S. A. Majetich
Department of Physics, Carnegie Mellon University, Pittsburgh, PA, 15213
Abstract - Nanoparticles were once ubiquitous in magnetic recording because they formed the basis of tape media. Today’s hard disk drives use thin films of nanograined alloys as media, but future information storage may use different technologies.
Self-assembly can create regular arrays with features smaller than those achievable by lithographic methods. Bit patterned magnetic recording media has been proposed based on self-assembling block copolymer templates, and arrays with a pitch of 27 nm have already been demonstrated. Scaling to a smaller pitch remains challenging, due to the driving forces for self-assembly of block copolymers. One alternative is to use self-assembling nanoparticle arrays, which could potentially enable a pitch as small as 5-6 nm. Here the nanoparticles would not be used directly to store information, since particles in self-assembled arrays have random crystallographic orientations. With appropriate processing, they can be used as an etch mask for an underlying film.
The nanomasking technique to prepare two types of structures, ordered arrays and isolated nanopillars, will be described. In some cases guided self-assembly is used to form more complex nanomask structures. Such templates can not only improve the quality of the ordering, but could help to create tracks on a disk. Strategies that combine nanomasking with nanoimprint are discussed.
Nanomasking is potentially useful not only for magnetic recording media, but also for more fundamental studies of the electronic properties of nanoparticles. Small nanoparticles are hard to study due to the difficulty of attaching electrical leads. Monodisperse particles made by chemical methods typically have organic surfactant coatings that complicate the nature of the electrical contact and interface quality. The nanomasking approach combines the advantages of thin film growth techniques, which enable multilayer structures with crystallographic orientation, epitaxial interfaces, and good electrical contacts, with the small feature size of a nanoparticle, where size-dependent changes in electronic properties are expected.
The electronic properties of magnetic tunnel junction (MTJ) nanopillars were measured using conductive atomic force microscopy (CAFM). Here a conductive scanning probe is used to locate and make electrical contact with the nanopillar. In CAFM the force rather than the current between the tip and sample is used as feedback to positioning. Once the pillar is located, the tip is brought into contact for two-point probe electrical measurements. The state of the free layer is determined by tunnel magnetoresistance. After modifying the commercial CAFM for operation with larger currents, it is also possible to switch the free layer using spin torque transfer.