Nano Letters

Self-assembled ferrimagnet-polymer composites for magnetic recording media

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A self-assembled magnetic recording medium was created using colloidal ferrimagnetic building blocks. Monodisperse cobalt ferrite nanoparticles (CoFe2O4) were synthesized using solution-based methods and then stabilized in solution using the amphiphilic diblock copolymer, poly(acrylic acid)-b-poly(styrene) (PAA-PS). The acid groups of the acrylate block bound the polymer to the nanoparticle surface via multivalent interactions, while the styrene block afforded the magnetic nanoparticle-polymer complex solubility in organic solvents. Moreover, the diblock copolymer improved the colloidal stability of the ferrimagnetic CoFe2O 4 nanoparticles by reducing the strong interparticle magnetic interactions, which typically caused the ferrimagnetic nanoparticles to irreversibly aggregate. The nanoparticle-polymer complex was spin-coated onto a silicon substrate to afford self-organized thin film arrays, with the interparticle spacing determined by the molecular weight of the diblock copolymer. The thin film composite was also exposed to an external magnetic field while simultaneously heated above the glass transition temperature of poly(styrene) to allow the nanoparticles to physically rotate to align their easy axes with the direction of the magnetic field. In order to demonstrate that this self-assembled ferrimagnet-polymer composite was suitable as a magnetic recording media, read/write cycles were demonstrated using a contact magnetic tester. This work provides a simple route to synthesizing stabilized ferrimagnetic nanocrystals that are suitable for developing magnetic recording media. © 2010 American Chemical Society.