off-specular angle in order to eliminate the background caused by substrate reflection. Averaged hysteresis loops from different agglomerates show a variety of unique structures. For our optical spot size of ∼15 m, multiple fibers, within a single agglomerate, are detected simultaneously, suggesting ScMOKE can distinguish local magnetization reversal fields that vary from fiber to fiber, as well as magnetic interactions between fibers.
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