Vertical polarization of quantum magnets in high density arrays of nickel dots with small height-to-diameter ratio

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Abstract
Using interferometric lithography and postexposure processing we have fabricated high density (1.3×1010/in.2) magnetic nickel dot arrays on silicon substrates. The dots have the shape of truncated cones and are 75 nm in height and about 120 nm in diameter. The arrays are characterized using magnetic force microscopy (MFM). We demonstrate that such shallow dots with a small height-to-diameter ratio of only 0.63, show single-domain behavior with vertical, out-of-plane magnetization, i.e., along their short axis. The coercive field of these dots is drastically enlarged due to shape anisotropy and is exceeding the dot interaction strength by about one order of magnitude. Local manipulation of the magnetization state using an additional external field and the stray field of the MFM tip is demonstrated.