Easy Axis Orientation of Chemically Synthesized L10 FePt Nanoparticles

S. Kang, Z. Jia, S. Shi, D.E. Nikles, and J.W. Harrell
Center for Materials for Information Technology
The University of Alabama, Tuscaloosa, AL 35487

Chemically synthesized FePt nanoparticles have attracted considerable interest because of their potential for ultra-high density recording media. The synthesis originally reported by the IBM group of Sun and co-workers results in randomly oriented, chemically disordered particles that are magnetically unstable [1]. High-temperature annealing is required to convert the particles to the high-anisotropy, chemically ordered L10 phase. Fully ordered particles have the potential for one bit per particle recording with storage densities nearly 1000 times that of current state-of-the-art hard disk media. One of the most difficult materials issues is to produce an array of chemically ordered nanoparticles with aligned magnetic easy axes. The most obvious approach, thermally annealing in a large magnetic field, does not work. We have recently succeeded in obtaining easy-axis alignment using an alternative approach similar to that used by the magnetic tape industry [2]. Our method is to chemically synthesize FePt nanoparticles in the L10 phase, apply a dispersion of the particles to a substrate, and allow the dispersion to dry in an in-plane magnetic field. Partially ordered nanoparticles with diameter ~6-7 nm and HC ~ 1.3 kOe Oe were synthesized by modifying the IBM method by using a high-temperature solvent and refluxing at ~360oC. The particles were dispersed in a commercial PVC binder and aligned in a magnetic field. The figures below show the in-plane parallel and perpendicular loops and the angular dependence of remanence for a 20 kOe drying field. This is the first report of significant easy-axis orientation of chemically synthesized L10 FePt nanoparticles. A model has been developed that is in semi-quantitative agreement with the measured dependence of the orientation on the orienting field.

[1] S.Sun, C.B.Murray, D.Weller, L.Folks, A.Moser, Science 287, 1987 (2000).
[2] S. Kang, Z. Jia, S. Shi, D.E. Nikles, J.W. Harrell, Appl. Phys. Lett. 86, 062503 (2005).