HANCOCK, N.H. The density of information stored on magnetic films has increased by a factor of 2 million since disk drives were introduced by IBM Corp. in 1957. Drives that store 70 Gbits/square inch are on the market now, and research projects have demonstrated densities three times as high. The dizzying pace, which outstrips the growth curve of silicon VLSI technology, has been sustained by fundamental materials research.
There was no sign of a slowdown in such innovations last week at the fall meeting of the Materials Research Society. Bruce Terris from the Hitachi San Jose (Calif.) Research Center described a process for creating cobalt-palladium nanoscale islands on silicon dioxide that he believes could carry magnetic media densities into the terabit/square-inch realm. While theoretical predictions point to a physical limit somewhere around 100 Gbits/inch2 for current approaches, the nano-island technique manages to avoid some of the factors leading to that limit. The process was developed in a joint project with researchers at nearby IBM Almaden Research Center.
The process has the ability to create vertically polarized magnetic domains, which reduce the crosstalk between neighboring bits. Previously, IBM researchers produced a medium using vertical domains that could store 200 Gbits/inch2.
The second innovation is a regular nanopatterning process that isolates individual magnetic domains on islands, which is essential in beating the thermal limit imposed by physics. The ability of magnetic domains to hold their magnetic polarization decreases linearly with their size, and the basic limit occurs when that figure matches the thermal energy of the magnetic medium. At that point, thermal noise would be able to switch magnetic orientations randomly.
Nanoimprint lithography was used to create an etch mask on a silicon dioxide substrate. Next, multilayers of cobalt and palladium were sputtered onto the SiO2 islands. Because the resulting domains were isolated from a continuous film, they showed higher values for coercivity and thus were more resistant to thermal effects.
Other projects are attempting to find nanostructured materials that isolate magnetic particles.
A research group at Rochester Institute of Technology presented a process that can integrate arrays of soft magnetic particles onto semiconductors for incorporation into circuits. Soft spinel ferrite nanoparticles were suspended in an alcohol solution containing magnesium nitrate, which binds to the particles and gives them an electric charge. The solution, deposited over a silicon substrate, is then subjected to electrophoresis, which uses an electrostatic field to distribute them over the surface.
The process was used to build integrated microtoroidal magnets and inductors. The researchers expect that the technique will be able to realize other devices, such as microtransformers.
Diblock copolymers are a popular medium for creating regular nanostructures because of their self-assembling properties. The polymers consist of two species that form a regular alternating structure on a molecular scale. One species can be opened up using a process called ring-opening metathesis polymerization, and a material of interest deposited into the cavity.
A project at the University of Maryland (College Park) used the system to create arrays of 5- to 15-nm-diameter iron-oxide nanoparticles suspended in the polymer and separated by around 37 nm.
The self-assembly properties of organic molecules are being used by a group at Germany's University of Duisburg-Essen to create regular nanoarrays of iron-platinum magnetic particles. Magnetic nanoparticles were first created by sintering, and then coated with organic phospholipids. When deposited on silicon substrates, the coated particles spontaneously organized themselves into a regular hexagonal pattern with a spacing of 1.2 nm. This regularity is critical to getting higher data storage densities, the researchers said.