Understanding the chemical origins of half-metallic ferromagnetism Ram Seshadri, UC Santa Barbara, DMR Half-metallic ferromagnets are a class of magnetic materials with the special quality that all the conduction electrons belong to only one spin direction. Fundamental understanding about why some materials and not others show this property is expected to lead to new materials for electronic devices that make use of the spin of electrons. Heusler compounds with the general formula XY 2 Z follow a general rule that the 24 electrons are non magnetic, while other electron counts (25, 26, 27, etc.) are half-metallic ferromagnets. The 27 electron compound NbCo 2 Sn does not obey this simple electron counting rule. One reason could be the structural change at low temperatures. It puzzling however, that even the hypothetical cubic compound computed by us is not half- metallic. The figure alongside displays the results of magnetic measurements. The moment at low temperatures (top left panel) is 0.6  B, which is close to what is computed (dashed horizontal lines) but is far from the expected value of 3  B. The work shown here was carried out by in part by an RET intern, Mr. Barnaby Dillon, from Dos Pueblos High School, Santa Barbara.

Understanding the chemical origins of half-metallic ferromagnetism Ram Seshadri, UC Santa Barbara, DMR The figure displays, for two different half-Heusler compounds, the electron localization function, and the electronic charge density. TiCoSb is a semiconductor and VCoSb is a half-metallic ferromagnet. These figures strongly suggest that the Co and Sb atoms form a covalently bonded zinc blende lattice, isostructural and isoelectronic with GaSb. The Ti4+ and V5+ ions are not a part of this covalent network. This analysis permits this class of half-metallic ferromagnets to be thought of as semiconductors stuffed with magnetic ions.

Understanding the chemical origins of half-metallic ferromagnetism Ram Seshadri, UC Santa Barbara, DMR Broader impact: In August 2006, the PI was lead organizer of a 2-week Materials Summer School on the topic of Porous Materials, sponsored by the ICMR program of the National Science Foundation. Graduate students, post doctoral fellows, and early career faculty from the US, as well as nearly 15 other countries attended. The summer school emphasized participation by students from countries that have not traditionally had a strong science infrastructure. Students from countries such as Argentina, Thailand, Uzbekistan, Lithuania, and Ukraine attended.