International audienceElectron-beam ͑E-beam͒ directed assembly, which combines the long-range phase and placement registration of e-beam lithography with the sharp dot size and spacing uniformity of block copolymer self assembly, is considered highly promising for fabricating templates that meet the tight magnetic specifications required for write synchronization in bit patterned media magnetic recording systems. In our study, we show that this approach also yields a narrower magnetic switching field distribution ͑SFD͒ than e-beam patterning or block copolymer self-assembly alone. We demonstrate that the pattern uniformity, i.e., island diameter and placement distributions are also important for achieving tight magnetic SFDs. Bit patterned media ͑BPM͒ magnetic recording systems at densities in excess of 1 Tb/ in 2 require an extremely tight lithographic bit placement accuracy and a narrow size distribution of less than 5% in order to achieve good write synchronization between the recording head and the patterned media. 1 Nanoimprint technology with master molds fabricated via e-beam directed assembly of block copolymer films is considered a very promising cost-effective approach for creating highly uniform magnetic dot patterns over large areas. 2,3 In addition to the tight lithographic specifications for BPM, a narrow switching field distribution ͑SFD͒ of the magnetic dots is critical to ensure exact bit addressability without overwriting adjacent bits

In order to adjust the reversal field of high anisotropy [Co(2.8 Å)/Pd(9 Å)]8 bit patterned media (BPM), one may increase the individual Co thickness to change the overall anisotropy or alternatively combine the high anisotropy multilayer with a lower anisotropy material, thus creating a heterogeneous laminated system. In the current study, we find that [Co/Pd]8−N[Co/Ni]N hard-soft laminated BPM allows tuning the coercivity while maintaining a narrow normalized switching field distribution (σSFD/HC), whereas homogeneous structures with increased individual Co thickness show a more pronounced increase in σSFD/HC. Possible reasons for the different behavior, such as changes in strain and texture, are discussed.

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Using a combination of synchrotron radiation based magnetic imaging and high-resolution transmission electron microscopy we reveal systematic correlations between the magnetic switching field and the internal nanoscale structure of individual islands in bit patterned media fabricated by Co/Pd-multilayer deposition onto pre-patterned substrates. We find that misaligned grains at the island periphery are a common feature independent of the island switching field, while irregular island shapes and misaligned grains specifically extending into the center of an island are systematically correlated with a reduced island reversal field.

Magnetic ͓Co/ Pd͔ N multilayers grown on assemblies of spherical polystyrene particles were used as a model system for studying the influence of magnetic coupling on the magnetization reversal by changing the size of the nanoparticles as well as the total thickness of the deposited magnetic film. The coercive field and the switching field distribution are found to be strongly dependent on the size of the nanocaps and on the total thickness of the magnetic layer, indicating a strong influence of the magnetic dipole-dipole interaction on the magnetization reversal of the entire array of nanocaps. Moreover, magnetic viscosity measurements allowed the estimation of the magnetic activation volume representing the effect of thermal activation on the switching process. It was found that the magnetic activation volume is substantially smaller compared to the volume of the nanocap and almost independent of the number of bilayers, and thus the total thickness of the nanocap, supporting an inhomogeneous magnetization reversal process.