This learning object centers around an article published fairly early on in the history of nanoscience (Sun, et al. “Monodisperse MFe2O4 (M = Fe, Co, Mn) Nanoparticles” J. Am. Chem. Soc. 2004, 126, 273-279. http://dx.doi.org/doi:10.1021/ja0380852). The article describes what has become a standard non-aqueous route to synthesize monodisperse spinel nanoparticles. The assignment asks students to analyze the solid-state spinel structure, count 3d electrons for the various metal ions involved, and compare the particles' magnetic properties. Students learn how to interpret a magnetic hysteresis loop and compare ferromagnetic and superparamagnetic behaviors.
As written, this literature discussion is best for students who already have some familiarity with d-electron counting and solid state structures. Anyone looking to extend the assignment could use the article to discuss X-ray diffraction, X-ray absorption, and nanoparticle ligand exchange.
After reading and discussing this paper, a student should be able to:
- diagram the crystal packing of oxide ions and the coordination environment of metal ions in the spinel structure
- identify the reducing agent and capping agents used in the synthesis of magnetic iron oxide spinel nanoparticles in organic solvents
- compare the magnetization behavior of superparamagnets and ferromagnets
In the spring of 2011, I used this literature discussion in my junior/senior level inorganic course as part of reviewing for the final exam. I also have used some of these concepts in an inorganic final exam question about the spinel structure and LFSE.
The assignment assumes students already know how to diagram solid state structures using a "layer diagram" (e.g. z = 0, z = 1/2...). Students should also be familiar with trends in the sizes of tetrahedral and octahedral holes in a solid state structure and trends in ionic radii. Students do not necessarily need previous exposure to magnetism concepts.
In the final exam question (not included here), I ask students to calculate the LFSE (in terms of the octahedral splitting parameter) for each of the ions in the structure. (The individual LFSE's will vary based on d-electron count and the geometry of the surrounding oxide ions.) One could also then introduce the concept of an "inverse spinel" in which the 2+ ion is in an octahedral hole and the 3+ ions are in octahedral and tetrahedral holes. Students can sum up the total LFSE for each structure and predict which structure will be more energetically favored.
I do not have a completely satisfying answer for the last question on the literature discussion. The answer centers around the concept of magnetic anisotropy. If there is anyone in the VIPEr community who knows more about this than I do, please leave a comment.