Magnetic properties of dot arrays made by block copolymer lithography

Two-dimensional arrays of magnetic particles have been made by a variety of lithographic methods such as electron-beam lithography and interference lithography. Block copolymers provide an alternative large-area nanolithography technique that is based on self-assembly. Block copolymers that micro-phase separate into a monolayer of domains can be used as templates to define arrays of close-packed magnetic particles, using a series of etching steps. In this work, we have used a block copolymer of polystyrene-polyferrocenyldimethylsilane (PSPFS) to form arrays of cobalt and permalloy dots with diameters of 30 nm and periods of 50 nm. The dots are formed by sequential etching of a (Co or NiFe)/W/silica stack using the PFS domains as an etch mask, to leave arrays. The magnetic films are made by evaporation and are polycrystalline, with thicknesses of up to 20 nm. The size of the magnetic dots, and the period of the structure, are controlled by the molecular weight of the polymer template. Magnetic measurements of patterned Co dot arrays with 30 nm diameter, 8 nm thick dots show that the array has an in-plane easy axis. The coercivity, measured by alternating force magnetometry, is 90 Oe at room temperature, increasing as the scan rate increases, characteristic of thermally-assisted reversal. For 15 nm thick Co dots of the same diameter, the array has an out-of-plane easy axis with coercivity 260 Oe; the change in net anisotropy is due to the increased aspect ratio and crystallographic texture of the Co dots. For applications such as patterned media, it is necessary to align the dots in specific locations on the substrate. This can be achieved by templating the self-assembly of the block copolymer by the use of substrate features such as steps. The authors show that alignment of dots can be achieved in one dimension by the use of surface topography consisting of steps in the substrate made lithographically. This causes the close-packed rows of dots to be aligned parallel to the steps. In this paper the time- and temperature-dependent magnetic properties of dot arrays will be discussed as a function of geometry, and compared with the behavior of arrays that are ordered using substrate features.