Self-Assembled Ferrimagnet-Polymer Composites for Magnetic Recording Media Qiu Dai, David Berman, Kumar Virwani, Jane Frommer, Pierre-Olivier Jubert, Michelle Lam, Teya Topuria, Wayne Imaino, and Alshakim Nelson* IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120 ABSTRACT A self-assembled magnetic recording medium was created using colloidal ferrimagnetic building blocks. Monodisperse cobalt ferrite nanoparticles (CoFe 2 O 4 ) 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 CoFe 2 O 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. KEYWORDS Ferrimagnetic nanoparticles, nanoparticle-polymer composite, magnetic recording T he synthesis and self-assembly of magnetic nanopar- ticles to form stimuli-responsive materials generates significant interest for a host of applications that include magnetic recording media, 1 magnetic sensors, 2 nanomedicine, 3 and ferrofluids. 4 The composition, shape, and size of the nanoparticle dictates its magnetic character, as well as its potential applications. Superparamagnetic nanoparticles (SPMNPs) (at 298 K) have magnetic dipole moments that fluctuate in the absence of an external magnetic field, and hence, are suitable as magnetic reso- nance imaging contrast agents and biosensors. Magnetic storage media require ferro- or ferrimagnetic nanoparticles (FMNPs), which act as individual nanomagnets at 298 K in the absence of an external magnetic field. Methodologies for the solution phase synthesis of nanoparticles enable the formation of particles with defined composition, size, shape, and magnetic properties, although the majority of these reports focus on SPMNPs. 5 A practical solution-based route to FMNPs that affords nanocrystals of uniform shape and size would benefit the design and formation of self-as- sembled magnetic media for high density storage applica- tions including hard disk drives and magnetic tape. Unlike SPMNPs, FMNPs are subject to magnetically induced particle aggregation, which hampers both the solution phase syn- thesis of FMNPs and their self-assembly. These interparticle attractive forces pose a significant challenge to the stabiliza- tion of these particles in solution. 6 Several groups have reported strategies that utilize poly- meric molecules in the synthesis and/or the assembly of FMNPs. For example, Pyun and co-workers synthesized polymer-coated cobalt nanoparticles using polymers func- tionalized with end groups that interact with the growing FMNP surface to function as a steric buffer between particles and decrease interparticle magnetic interactions. 7 In this case, the polymers mediated the growth of the metallic cobalt nanoparticles and also enabled stable dispersions of the particles in solvent. In an alternative strategy, Sun and co-workers demonstrated the formation of multilayered assemblies of alternating FePt nanoparticles (in their super- paramagnetic amorphous phase) and poly(ethylene imine) using a layer-by-layer process. 8 This approach enables the self-assembly of these particles while simultaneously cir- cumventing magnetically induced aggregation associated with FMNPs. Upon the formation of the desired number of layers, the MNP-polymer composite was annealed at ∼1000 K to transform the FePt nanoparticles into their L10 phase with ferromagnetic behavior (298 K). Herein, we present a strategy for self-assembling FMNPs for magnetic storage devices which involves (1) the synthesis of monodisperse CoFe 2 O 4 FMNPs via a solution phase colloidal route, (2) the formation of a diblock copolymer shell around each individual FMNP, and (3) the assembly of the FMNPs and subsequent physical realignment of their mag- netic easy axis in an externally applied magnetic field. The * To whom correspondence should be addressed, alshak@us.ibm.com. Received for review: 06/29/2010 Published on Web: 07/27/2010 pubs.acs.org/NanoLett © 2010 American Chemical Society 3216 DOI: 10.1021/nl1022749 | Nano Lett. 2010, 10, 3216–3221