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Experimental interest into nanoparticles has maintained its momentum over recent decades, with scientists discovering more about their unique properties, how they can be used in a growing number of applications, and developing advanced techniques for producing these particles.
Back in 2000, scientists developed a method of producing ferromagnetic FePt nanocrystal superlattices which were believed to hold much promise for the future of ultrahigh-density magnetic recording media applications.
Below, we discuss the method of producing these nanocrystal superlattices as well as their current, modern applications.
How are Ferromagnetic FePt Nanocrystal Superlattices made?
A team of scientists from Watson Research Center and Almaden Research Center in New York and California developed a method of synthesizing iron-platinum (FePt) nanoparticles by reducing platinum acetylacetonate and decomposing iron pentacarbonyl in oleic acid and oleyl amine stabilizers. The method was reported to create FePt nanoparticles with controlled size and composition along with ferromagnetic FePt nanocrystal superlattices with tunable inter-particle spacings.
First, to produce the FePt nanoparticles, the team used oleic acid and oleyl amine to stabilize and prevent oxidation of the monodisperse FePt colloids. Next, the metal salts were reduced to metal particles by the polyol process. Following this, Fe(CO)5 was thermally decomposed to produce Fe particles. Both these processes took place in the presence of oleic acid and oleyl amine, resulting in the production of monodisperse FePt nanoparticles.
The team demonstrated that the resultant FePt nanoparticles could be readily controlled. In controlling the molar ratio of iron carbonyl to the platinum salt, the researchers showed that the composition could be adjusted. The particle size was amended by growing monodisperse seed particles and adding reagents to grow the seeds to the required size. Finally, the particles were purified and isolated by adding a flocculent and centrifuging.
The next stage, where the FePt colloids are dispersed onto a substrate allowing the solvent to evaporate, creates the FePt nanoparticle superlattices. Studies have demonstrated that the particles created are monodisperse and readily self-assemble into 3D superlattices.
Applications of Ferromagnetic FePt Nanocrystal Superlattices
Once the method of creating ferromagnetic FePt nanocrystal superlattices was established, scientists foresaw their use in a number of applications, particularly in optical and electronic devices. Their properties of good chemical stability and large uniaxial magnetocrystalline anisotropy lend themselves to being integrated into permanent magnetic applications.
Their characteristic of having magnetic stability of their individual particles that scales with both the anisotropy constant and the particle volume, it was concluded that these particles would likely be influential in the development of ultrahigh-density magnetic recording media applications of the future.
More recent studies, however, have highlighted the issues that hinder the use of these superlattices in magnetic recording. It has been found that FePt has a high coercivity that vastly exceeds that of the writing field of available heads, which the head materials limit. Therefore, scientists are exploring a way to reduce the writing field to overcome this limitation.
Currently, the most promising technique is in exchanging coupling between the soft magnetic and the hard magnetic phase. However, to realize this, at least two phases are required in the composite. Recent research has made progress in this area, with papers being published within the last few months which demonstrate viable methods for exchanging coupling and therefore controlling the magnetic properties of the material.
It is likely that a number of magnetic applications will benefit from the progress being made in this field, however, it is probable that it will take several years before these applications are realized, with much more research being required before the method is optimized and ready for use outside of the research setting.
References and Further Reading
Dong, A., Chen, J., Ye, X., Kikkawa, J. and Murray, C. (2011). Enhanced Thermal Stability and Magnetic Properties in NaCl-Type FePt–MnO Binary Nanocrystal Superlattices. Journal of the American Chemical Society, 133(34), pp.13296-13299. https://www.ncbi.nlm.nih.gov/pubmed/21800910
Sun, S., Murray, C., Weller, D., Folks, L. and Moser, A. (2000). Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices. Science, 287(5460), pp.1989-1992. https://science.sciencemag.org/content/287/5460/1989
Yu, J., Xiao, T., Wang, X., Zhou, X., Wang, X., Peng, L., Zhao, Y., Wang, J., Chen, J., Yin, H. and Wu, W. (2019). A Controllability Investigation of Magnetic Properties for FePt Alloy Nanocomposite Thin Films. Nanomaterials, 9(1), p.53. https://www.mdpi.com/2079-4991/9/1/53/htm