Magnetic hysteresis in array of magnetic nanostructures by block copolymers

Summary form only given. Synthesis of nanopatterned magnetic materials offers advanced capabilities in tailoring material structures and opens up new opportunities for engineering innovative devices (i .e . electronic and biomedical) . In the frame of magnetic materials, the most demanding application consists in fabricating high-density arrays for use in data storage and magnetic sensors for spintronics [1] . In the last decade, many routes for the reliable fabrication of magnetic nanostructures have been extensively investigated, including top-down lithography and bottom-up self-assembly processes . Conventional electron beam lithography (EBL) soon turned out to be limited by low-speed and high costs while self-assembling emerged as a viable and handy alternative technique for designing nanostructures over a wide area on magnetic thin films . In the last decade, nanolithography routes based on self-assembling of polystyrene nanospheres resulted to be a viable and easy-to-use process to pattern a variety of magnetic thin films with mean nanostructure diameter as low as 70 nm [2] . To overcome such a limitation, the capability of soft materials such as block copolymers (BC) to form a rich variety of low-dimensional, uniform periodic patterns have been exploited . This class of polymers offers unique opportunities to develop large area nanometer scale features having domain spacing typically dependent on molecular weight, segment size, and the strength of interaction between the blocks . In this work, block-copolymer (BCP) - based lithography has been exploited to fabricate uniform, densely spaced nanometer - scaled on Ni₈₀Fe₂₀ and Co sputtered thin film having 10 nm thickness by a novel process shown in Fig 1 (panel 1) . In particular, a Random Copolymer (RCP) brush layer was grafted on the Si substrate in order to obtain the surface neutralization . A PS-b-PMMA Block Copolymer (BCP) film was subsequently deposited on the RCP, obtaining cylind- ical features perpendicularly oriented with respect to the substrate . In this way, a patterned nanostructure having cylindrical features has been realized . The self-organization is promoted by means of a thermal annealing higher than the glass transition temperature of the BCP [3] . The magnetic layer is then deposited by RF sputtering . A careful evaluation of the propagation effect of the nanometric pattern to the magnetic thin film has been performed with the aim to optimise nanostructures and preserve the magnetic properties of the continuous film . A systematic morphological study has been made by Scanning Electron (SEM) microscopy . A SEM image of a Ni₈₀Fe₂₀ dot array is reported as an example in Fig . 1 (panel 2, mean dot diameter 17 nm) where a very fine, well ordered dotted structure arranged in an hexagonal lattice is observed . Room-temperature magnetic behavior has been studied by magnetisation measurements by means of ultra-sensitive magnetometry techniques following the hysteresis loop changes at each stage of the synthesis process . Hysteresis loops of patterned films have been measured as a function of temperature in the interval 5 - 300 K . Selected loops indicating their temperature behavior are reported in Fig 1 (panel 3) . A sudden increase of coercive field is observed in the curve at 5 K . In particular, coercive field behavior as function of measuring temperature is characterised by an almost constant value up to reach 50 K followed by an increase up to the lowest temperature . Micromagnetic simulations have been performed to analyze the effect on hysteresis loops of the patterned film morphological properties taking into account different magnetic anisotropy values and investigating the influence of non-complete hexagonal order (presence of nanodomains and local defects, as dot interconnections) on magnetic behavior.