Magnetic Nanoparticles: Applications for Granular Recording Media

Authors

J. W. Harrell Department of Physics and Astronomy, University of Alabama

Publication Date

7/15/04

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Abstract

Moore's Law has been the most famous symbol of the power of high technology in the last half of the 20th century. In 1965, Moore predicted that the number of transistors on a computer chip would double every 18 months. Indeed, the number of transistors has grown at a compound annual rate of 39% per year for more than 30 years. Meanwhile, the data storage density of magnetic hard drives has matched the pace of growth in data storage density (36% per year) until 1990, when it accelerated to 79% per year. Both technologies have grown at an amazing pace, providing higher-performance computing and higher-capacity data storage at lower costs, thereby fueling growth of the Internet. However, both technologies face scientific challenges that threaten to limit continued growth.

Modern magnetic data storage devices record data as magnetized bit cells in thin films, either polycrystalline metal films or particulate films (acicular iron particles in a polymer matrix). Each bit cell contains many grains or particles, and the signal-to-noise ratio depends on the number of particles in the bit volume. The data storage density of state-of-the-art hard disk drive systems is 100 gigabits per square inch, and the information storage industry is now pursuing basic research aimed at densities beyond 1 terabit per square inch. Increases in data storage density are achieved by scaling down the bit size, i.e., the bit length (increasing the bits per inch), the track width (increasing the track per inch), and the film thickness. In 2002, the bit cell dimension in state-of-the-art hard disk media was approximately 50 × 400 nm. If densities were to increase at a rate of 41% per year over the next decade (less than the last decade), then the bit cell would be 4 × 4 nm in 2012. This is less than the size of a single grain in current media and would necessitate a new paradigm in magnetic recording. The reduction in bit size using conventional recording methods means that magnetic grains and particle sizes must be decreased. However, the superparamagnetic limit threatens to limit the bit sizes that can be achieved using conventional recording methods. Superparamagnetism arises when the thermal stability factor (KV/kT) becomes small, where K is the anisotropy constant, V is the particle or grain volume, k is the Boltzmann constant, and T is the temperature. KV gives the magnetic anisotropy energy for a magnetic grain or particle, and as the volume decreases, the anisotropy energy decreases. As the ratio of anisotropy energy to thermal energy (KV/kT) decreases, the thermal stability of the magnetization decreases (i.e., subject to thermally activated magnetization reversal). When KV/kT is less than about 60, long-term data storage (> 10 years) is no longer reliable. Of course, the grain size can be decreased if K is increased. This has led to increased interest in materials with high values of magnetocrystalline anisotropy, and the L10 phase of FePt has found particular scrutiny because of its very high value of Ku (6 to 10 × 107 erg/cm2).

Here we report our progress on solving basic materials science problems, whose solution would enable self-assembled FePt nanoparticles to be used in high-density magnetic recording. We describe the synthesis of FePtAg, FePtAu, FePtCu, or FePtCo ternary alloy nanoparticles. The effect of added Ag, Au, or Cu on temperature required to transform the particles from the fcc phase to the L10 phase is reported along with the effect of added Co on the magnetic properties.