Ordered arrays of identical sub-micrometric elements of a magnetic metal are attractive both for fundamental studies and for technological applications. Their study allows one to clarify the effect of dimension shrinking on the overall magnetic response of the array. Moreover, ordered magnetic lattices are promising candidates in data storage technology for high-density magnetic recording media [1]. In this work, the fabrication of ordered arrays of magnetic dots through electron-beam lithography (EBL) will be discussed.
Arrays of dots with square and hexagonal symmetry have been realised in order to study the effect of array geometry and dot size on the magnetic properties. Three lattice spacing values (0.7, 1 and 1.3 μm) were examined. In all these cases, dipolar interaction is negligible and nearly independent reversal of the magnetisation is achieved. The dot diameter was reduced from 500 to 300 nm with decreasing the exposure dose. Patterns of 100x100 μm² area were produced on Si (100) wafers covered by a 250 nm layer of PMMA as electron-sensitive resist. The magnetic dots have been obtained after the deposition of a thin film and a lift-off process for pattern transfer [2]. Fe, Co and Ni films with thickness of 50 nm have been grown by e-beam and thermal evaporation under vacuum of the pure metals. The morphological characterisation, performed by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM), revealed in most cases a good quality of the patterns. In addition, Magnetic Force Microscopy (MFM) has been exploited to investigate the magnetic domain structure of a single array element. Magnetisation curves were obtained at different temperatures using an alternating gradient magnetometer (AGM). The relation between magnetic response and geometrical properties of the array will be discussed.
[1] C. Haginoya et al. J. Appl. Phys 85 (1999), 8327
[2] J.I. Martin et al. J. Appl. Phys. 84 (1998), 411

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2. Low-temperature magnetic softening in sputtered FePtAg granular multilayers